Why Fast? Part One – Weight Loss

“When a person has nothing to eat, fasting is the smartest thing he could do.” – Herman Hesse, Siddhartha.

I like that quote. It’s making (non-caloric) lemonade out of lemons, and for all the transcendental insights contained in Hesse’s book, this line strikes me as a really cool, no-nonsense way to make the best out of a bad situation. No doubt about that. But how useful is it, really, to today’s readers? Very few of us ever have “nothing to eat.” On the contrary, food is ever at our beck and call, with very little effort required to obtain it. Actually, that’s not completely true. Processed junk and fast food is readily available, while the good stuff – fresh meat and veggies, actual, you know, food – requires prep work, cooking, time, and the doing of dishes. But the main point stands: we rarely go without.

That doesn’t mean the quote is useless. In fact, with a few slight modifications, it becomes extremely effective weight loss advice. Check out my version:

“When a person has had too much to eat, fasting is the smartest thing he could do.” – Mark Sisson, Mark’s Daily Apple.

If that sounds harsh or even unrealistic, consider the story of the Scotsman. Back in 1965, an obese Scotsman of 27 years and 456 pounds came to the Department of Medicine in Dundee, Scotland, with a problem. He needed to lose weight. A (1/8 of a) ton of it. The doctors suggested maybe not eating for a few days could help. It was just an offhand recommendation, but our Scotsman (known only as “AB”) really took to it. He stayed at the hospital for several days, taking only water and vitamin pills while undergoing observation to ensure nothing went wrong. When his time was up, he continued the fast back at home, returning to the hospital only for regular monitoring. After a week, he was down five pounds and feeling good. His vitals checked out, blood pressure was normal, and though he had lower blood sugar than most men, he didn’t seem particularly impaired by it. The experiment continued… for 382 days.

Yes, AB fasted for 382 days, drinking only water and taking vitamin, potassium, and sodium supplements. All told, he lost 276 pounds, reaching his target weight of 180 pounds and maintaining the bulk of his weight loss. Over the five following years of observation, AB regained just sixteen pounds, putting him in excellent, but underpopulated territory (at least 80% of dieters eventually regain all the lost weight). Other doctors paid attention. Maybe it was the fact that it was the 60s, and all sorts of crazy stuff was going on – rebellion in the air, good music being made, a war in Vietnam, Ken Kesey and his Merry Pranksters blazing across the U.S. in a beat-up school bus, spreading the good weird word, Kevin Arnold and a young Marilyn Manson coming of age in Anytown, USA – but for whatever reason, placing obese patients on extended and short-term fasts became relatively common practice.

But could this work for the average person looking to lose weight without submitting to constant medical observation?

Absolutely. Study after study shows that whatever you want to call the protocol – intermittent fasting, fasting, alternate day fasting, or alternate day caloric restriction – it works very well for weight loss. A few recent ones:

So, yes: it works. But does fasting work solely through caloric restriction, or is it doing something special?

That’s the real question. There’s no question that fasting causes weight loss through caloric restriction. Obviously, when you don’t eat anything, your body turns to its own stored energy reserves, reserves that take up physical space and have mass. Depletion of those energy stores reduces mass and thus weight. Total and absolute caloric restriction. That’s elementary stuff and the studies from the 1960s show that.

To dig a bit deeper, let’s look at how weight loss occurs during a fast. I’ll stick to research involving humans only (sorry, rodent personal trainers).

Secretion of growth hormone, one of the premier fat burning hormones, increases during a fast. In a five-day fasting protocol, men experienced increased GH secretion on day one and day five (the only two days where GH was measured). A later study showed that during two-day fasting sessions, growth hormone secretions increased in both frequency and intensity in men. They experienced more frequent GH bursts and each burst secreted a higher mass of GH. A more recent study found that 24-hour fasts increased GH by 1300% in women and almost 2000% in men.

Fasting decreases fasting insulin levels. The presence of insulin inhibits lipolysis, the release of stored triglycerides (body fat). Without lipolysis actually releasing stored body fat, it’s rather difficult to, well, burn that body fat for energy. During a fast, fasting insulin decreases and lipolysis increases. This insulin-blunting aspect of fasting quite literally allows the fast to be successful, because without the ability to access stored body fat for energy, making it through a period of zero caloric intake will be nigh impossible.

Fasting improves insulin sensitivity. 20-hour fasts were enough to improve insulin sensitivity in men.

Fasting increases the catecholamines, both adrenaline (epinephrine) and noradrenaline (norepinephrine). Both catecholamines increase resting energy expenditure during a fast, and guess where your fasting body finds the energy to expend? From body fat. Catecholamines activate hormone sensitive lipase present in adipose tissue, spurring the release of said fat. This makes intuitive sense, doesn’t it? If you’re hungry in the wild, you need to hunt (or gather, or fish, or somehow procure food) and you need energy to do it. The catecholamines help provide some of that energy while burning fat in the process.

Hmm, notice anything? All those mechanisms dealt with fat burning specifically. While there may be some weirdo out there who’s interested in reducing bone mineral density and muscle mass while maintaining fat tissue, I would wager that what most people mean by “weight loss” is “fat mass loss.” From the stuff I just linked, it looks like fasting burns fat, rather than just weight. But what about Conventional Wisdom which claims that fasting increases muscle wasting – maybe because your body will totally recognize the lethal nature of all that arterycloggingsaturated animal fat and choose to break down muscle instead? Is it true?

Let’s go to the research:

In one study, normal weight subjects ate just once a day without reducing overall caloric intake. Weight didn’t change, which isn’t really surprising, but body composition did change – and for the better. Body fat decreased and lean weight increased (in addition to a bunch of other beneficial changes) without an overall reduction in calories.

recent review of the relevant literature found that while fasting and caloric restriction are “equally as effective in decreasing body weight and fat mass,” fasting is “more effective for the retention of lean mass.”

Conventional Wisdom strikes out again.

In closing…

It appears that fasting “works” in several different ways:

1. It decreases caloric intake. In order to lose weight, you need a caloric deficit. That really isn’t in contention here, folks.

2. It increases fat oxidation while sparing lean mass. Since what we’re trying to do is lose fat (rather than just “weight”), the fact that fasting increases hormones that preferentially burn fat and decreases hormones that inhibit fat burning is extremely desirable.

3. It improves adherence. In most of the studies surveyed, participants found fasting to be an extremely tolerable way to diet, especially when compared to outright caloric restriction. Even AB, the fasting Scotsman, reported very little difficulty throughout his 382 day fast. If fasting is easier for you than trying to laboriously count calories, fasting is going to be the more effective weight – er, fat – loss method.

All in all, fasting is an effective way to lose body fat. It’s not the only way, and it isn’t “required” for Primal weight loss, but many in the community have found it to be very helpful and the literature backs them up. If you’re looking to jumpstart your fat loss, fasting may be just the ticket. To get some ideas, be sure to check out my post on various fasting methods.

In subsequent installments, I’ll highlight some of the other benefits of fasting. There are a ton, and new research is being released all the time, so I expect I’ll have a lot to discuss. Until then, I’d like to hear about your experiences with fasting for fat loss. Has it worked? Has it failed you? Let us know in the comment section!

Thanks for reading, everyone!

Here’s the entire series for easy reference:

Why Fast? Part One – Weight Loss

Why Fast? Part Two – Cancer

Why Fast? Part Three – Longevity

Why Fast? Part Four – Brain Health

Why Fast? Part Five – Exercise

Why Fast? Part Six – Choosing a Method

Why Fast? Part Seven – Q&A

Dear Mark: Women and Intermittent Fasting

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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365 thoughts on “Why Fast? Part One – Weight Loss”

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  1. Very interesting article, especially since I am trying to lose some weight. It seemed to me that I read somewhere that you told people not to fast until they had their weight under control, but obviously I must have that wrong.

    I personally have found that fasting happens almost effortlessly once you start eating Primal. I often have coffee for breakfast and then start getting hungry and notice I’ve worked right through lunch, and since it’s usually about 4 or 5, might as well wait and have an early supper at 6.

    Haven’t tried fasts of any longer than that yet, but since it seems to work so well for weight loss I’m certainly thinking about it now.

    1. Good observation. It’s very easy to go from Primal/Paleo to fasting because both rely on lipolysis to fuel the body. With these fat-burning enzymes already up-regulated, going from lipolysis while eating to lipolysis while fasting is seamless.

      You can imagine how many people eating a SAD (standard American diet) might try fasting and have a terrible experience because their body just does not have enzymes needed for lipolysis in the fasted state because their bodies have up-regulated enzymes to run on glucose (sugar fixes) and not fat.

      1. There is another concern for fasting among those who aren’t primal/paleo. Sympathetic tone (aka fat burning hormone influencers) goes down with fasting. This happens at a quicker rate in those without experience missing meals.

        Because of this decrease in sympathetic tone, people may feel more lethargic than just their altered brain fuel mix may lead to. And if you try to buzz yourself with some coffee to increase sympathetic tone, lightheadness may crumple you!

        Not to mention that SAD dieters are more likely to eat things that contain artificial sweeteners, which for some may act as excitotoxins in the brain and screw up the mental picture even more.

        Not to mention that

      2. I was wondering about how fasting went against common wisdom and studies that showed how muscle and bone wasting happened faster than fat loss.

        I’m also wondering if the faster muscle loss was for people who used their brains more than their bodies. Doesn’t the brain need fat, while the other systems can use any fuel, making the fat the most valuable resource to keep the brain going?

        Wild speculation on my part, but worth looking into further.

        1. Actualy, the brain relies purely on glucose for energy. Glucose can be synthesized by protein intake (during fasting muscle catabolism is used for this)

        2. Only a fraction of the brain’s cells require glucose. I think I have read that it’s around 20%. The long, thin nerve cells in the brain have sections that don’t have mitochondria, so they need glucose for fuel.

        3. The brain can run on a mix of ketones and glucose. Up to 60% of the brain’s function can be supported by ketone bodies, the product of lipolysis alone; the remaining 40% requires glucose.

        4. Lojasmo: Wrong. Just like the rest of your body, the brain runs on glucose ONLY IF YOU FEED IT GLUCOSE from carbs. If you cut carbs completely out of the diet, which has been done MANY times for up to a year or more with NO negative effects, your brain runs just fine on protein and fats. The most famous of these experiments was done in 1930 by Vilhjalmur Stefansson. Click on reference #12 to go to the original published paper here: https://goo.gl/Ex5PE

          “Once again, the “experts” have misled us. First, they told us that our brain wants to run exclusively on sugar (glucose) and that sugar is its preferred fuel. This is absolutely incorrect. Your brain wants to run on ketones produced from burning your own bodyfat, NOT sugar. When carbohydrates are minimized, the brain naturally obtains all the energy it needs from running on your own bodyfat. It is only when overdosiing on carbs that this mechanism is short-circuited.” https://goo.gl/GTSmP

          “Surprise #6 — We are told that ketosis will cannibalize our muscle tissue. Medical Fact: After just 3-5 days of fasting, our body requires only
          1/3 the amount of glucose it has been forced to tolerate during “the great
          carbohydrate eating experiment” — we could eat less than 1/3 of a bagel a
          day and maintain superb health. The brain and nervous system start
          to use ketones, because they finally get them.” https://goo.gl/iY4mu

          And From Richard Feinman:
          “Under conditions of starvation or carbohydrate restriction, acetyl-CoA can be effectively transported from the liver in the form of ketone bodies. Ketone bodies, then, are a source of acetyl-CoA that can be used by brain and CNS. Red blood cells are still dependent on glucose but the brain’s demand for glucose is reduced by the availability of ketone bodies.”

        5. Your brain does need a little glucose but you don’t need to ingest that. The liver makes just enough glucose out of protein to keep the brain happy (gluconeogenisis). Ketones take care of the rest of the brain’s needs.

      3. Hi can I fast for 3days straight and then eat? At what point will I screw up my metabolism? I don’t want that to happen! What fruits must I avoid?

    2. No, don’t believe any of the things Mark says about fasting and weight loss. It is HORSESHIT HE PULLS OUT OF HIS ASS AND MADE-UP STUDIES THAT ARE CONTRADICTED BY OTHER STUDIES AND THE MEDICAL ESTABLISHMENT. Eventually, people will get hungry and feel the need to eat–a basic biological function–and trotting out anecdotes like the Scotsman in 1965 are scientifically worthless. Besides, how is it possible to get nutrients without eating REAL food, since vitamins have not been tested by the FDA or anyone else for proff that they really do get aabsorbed and digested?


      1. You seem like you have just enough knowledge to get yourself into arguments, but not quite enough to win them.

      2. Hi Jeremy, I didn’t believe that vitamins were approved by the FDA either, until a pharmacist recommended taking a prescription vitamin called prenate essential or prenate elite because their vitamin c, folate etc content is higher. These vitamins are FDA approved which must mean they aren’t a waste of money. They are just more expensive. I believe prenate essential is over 100$ without insurance.I pay 50$ each month for the vitamins, because my insurance, anthem/ blue cross, pays the rest. My co-pay for meds and doctors is 30 percent, so maybe the vitamins are more expensive.
        I don’t know if men can take prenate elite or prenate essential. I would like to find out if there are prescription vitamins which are not prenatal. I keep hearing that anything that’s not fda approved is questionable…which is scary.

      3. If interested, take a moment to look up “Features of a successful therapeutic fast of 382 days’ duration” by W. K. STEWART, M.D., F.R.C.P.E., M.R.C.P. Lond., and LAURA W. FLEMING
        B.Sc., published in Postgraduate Medical Journal (March 1973) 49, 203-209. The Abstract of this scientific article written by the doctors of the above mentioned experiment reads: Summary. A 27-year-old male patient fasted under supervision for 382 days and has subsequently maintained his normal weight. Blood glucose concentrations around
        30 mg/100 ml were recorded consistently during the last 8 months, although the patient was ambulant and attending as an out-patient. Responses to glucose and tolbutamide tolerance tests remained normal. The hyperglycaemic response to glucagon was reduced and
        latterly absent, but promptly returned to normal during carbohydrate refeeding. After an initial decrease was corrected, plasma potassium levels remained normal without supplementation. A temporary period of
        hypercalcaemia occurred towards the end of the fast. Decreased plasma magnesium concentrations were a consistent feature from the first month onwards. After 100 days of fasting there was a marked and persistent
        increase in the excretion of urinary cations and inorganic phosphate, which until then had been minimal. These increases may be due to dissolution of excessive soft tissue and skeletal mass. Prolonged fasting in this patient had no ill-effects.”

      4. After three days you are no longer hungry. Hunger comes back only when its critical to eat. I fasted for 90 days on just water. It was really easy. I walked 5 miles a day and worked in the garden. I only lost 60 pounds while most people would lose 90. I was 179 and ended up 119. I went up to 135 and kept that weight for four years. Stopped exercising and it all came back. Easiest way to lose weight that there is if you have the time to be off work. I have done several short fasts since (around 21 days) and the hunger always completely disappears after three days. No matter what I do 15 pounds always comes back fast.

      5. This idea is actually incorrect. The feeling of hunger quickly goes away if ignored because it s based heavily on daily routine. If you eat very early in the morning your body will want to eat then because it has become accustomed to doing so.

    3. Eating Primal has removed the need to eat at set times. I have different work/play hours every day, so going several hours between meals is no issue as I don’t have the cravings I used to while eating SAD and taking metformin. I have cut my metformin in half, sugars normalizing so will be able to come off metformin completely soon (started one month ago), weight loss 20 lbs so far, not starving, loving the food, don’t miss carbs. When I don’t have “good quality” food around, I just don’t eat. In addition, I now recognize true hunger, which peeks in only every now and then on my Primal diet. I don’t want to be extreme, so my fasts will be delayed meals caused by busy with work or play rather than scheduled,

  2. Every part of me wants to be wary of this, even though the evidence is pretty solid. I guess my semi-irrational concern, comes from meeting many anorexics who use this kind of info to justify very long “fasts”, i.e. the scary “ABC” diet. Then again, any information can be used for good or bad. Great article


    1. I come across this stigma all the time. People think of gaunt, zombie-like, figures when they hear the word fasting. I practice IF and I have gained strength and muscle, while losing fat. Check out my website, at the bottom of my “about me” page, and you will not find a picture of a gaunt zombie. I promise. In fact you might even say I look very healthy. 😉

      1. Yup Its true I have a friend that does IF and he looks like a Spartan lol

    2. I understand where you’re coming from on this, but IFing and anorexia are different. Do a google search on anorexia, and read some of the stories. One I read was about a woman that would drink a coke or two and smoke, but not eat anything else during the day. That is NOT what paleo or primal folks do when they fast! I would drink coffee, tea, or lemon water during the fast, and always break it with a decent sized meal (at least a half pound of meat, veggies, maybe some starch thrown in).

    3. An anorexic doesn’t need articles like this or any other to justify their actions. IFing is not close to being anorexic. When you suffer from anorexia you don’t ever had a “re-feeding” period. When you do eat, its so small and not much substance. When your feeding window opens for IFing you EAT and in my case eat a lot. I have been following a 16 hour fast/8 hour feeding window for almost 5 weeks. During my 8 hour feeding window I still only eat two meals, maybe 3 if its a hungry day. On average, just two Primal meals. My body fat has gone down, my lean muscle has remained. My CrossFit workouts have improved in all areas, strength, cardio, endurance…. I also feel better mentally. Can’t describe why, just an amazing side effect. 😉

      1. This is exactly how I feel as well. I’ve been dabbling with 24 hour fasts once or twice a week with the ocassional 16-8 (when I forget to eat) and I’ve never felt better. My workouts have improved and so has my body!

      2. Hi Tracy – could you please specifically describe the 16hr/8hr fast you do? I’m on hour 11 of my fast. I need to cut fat but need to keep strenght up for my training.

    4. I know what you are thinking, I am recovered from an eating disorder and I keep my IF between my husband and myself since I do not want anyone else labeling me or lecturing on how bad it is. The whole key to this is only fasting intermittently. You do not just fast indefinitely and then eat as little as possible. The key is to eat regularly and break up that pattern with the occasional fast.

    5. Just remember that anorexics tend to eat almost nothing all the time – nothing with substance or real nutritional value. They have weights that are dangerously Lowe…but their bodies finally fail, but their brains are the last to drop out, coma etc. Worth noting? And it is extreme, abnormal. IF is not extreme.

  3. Inspired to do a 20 hour fast…starting…now! (Note to self: wife’s turn to cook dinner, so I should let her know I won’t be eating tonight.)

    A couple of questions, though.

    What about impact of caffeine during a fast? The idea of no food AND no coffee really strikes terror in me.

    Workout intensity? Should I just be a slug during the fast, normal activity (long, slow walks or bike rides), intense workouts? Intense workouts usually make me hungry shortly after.

    1. Caffeine will actually increase catecholemine levels, and burn even more fat. So drink up. Black is best, a “splash” of milk won’t hurt, and sugar will hurt.

      You should keep activity low to moderate. Walking is great and will help to burn even more fat. Extended sessions of “cardio” will only be detrimental. Intense activities are great in the fasted state. Some anecdotal evidence supports increases in strength, probably because of the epinephrine. However, you should eat after an intense workout, because you may experience a drop in leptin, which will slow your metabolic rate and cause you to burn muscle. Contact me through my website if you need more advice.

      Matthew Caton

      1. I’ve heard (and kind of experience) that coffee holds off hunger too. Why is that?

      2. You’re describing my regime to a ‘T’! I started experimenting with IF recently having followed a primal diet for three years, but loaded a bit of extra weight over last Christmas which wouldn’t shift. At first I was worried about ‘lightheadedness’ or hunger, which was apparent during the first couple of goes. But it disappeared quickly and now I don’t notice that I haven’t eaten. In fact what I do notice is a heightened alertness and awareness which doesn’t necessaarily mean caffiene (although I drink @ 4 cups of black coffee a day), I’m buzzing before I have a cup! It’s become a routine for me now during the week but I eat healthily and heartily in the evenings and at weekends. I’m 179cms tall and weigh in at 73 Kg. Feeling good!

    2. I just did a 20 hour fast and had a good workout 4 hours before breaking it. I felt great all the time. Felt like I could go on like this for a very long time, but moderation is better than fanaticism, so I broke it and will repeat in a week or so. I’ve been primal since 3 months, so it came very easily and naturally to me.

    3. I prefer to workout intensely during a fast since it boosts the body’s production of growth hormone. Read Mark’s articles about fasting after workouts for reference. Of course this is all about personal preference, but it can help you get the most out of your fast. As a bonus, pushing past hunger helps you appreciate your food much more.

    4. Hey I start my fasts after lunch usually counting it from about 6pm to 6pm ..for ex lastnight to now about 21 hrs into my fast i weigh 4 pounds less… but i do exercise everyday which accounted for 2 extra pounds of weightloss and i have been eating primal for almost 2 weeks before this fast so my body isnt in shock to the fasts anymore… hoping to lose a good 15 pounds before i stop the fasting ..hopefully by sunday for football party lol

      1. point was i always have coffee! it kickstarts my metabolism so i workout for an hour after 2 cups and like i said i continued to lose 2 extra pounds after the coffee and workout

    5. While caffeine may suppress appetite and help to burn fat – you should be reminded also that it places a lot of strain on your internal system and especially liver and kidney. If you must take caffeine – take the green coffee extract and/or guarana.

      It appears that many seem to view weight management and nutritional supplements as an activity that can utilise individual approaches. We need to bear in mind that the human body functions via an intricate relationship of complex systems that ideally work to maintain homeostasis.

      For this reason alone there are always pros and cons of ingesting anything. The best thing you can ever take while fasting is WATER. Not distilled water either.

      Why? Because fasting is a detoxification process and will release feel good into the brain – but you will need to flush out all those toxins that your body has stored to gain full benefit. If you can’t handle a full water diet, try miso soup in the evening (full of nutrients) to aid your liver and kidney function.

      After 48 hours appetite will decrease and supposedly the production of grehlin and if you make it to day three you may have halved your GI.

      If you break out and celebrate by a great sugar – insulin spiking feast you will find you quickly regain your appetite and lose any benefits you gained. Yet if you want to continue the system balancing approach – try day 4 onwards to wean yourself back onto foods via vegetable juices (and fruits in balance). Include some fibre and protein and try to stick to this combination to maintain strength and rebuild your metabolism. This is another area where there is conflict opinion that juices etc (fresh not processed) create an insulin spike and should be avoided for weight training. If the vegetable and fruit juice is a balance of the right nutrition – it can in fact assist the body. For example – Carrots are listed with a high GI rating and so are Beets. Yet Beet juice included in your mix will actually help to balance blood sugar. Carrot juice is well known antioxidant and for Vitamin A and B group vitamins – but is also a lesser known phytonutrient – currently being researched for its ability to detoxify and prevent colon cancer. I won’t bore you with the values of other veggies and fruits. The point is – that when in a non processed juice format (some fibre retained) the insulin hit is actually supporting the live energy and nutrients to get quickly into your blood stream, wake up and activate your body systems. Less energy is needed to break down the nutrients from “waste” and that allows greater absorption of the good stuff to give you more power and energy to spend on your work outs. If really worried about the increased insulin spike, combine with protein powder and ice to make a balanced smoothy. Its like high octane instead of 2 stroke.

      If you are only interested in fasting for fat burning and not detoxing and improving your health – remember that detoxing is actually supporting your body systems so that you can process and burn those fats. Fasting on water alone for three days will maximise the benefits if you don’t blob out afterwards. Water will also assist you from side effects of fasting – such as mild headaches etc. from caffeine withdrawal and other toxins.

      If you fast for spiritual purposes or other “endurance” fasting over a long period you will lose weight but you also train your body to adjust to “starvation mode” – this is why the lady who wrote above lost less weight than she should. Alternate and intermittent fasting combined with a health supporting diet that utilises nutritional value will do more than any other type of regime. Exercise before breakfast will also help to curb the hunger cycle during non fast periods. The other important factor to note is that just as our finger prints are individual and unique – so are our bodily responses to diet and exercise. There is no “one” perfect way for everyone. We need to listen to our own bodies – also to evaluate our current position (what is out of balance through noting health issues and conditions) and then working out the relationship plan best suited to us. This takes time and we can benefit from a whole system approach and advice via naturopath and nutritionist who is also sympathetic to our motives i.e. – health, or fitness, or physical appearance.

      A few years ago someone in my neighbourhood died – someone who was a health and fitness instructor – who also was only in his thirties and had been thought to be in perfect health. This person had apparently been approaching weight training with extreme approaches and popping some “health” pills – causing renal failure. If you are going to fast and do HIIT or other intense cardio etc. I recommend you check your heart and organ health before. Otherwise, consider using it as a tool to detox and rebuild vitality for some serious post fast fat busting and workout.

      Just some alternative input to think about.

      Best wishes

  4. I have been IF:on on and off for almost a year now but recently I was told that long term fasting raises cortisol-levels which stress the body if done for to long. I remember it having something to with the circadian rhythm.

    If somebody could shed some info on this it would be appreciated.

    1. I am also interested in thoughts on this. I do the occasional IF or or restrict eating to an 8 hour window, but have been told that anyone suffering from burnout or thyroid issues shouldn’t do this as it raises cortisol levels and puts more pressure on the body.

    2. I’ve read that the study this information came from was taken out of context. Cortisol will increase during the fast, which is natural and a part of keeping blood sugar stabilized. However, mean cortisol levels did not increase. Only “short-term” increase were found.

      Due to the circadian rhythym, I find that fasting is best in the morning and early afternoon, and eating is best saved for the late afternoon and night. This way eating patterns will be in-sync with the diurnal rise and fall of cortisol. Cortisol is high in the morning and drops at night. Fasting in the evening may not be the best thing because then you will be elevating cortisol at night.

      1. This reminds me of The Warrior Diet philosophy. It’s been forever since I read the book though, but I do recall it speaking about this way of eating. Have you read that book?

    3. Intermittent fasting was one of the changes that helped me get rid of my (admittedly minor) asthma. Thus I would expect that it actually helps to reduce long term cortisol-levels or at least inflammation.

    4. I don’t know if anyone answered you yet, but what your talking about is the actual fast. Like a 48hr fast vs a 16-20h fast. If you break your fast within even 24 hours you don’t have to worry about that. 🙂 VERY long fasts are what raise cortisol levels.

    5. I’ve been very interested in and have pealrnosly conducted fasts on my own for the past 25 years. I am currently an alternative health practitioner and have helped people work through many health issues using fasting. For these reasons, I buy every book I can find on the subject. Juice fasting, has been my favorite type of fasting so I was particularly interested in this book prior to receiving. Unfortunately, I wasn’t as enthusiastic after reading it. I found this book in congruent, disjointed and incomplete. It had a lot of anecdotal and miscellaneous information that might be interesting for trivial knowledge about the subject but not congruent enough to be useful when conducting a proper fast. If you want to read a book on juice fasting that’s complete, to the point and relevant, I’ve never read anything better than the old standard by Paavo Airola How to Keep Slim, Healthy and Young With Juice Fasting . It’s amazing how ahead of his time Airola was, after all these years, it still the best book on the subject.

  5. “They were also monitored while eating a normal diet during an additional 24-hour period.” I’d love to know what this “normal diet” consisted of.

    Also, it mentioned the increase in HGH after the fast. Did these people exercise fasted at all? Is it recommended to break your fast THEN exercise?

    1. Exercise fasted, it further boosts your HGH production. In fact, people who don’t participate in regular fasting should always train in the morning before they have eaten. I always work out before I eat. Check out the website “Lean Gains” for fasted training information

  6. So much of the fasting information I’ve read has to deal with weight loss or body fat loss. Are there benefits of fasting for a lean, athletic female (or male for that matter) that doesn’t have trouble staying lean in their current primalesque lifestyle?

    1. The study Mark referred to as improving insulin sensitivty was done on young men with who were not obese. It also includes results that show increases in other bio-markers as well.

      1. I understand that the study was in non-obese men but it still measured fat oxidation and weight loss. If anything, I trend toward too lean and I don’t need to lose any weight. The study on the non-obese men also mentions that they were hungry during the fasting days. I also don’t like being hungry!

        So I was referring more to the benefits of fasting in lean people at a healthy weight that they have maintained for years that don’t have fixations on meal timing and quantities.

  7. Hi all – can anyone recommend the best book about diabetes and primal diet? my co-worker is so sure he needs to constantly eat to keep his “blood sugar” up so he doesn’t “pass out” that he would never consider fasting. I am doing my best to educate him about primal wisdom (and maybe he will read PB if I shove it at hiim), but I will buy him a book if it specifically in detail discusses diabetes and carbs! thanks!

    1. Google this article, you can access it free on-line:

      “Role of glycemic index and glycemic load in the healthy state, in prediabetes, and in diabetes.” Am J Clin Nutr. 2008 Jan;87(1):269S-274S.

    2. Fiber Menace by Konstantin Monastyrsky (https://gutsense.com/fibermenace/about_fm.html) discusses, among other topics, diabetes and it’s relationship to carbs/fiber. I read this book before I found MDA & the Primal way of life and I found it enlightening. LOTS of parallels with the Blueprint…

    3. I can tell you from the personal experience of managing my son’s type 1 diabetes mellitus that the primal lifestyle is absolutely an option and a good one. The key is to watch the insulin levels carefully because you’re going to have to adjust them downward, and fast. My son went from using nearly 55 units total per day to now being in the low 30’s, almost half, which is great from two perspectives 1) more insulin means more fat storage and more hunger pangs and 2) the law of small numbers means that injecting for 10 carbs at a meal is much safer than doing the same for 100. This is because, if you’re wrong, if it’s really 8 carbs and not ten, then you’ll be a tiny bit low… but being wrong by the same percent at 100 carbs can mean a dangerous life threatening low (a major cause of premature death for type one diabetics).

      Obviously any diabetic should consult with their endocrinologist before changing your insulin but I recommend looking into the paleo/primal/ancestral lifestyle. It’s more than just possible, we think it’s great. He’s a lot more stable than he’s ever been before (and thriving, he’s already almost taller than I am and he’s 11) and that’s a very good thing.


  8. I like intermittent fasting, but I think you’ve gotta really be adapted to a primal type of diet for it to work. You’ve gotta have good blood sugar control, and stress and sleep under control for it to work well in my opinion.

    1. A normal metabolism would be helpful, but just extending the time about 30 minutes every morning before eating is an easy way to break into fasting with a bad metabolism.

  9. I find intermittent fasting to be so easy to do compared to calorie restriction.

    Also, if there isn’t anything Primal around to eat, I have no fear that my muscles will wither away if I simple don’t eat until I can eat Primally!

  10. I have been eating one meal a day (dinner) and I found after a few days, that I felt more energized and stronger. I workout 6 days a week and have not felt weak, tired, or dizzy because of it. I found the hardest part about it is enduring to constant barrage of CW and explaining why what I’m doing is ok. But its easier when I look in the mirror or when I visit the doc and my resting BP is down. The key for everyone is to try and see what works for their bodies. This works for me and I thank Mark and my Primal family for sharing stories and giving support. This has definitely changed my lifesyle for the MUCH better.

    1. Isn’t funny how people with metabolic and health ailments can look you in your healthy face and say what your doing is going to kill you? It’s sad that some people are so bound by stigma.

    2. I could not have put it better been doing the same for a month now lost 22 pound in two months and have never felt so good when exercising but it’s not for everyone I’m a shift worker and it’s the only eating routine that has ever worked all the best Greg rees

  11. I fast 16/8 pretty much every day. Sometimes my fasts extend to 20/4 or something like that… definitely don’t be scared of fasting.

    However, I have not ever, not once, gone over 30 hours. I’m basically at my ideal weight, so I really have no desire to do it for fat loss, but I would consider it just as an experiment of sorts — however, I largely enjoy eating, so I’m not sure if I will at this point in time.

    Lastly, Marilyn Manson wasn’t on The Wonder Years – per snopes.com

  12. I am trying more/longer fasts now – but without getting too crazy about it – what happens if you have ONE handful of nuts, or one slice of raw cheese, or some kombuchka – does that entirely elimiate all the benefits of the entire fast????

    1. I don’t think so, since foods lacking carbohydrates will have little to no effect on insulin levels and sensitivity. It may blunt some of the effects, but probably not all of them.

      1. Fasting is fasting. The whole point is to give your digestive system a break. Even coffee is not ideal because it tells your body food is coming – digesting coffee is using digestive system. If you want to experience the full benefits of fasting, it needs to be done for at least 24 hours at least once a week. It gets easier every time. READ what Mark has written. Stop asking foolish questions and take the time to read what he has prepared. Another wonderful writer on this subject is Brad Pillon of Eat Stop Eat. Thank god for Mark and Brad.

        1. I doubt you’ve actually READ a WORD of Pilon’s writings about fasting. If you had, you’d know that he drinks black coffee daily on his fasts AND he says it is ok for people to drink unsweetened teas and coffee on the fast.

    2. +1 about the kombucha! I’m in the middle of a 20 hour fast right now and I have a bottle of kombucha right next to me… Calling me… Begging to be consumed…

    3. Your insulin levels will still rise, definetely not as much as if you eat ice cream though. Depending on how much or how little you ate it would probably take a few hours before insulin levels fell back to fasting levels. For further advice contact me via my website.

    4. I recommend checking out leangains.com for more information about the 16/8 fast – it suggests a splash of cream in your coffee is ok, but anything even getting around to 50 calories is not good for the fast. I would not risk eating the handful of nuts or cheese, it’s not going to temper your appetite at all and will probably make you hungrier. Snacking is more of a psychological mechanism, just don’t do it 🙂

      1. The guys sitting next to me at Mark’s presentation just mentioned the same site… And it seems to line up with Mark’s bullet points on shifting/conforming eating patterns to your body (eat when you’re hungry) as opposed to a set of societal/customary norms (“Breakfast is the MOST IMPORTANT meal of the day”… “Eat 6 small meals a day”… etc.)

  13. I’ve been doing daily 16 hour fasts with a relatively low-carb paleo diet during my 8 hour feeding window. Is this long enough of a fast to get the fat-loss benefits or should I try alternating days like in some of the studies mentioned?


    1. Alternating days can be hard if you are trying to maintain muscle or train in the Primal context.

      You will start to burn almost entirely fat after 12 hours of fasting, however, after 18 to 20 you will start to break down protein to stabilize your blood glucose. Not a significant amount, but it’s something to think about.

      If you want to split the difference you could do a 20 hour daily fast, but you should eat about an hour after you workout. The 20 hour fast would give you 8 hours of pure fat burning time everyday. Of course you could just change the feeding window back and forth anywhere from 4 to 8 hours whenever it suits you. For further advice contact me through my website.

      1. I’m currently eating in an 8 hour window that changes from day to day. Sometimes I reduce the window to less than 8 hours. Seems to be working well so far. However, when it’s time to eat, I really enjoy eating at least 1 lb. of grass fed beef or 8-12 pastured eggs! I absolutely could not do it (I don’t think) if I wasn’t eating at least 1 pretty big meal per day.

  14. Worked like a charm for me to lose the stubborn fat. Esp when combined with low intensity exercise in the fasted state. The high intensity stuff on the otherhand left me feeling a little less inspired.

  15. One of the best benefits of fasting is that you learn that you can decouple yourself from the 3 meal a day norm and not have your world come crashing in. Fasting can help establish a different relationship with food that makes you realize that you are in control of the Sara lee frozen cheesecake rather than the other way around

  16. Wendy, from personal exp I find that snacking during the fast only seems to stimulate my appetite and complicates what is typically an effortless fast.
    You’re bound to inhibit some of the hormonal benefits as well.

  17. Did anybody else picture Fat ‘B’ from Austin Powers when reading about the obese Scotsman?

    1. LOL “Get in my belly!!”
      Having been primal for 32 days, I find my hunger is usually much less insistent (which is really liberating)… but I haven’t tried IF yet. Does the hunger stay mild? Does energy stay OK if you don’t get too strenuous? Thanks Mark, and for all the info Matthew Caton, I’ll check your site next.

  18. I’ve given alot of thought to fasting but am not sure how/whether to proceed since I am diabetic (type II) and take daily long-term acting insulin injections. I’ve suspected the insulin is inhibiting the positive effects of the primal lifestyle. Any thoughts and suggestions would be most welcome.

  19. Just a question – one of the related posts was Ten Sure Ways to Sabotage Weight Loss, and it includes things like skipping breakfast and eating one meal a day as ways to sabotage your weight loss. That seems inconsistent with the post here.

    1. I’m confused about this too. I struggle to eat something for breakfast each day. Now I read that it’s better to skip it? Put us on the right path please!

      1. The association between breakfast and weight loss is just an association – as we saw in the previous post about red meat, associations cannot prove that A causes B.

        The observation is that people who eat breakfast tend to lose more weight than people who don’t. Why is that?

        *Maybe breakfast helps weight loss.
        *Maybe the people who have time to eat breakfast are less busy and less stressed than those who don’t eat breakfast.
        *Maybe the people who don’t eat breakfast at home tend to snack on bagels and doughnuts instead.

        All valid explanations as to why there is an association.

    2. I think that post was from 2006 (the year May 22 was a Tuesday). Marks thinking has evolved. Unfortunately, he doesn’t year mark his posts, so it’s hard to tell sometimes what his latest thinking is.

      1. He doesn’t year mark, but the first comment will have a year on it.

  20. I’m doing John Romaniello’s Fat Loss Forever diet (lifestyle) and having success… check it out

  21. just started tinkering with 1 meal a day and its been great…. sleep is prolly the aspect of my life that benefits the most from fasting

    1. When do you have your meal? I am interested because I would like to mess around with fasting, but I don’t want to eat such a large meal near my bedtime.

      1. Usually around noon or 1 pm… by the time I’m ready for bed I start feeling a little hungry but its not a problem and I fall asleep much easier and get into a really deep sleep

  22. I appreciate that you are writing for what you think is a relatively mentally healthy group, but please consider the ramifications of what you are saying on the unhealthy, in particularly, people with eating disorders. These sick folks want any excuse to think that they are doing the right thing by starving themselves. That story of the Scottsman just scares me because I’ve learned a lot about anorexia and it is a deadly disorder when people of a healthy weight starve themselves for months on end or severely restrict their calories. It is a serious mental disorder and I know that they aren’t the audience you meant this recommendation for, but you are on the internet 🙂

    If you want to understand the mindset that leads to anorexia, I highly recommend Portia de Rossi’s book “Unbearable Lightness”.


    1. Intermittent fasting is not in any way related to anorexia. (I didn’t go to your link so this doesn’t directly address that) I was aquainted with a women who suffered from anorexia. She didn’t IF….she starved herself. Allowed herself a couple of bites of food over the course of the entire day and then would work out like a fiend. As you said it is a serious mental disorder. The concept of an intermittent fast doesn’t create anorexics

    2. You can’t keep truth away from people because someone might misuse it. I think conversely, an anorexic reading this might find a little education about how to reach their desired end in a healthy way. If anorexia nervosa really is a mental condition, they don’t need an excuse and pointing to someone saying an occasional fast in the context of natural eating will produce optimal health, would not really be reading this anyhow. You can’t control how people will misunderstand your message.

  23. Awesome, Mark. I used to fast once a week (the feed/fast approach advocated by John Romaniello). I loved the increased energy during my fasts, so I have stepped it up and now do a “mini-fast” of 16 hours on workout days and 24 hours on rest days. Conventional wisdom be damned–it hasn’t hurt my workout intensity, muscle mass, or energy/productivity. If anything, I’m more productive and energetic during my fasts. I’m happy to see the “4 meal a day” recommendation be debunked, because in retrospect it was a *total* pain-in-the-ass.

  24. I’m a little over 400lbs, and have been just eating one large meal a day for a few weeks now. Not due to weigh for a week, so no idea about weight loss yet, but I now have much more control over my food – I had been really struggling with craving, but somehow eating one meal a day gives me more self control – not sure why, but as its working, who cares why!

  25. I have been using intermittent fasting with great succeess to lower and maintain my bodyfat percentage. My last measurement from skinfold calipers was 7.3%. I’m actually in the fasted state as I type this.

    If you have never tried fasting you will notice the effects of the catecholemines almost immediately. The catecholemines epinephrine and norepinephrine will reach a plateau after around 12 hours of fasting and will remain elevated after that. As Mark said the epinephrine will give you energy, but I would like to add that you will also feel very focussed. Coupled with the mentally calming effect of the norepinephrine, you will experience a natural high. You will feel calm, focussed, and energetic. You will become lost in anything you put your mind to. Personally, I look forward to my fasted state every day.

    Tip: Drink a cup of coffee in the fasted state and you will increase your serum level of these catecholemines.

    What’s not to love about IF?
    Increased growth hormone, autophagy, glucogan, insulin sensitivity, catecholemines, with decreases in bodyfat, and serum insulin.

    It’s terrible that fasting has such a negative stigma on it. Most people think that my daily fasting period is bizarre and I’m going to lose all my muscle mass. If you check out my website you’ll see that definetely isn’t the case. In fact, some research indicates that training fasted can lead to greater protein synthesis post-workout.

    Contact me through my website, linked through my name below my picture, and I can help you create the best fasting routine for you.

    Matthew Caton

    1. Last year I went on a 10 day water fast in an attempt to find out if I had food/sugar allergies. After the first 48 hrs the hunger pains went away and it actually got easier and easier as the fast progressed. More to the point, the catecholemines release caused a clearing of my mind that was almost spiritual. I now have zero qualms regarding IF and have had great strides forward in my health after fasting.

      1. That’s great MadMav. But do you have a website we can contact you through?

      2. After reading an article in ‘Harper’s’ magazine a month or two ago, entitled, “Starve yourself to vigor”, I embarked on a 7 day fast. I am 56 years old, have been a vegan for 7 years, take exercise very seriously, both cardio and some weights and am not overweight at all. I felt euphoric from day 2 on and had boundless energy, doing all my normal activities, including running, playing tennis and doing weights. On the morning of the fifth day I decided to call off the fast because I had lost 13 pounds and thought that was enough. I really felt no hunger pangs and felt like I never had to eat again. Since then, I have done numerous one day fasts and find that they are the only way to curb eating that works for me. When I became a vegan 7 years ago my weight dropped to what it was when I was 17 (172 lbs and 6 ft tall). Now it sits at 163 lbs and I feel great. I also feel a spiritual aspect to fasting, a calming of the mind and body. It is great to hear from people who have had similar pleasant experiences.

    2. You fast every day?

      I would love to learn what a good fasting routine would be for me but I can’t pay for a consultation. It’s great that there are people like you out there that can provide real help though. Thanks.

    3. Matthew-
      No, it’s not such a bad thing that people are a little slow to hop on the latest bandwagon.

      I skip meals myself, and it does work, but I waited until I knew metabolic markers were in line, like rT3, leptin, insulin, and some others.
      People who love their IF just need to realize that it can be a very foolish thing to do if one has a lot of weight to work on and their metabolism is wrecked. Everyone is a bit different.

      I just wish Mark would be a bit more responsible and tell the whole story about when someone may not be ready. Some of us are not 5% away from ideal weight, or trying to get from 13% body fat to 11.5% or whatever!

      I just heard a good interview that Moore did with Kresser about dietary causes for low T3 thyroid hormone and one of them is fasting. Know your rT3.

  26. Okay I have been always weary of fasting from my past history with conventional wisdom. I always thought it was for those people you see on Oprah with eating disorders.

    My coworker and I were just talking about this topic last night! I swear Mark can hear us talking and posts with all the information we need (and the points to pretty much prove that everything I was saying was wrong haha). Also, this post uses science, which I am very found of being a biologist.

    All in all, I think I am going to give intermittent fasting a go. Slow at first and then see where it takes me. The great part is it will be fun breaking my fast with a big ass steak!

  27. For most of the past year I thrived on IF (intermittent fasting) and in the past month I have experimented with ADF(alternate day fasting.) For me, fasting is very calming. My gut is very quiet and the other bodily systems follow suit and yet I have high energy. I don’t suffer any weakness at all and the only “weird” comes in realizing just how much time I typically spend preparing, eating and cleaning up after meals.

  28. I’ve been paleo for about a year now and have lost about 30 pounds… pretty typical story. But it seemed like I had stalled out, with some obvious fat yet to burn. I came across Richard Nikoley’s book “Free the Animal” and was reminded of intermittant fasting. I’ve been doing a 24 hour fast 1-2 times a week for the last month, and have found this very effective in burning fat that I have been having trouble getting rid of. I’m down about 5 pounds, but aside from that, I can see a big difference in how I look. My muscle tone in my arms and shoulders is much more defined, and my gut is slowly shrinking away, much faster than it had been for a while now.

    1. Congrats on the action and results.

      Not sure what you mean by “24 hour fast”, but if you don’t eat from, for example, Sun night to Tues morning, that’s actually more like a 36 hour fast. I would recommend one meal – either breakfast or dinner, NOT lunch – on fasting days, as it is after about 24 ours that you start losing muscle.

    2. Yes…actually that is strangely similar to my experience. I’ve been primal for about a year, and lost about exactly 30 lbs. I have to give credit to the leangains program for outlining a sensible approach to IF. It’s easy, I just skip breakfast and make a big lunch. It drives me crazy when I hear that a paleo diet is hard to maintain. How hard is it to skip breakfast, and have bacon for lunch?

  29. Fasting is absolutely helpful. I first got serious about fasting after I had an eating disorder and gained a bunch of weight (I got over the ED first, mind you, that’s the first step). I lost the excess fat effortlessly, and as I experimented more and more with fasting, I found that a fasted state made me feel better, more alert, and more productive. Nowadays I try to fast for at least 12 hours overnight, every day, with occasional 18-24 hour fasts thrown in on a really good day. I also do all exercise fasted, except for Kung Fu. I’m a major proponent of fasting for fat loss and general well-being.

  30. I tried a 24 hour fast for the first time yesterday and my enegery was through the roof! As soon as I got home from work I began my workout and then waited an hour before eating my meal. I plan on doing this from now on every 3 days, which will also be my workout days. Every other day I fast 16 hours and eat 2 meals in a 8 hour window.
    I’ve lost 40 pounds since late December following the Primal Blueprint and I love it!

  31. OK, so I am not opposed to intermittent fasting, or any fasting, per se … but where do you draw the line between fasting and anorexia?

    1. Anorexia is a psychological disorder. If you have or are prone to such a disorder, you may glom onto any information such as this to bolster your rationale for your disorder.

      But this post isn’t about folks who are sick with anorexia.

    2. Anorexia is continuing to lose weight through extreme dietary measures when one is at 85% or less of their ideal body weight. So, as long as you aren’t underweight, by this definition you can’t be anorexic.

  32. I’ve always felt the “three meals a day” mantra was a bogus modern construct not based on any science.

    We are so over-fed and food-obsessed in Conventional Wisdom that we call any period of not eating a “fast” and not a normal behavior. But we really don’t know shit.

    Those of you asking how to fast, when to fast, wanting a plan layed out for you – go experiment on yourselves. Do it your way, see what happens.

    1. Here here!

      We tend to fast on the weekends. We cook food in the morning and just let our kids graze as they wish. It amazes me at ages 2 & 5 how in tune they are with their bodies. Many nights they just don’t feel like eating dinner–so they don’t!

      For me, fasting was more getting over the mind-set that I had to eat. Having been a diagnosed hypoglycemic since I was a kid, I had trained myself to eat every 2-3 hours to prevent low blood sugar. Was I ever a slave to the clock! My first time trying a fast (eating dinner and then not eating until noon the following day) , I had to get over watching the clock and to just listen to my body. Like many people have attested in this post, I felt energized and alive! What a great feeling.

      I do find, as I still turn to sugar in times of emotional turmoil, that it takes me a few days after a binge before I can try fasting again. I can really feel it in my body that it is not ready for a fast.

  33. I also switched over to the “Warrior Diet” of 1 meal per day several months ago and find it to be the most liberating choice I can remember. Depending on mood I might have 1 or 2 very small snacks, but almost all of my calories come in the form of one giant 4 hour feast at the end of the day. Took a little while to adjust to eating that much, but it was well worth the effort and it’s so nice not constantly worrying about my next meal during the day. A Primal based Warrior Diet is something everything should look into. That being said, I’ve been eating 20/30/50 C/P/F for some time now and CF’ing 5 days a week and can not get lower than 12% bodyfat. So while I love this style and convenience of eating, for some reason I don’t seem to reap the same benefits as others do with the easy fat loss. Ehh, just my input from a few months of experience with daily fasting.

  34. Without a doubt, fasting is one of the things that really helped me drop about 40 pounds of fat last year. I first tried a full day fast in June (I think it was about 36 hours), and really took to it. I did feel hungry, but at the same time, felt better when doing it. I started skipping breakfast during the week and doing at least 1, sometimes 2, 24 hour fasts in a week. Not sure, but I think I may have been overdoing it a bit toward the end of last year, and I’ve been eating more frequently recently. I still have another 40 pounds of fat or so to get to where I want to be, and I think I’ll get back to more regular fasting again after this weekend.

    But yes, I found it very effective, and quite enjoyable and empowering too. It was great to know that I could easily skip meals if need be. Basically, I used a 5-8 eating window during the week, threw in 1 24 hour fast, and ate a bit freer on the weekends (although I usually just ate breakfast and dinner).

  35. I water fast every Monday, Wednesday, and Friday. I eat a light lunch around 2pm on my eating days, and eat a substantial dinner around 8pm or so. I may have a few macadamia nuts or a bowl of kefir in between lunch and dinner. So essentially, my fasts are about +/-42 hours and my every other day eating window is about 6ish hours long. Sometimes the fasting period is a couple hours shorter, and I’ll eat lunch earlier (or eat a bigger lunch) depending how I feel.

    I feel fantastic! I laugh when people say it “isn’t sustainable” or that I will “gain weight when I go back to eating normally”… I will ALWAYS maintain an IF schedule because I feel so good, it costs less at the market, and I am just NOT hungry outside of my eating days. The weight loss is good, and my energy is excellent.

  36. Um, to everyone pointing out the Marilyn Manson wasn’t really on the Wonder Years, you do realize Mark was joking right? I also feel compelled to point out that Kevin Arnold didn’t REALLY live in the 60’s either, seeing as he’s a CHARACTER. How did you let THAT get past you?

  37. I have one meal a day, most days. It’s making a huge difference in strength gains, fat loss and mood stability. I also love not having to pack anything for lunch.

  38. I’ve always heard if you fast you put your body in starvation mode-so when you do eat it hoards those calories and converts them into fat (to store for the next potential fast). Is that true? What happens to the body after breaking the fast?

    1. I’ve also head this as well. I think Mark has covered this before, but I can’t find the article. I’m also very interested in what happens to the calories you consume after a fast.

  39. It works! been applying intermitent fasting (one two hour window) 5 or six days per week for the last year. I am down 110 lbs and have more muscle then ever! I’m hardly ever hungry too! Nice post mark can’t wait to read part 2!

  40. People who want to lose weight commonly, and first of all, think about reducing the amount of food they eat. This may be quite a solution but not exactly the best there is. In fact, depending on the amount you reduce in your food intake, it may even be dangerous to one’s health. So how does one lose weight effectively and safely? Here are some points one should consider when trying to lose weight:


  41. Anyone reading this post do bulletproof coffee? I’ve started drinking coffee with unsalted butter, coconut oil, and cinnamon in the mornings as Dave Asprey (bulletproof exec) claims it will not ‘break’ your fast. Seems some others on here who have posted would disagree?

    1. I intend to try Bulletproof soon! It sounds great! I’ve already blended grass-fed butter into my coffee and it’s way thicker and richer than milk. Mmm!

    2. Yes! That butter coffee thing is the BEST nutritional tip I’ve ever found. On two 16 ounce mugs of butter coffee per day I can eat every 2 or 3 days whether I need to or not. NO fatigue or cravings for food of any kind because butter IS food, just a pure, concentrated fat type of macro nutrient.

      For each 16 ounce travel mug of coffee I add 2 tablespoon of unsalted butter at 100 calories per and 1 tablespoon of coconut oil at 110 calories per, so my total fat intake for 2 – 16 ounce travel mugs of coffee is around 620 calories, WAY below the 1800, 2200 or 2500 calorie intake that is taken for and preached as nutritional gospel by the FDA, mainstream medicine, carb addicted dieticians & nutritionists, etc.

      On the butter coffee I’m learning how to eat to live rather than the other way around, and with my current (temporary) mostly sedentary lifestyle I just don’t need much food anyway. I do make sure to take a full set of chelated multi vitamins and chelated multi minerals and my 2.6 grams Omega-6 evening primrose or organic FLORA brand sunflower oil balanced with 1.3 grams of Omega-3 flax for cellular nutrition, but otherwise no other food.

      What this type of butter fast amounts to is MACRO NUTRIENT restriction rather than calorie restriction, and that’s why I seem to be able to go long periods without any other foods. I’m still getting some calories but still FAR less than normal or average American intake levels.

      Remember that fats at 9 calories per gram have twice the caloric density than carbs and proteins, but produce around 6 TIMES more ATP energy at the cell level. A 6 carbon glucose molecule (from carbs) produces around 36 or 38 molecules of ATP, but due to metabolic inefficiencies there’s a loss to around 30 ATP’s per, whereas an 18 carbon fatty acid produces 147 units of ATP. LOTS of energy in fats, and it’s one more reason and proof that at a certain point calories don’t matter.

      I realized the best way to keep my body in fat burning mode is to never fall out of it by eating carbs that stop fat burning, Mark’s Carbohydrates Curve chart shows how many grams of carbs you can eat and still stay in a ketotic fat burning state. Just 4 or 5 ounces of carbs (100 grams) is all it takes to stop fat burning, so if you stay under that your body keeps burning fats, including the ones you eat, for energy.

      As a side note went to Denver last week & for some reason was hungry despite the 2 butter coffees, don’t know if it was the altitude or just a different attitude from the change of routine, but I ate twice a day for that week, including more carbs than usual. Felt like I was blowing up but when I checked my weight after I got home I was still at 205 where I was before I went, so no harm no foul, amazing.

      Your mileage may vary, but I HIGHLY recommend it, even if just to learn that your body can run on fats pretty much exclusively and it won’t kill you. Nice to know I can not only survive but flourish even in lean times with macro nutrient mastery.

      1. That butter coffee sounds great! I have the grass fed butter already and will pick up some good coconut oil today.

    3. It won’t break your fast depending on what you are fasting for. If you are fasting for the ketogenic affects of fasting then fats added to your morning coffee or tea supports, not detracts from this. I think MCT oil is a better choice for producing ketones than coconut oil. You will notice a big difference if you use MCT oil.

  42. Please, please, please be careful before doing this. I generally wouldn’t recommend it if you’re young, especially young and active, easily stressed; and unless you’re overweight, I wouldn’t recommend it at all. I know this is the point of this post and that Mark has posted cautions about fasting before, but some people might just decide to recklessly jump into it ‘for the benefits’. The way I did.
    I feel better when I eat regularly; have a look at this: https://www.cheeseslave.com/how-intermittent-fasting-caused-my-insomnia-and-belly-fat/. I personally didn’t gain fat, but IF caused a myriad of problems for me – it wrecked my appetite (I am 5’10, very active, and 20 years old, and should have a healthy appetite), I think it’s messed with my thyroid and definitely made me stressed.

    1. WHat was your diet??? This is very important. Did your diet support fasting? Also, ketosis results in a lack of appettite because your body needs less food. I have been doing intermittent fasting for years and I have a healthy appetite. Not like the insane appetite I had 11 years ago but an appetitie that I can manage without overeating the wrong food. I also fasted many times naturally when I was in college and that scared me. I thought there was something wrong with me because I went 30 hours without eating…It is normal to go 20 hours without eating because our bodies are already built to go a long time wihtou food. We screw that up by eat 4-6 meals a day with snacks. THis not only contributes to obesity but also insulin/glucose instability.

  43. What actually counts as “fasting”? Can I drink green/herbal tea? Take multivitamins? Black tea with a bit of milk? Clean my teeth with toothpaste!?
    I’ve always though eating under, say, 50 calories in a day is as good as fasting (assuming those calories come from those similar to above, not food).

    1. I think that your 50 calorie rule is pretty close. I drink coffee (no cream or milk), tea, and lemon water during fasts, and had great results. I’ve heard of people using a bit of milk or cream in their coffee and still seeing good results. I would take a magnesium malate supplement as well, but I take any other vitamins with meals. During Ramadan, Muslims fast during the day and don’t take in ANY liquids. If you’re mainly fasting for health, and not religous or other reasons, water and tiny caloric intake is fine.

  44. This may be a silly question, but is it a sign that I should avoid fasting if my stomach is growling?

    I’ve only been going Primal for just over a month now, so maybe my body hasn’t adjusted enough yet?

    I also have experienced days where it was easy or unintentional to skip a meal, and felt fine if not great! But today is not one of those days.

    1. Not silly at all. I’ve noticed hunger and some stomach growls during a fasting and continued to do it. It really is about intensity. If it’s just mild hunger, you can fast right through it. But if it’s GNAWING at you, you should break your fast. I think benefit of fasting is getting more in tune with your body. Break your fast early, but certainly try again if you want to. Also, you can start by skipping a meal, then two meals, then 12 hours, then 18 hours over a few weeks.

    2. Another thing I and some friends have noticed… fasting is much harder after a “cheat” day. Try eating 100% primal for a day or two before starting a fast, and it may be a lot easier.

  45. My great grandfather who I met as a child fasted one day a week for religious reasons. He did not like bread or sweets of any kind. He was a dairy farmer and ate mostly raw whole fat milk, animal fat, meat and very little vegetables because he didn’t like them either. He died at the age of 106 years. In addition his brother in law followed the same diet and fathered a child at the age of 80. these are true stories from my family history. Now the family lives in the big city. Lots of stress, carbs and plenty of food. His great grandchildren all suffers from degenerative diseases. His great great grandchildren all are on meds for hyper active behavior, depression and bad behavior. This makes me think of Dr. Pottenger’s experiment with cats. I am the only family member interested in nutrition and paleo nutrition. I am also the only family member without illness or medication and I am going toward 80. I fast 2 days a week from sundown to sundown. I also trade options and equities to support myself and have money to donate to charity.

  46. Does anyone know if there are any repercussions with fasting during pregnancy?

    1. Just from the common sense perspective, the fetus should be fine but your body tissues have a good chance to be used tor the needs of pregnancy. I wouldn’t do it.

      1. I definately would recommend against it. Just think about Grockette. She probably nested, not hunting for long times with the guys (gathering fruits, probably). She probably get the better food too (at least I think Grock would save her the last porkchop, right? Or the fattier pieces).
        Also, the natural way of the female body during pregnancy is to get fatter. There’s no way around it, you’ll need the calories for nursing. The anatomic changes forces you to remain less active too. Women tend to nest. Men tend to protect their pregnant wifes.

        Ever seen pregnants lions in the discovery channel??

        Also, the fetus is a wonder of life and all, but behave like a freaking parasite, doesn’t it? Eating all the calcium in your bones and teeth and all. So I agree with Galina, everything the fetus need will come out from your body. Fat, sure; but also protein, minerals, your sanity…
        Pregnancy is NOT the time to lean out, period. Your biology makes you get fatter, because you need to get fatter. Like the analogy with polar bears hibernating that Taubes uses in his books.

    2. Most recommend against it, and I tend to agree. If you miss a meal because you’re not hungry, I think that’s fine, but don’t try and fast through hunger during pregnancy.

  47. The last few times I have tried to lose fat I have struggled, no matter how much exercise I got and how much I limited my carbs. I couldn’t understand why I had been so successful with low carb in the past (but fell off the wagon…never again!) and couldn’t have the same success this time around. What I realized, after finding this page and going 99% primal, is that I used to intermittently fast without thinking about it, and I had stopped doing that when my work hours became predictable and money wasn’t tight any more. I started IF about 2 weeks ago and the results have been nothing short of amazing! The best part is that everyone I talk to about it has been intrigued and supportive rather than conventionally wise…I guess I just run with a good crowd.

  48. As if those ancestors of ours had a table full of food every day..man was lean and mighty because he DID NOT eat every meal, every day…and many times it was a longer period of time. many Animals who live through Winter climates are forced to fast or eat very little. THEY are tough. So be a GROK..Don’t eat all the time..ITS OK TO BE HUNGRY. Remember your stomach is only fist size. GO STARVE a bit..it wont hurt..it actually kicks some body fat consumption into gear…ITS COMMON SENSE>..GROK ON>>>

    1. No, Grok definitely didn’t adhere to three-meals-a-day! It’s conventional brainwashing. Go Papa Grok!

  49. Edward, I found your posting quite interesting. I, too, am near 80, have all my own teeth, have never had surgery, and have see a doctor once a year. I find old age quite enjoyable and hope you do, also. Best wishes, Anna

  50. Really, there’s a lot to be said for proof reading. I think I meant to say “have seen a doctor once a year”. So all you young guys out there, cheer up, it’s all good.

    1. I’m not going to pretend to know any science about it, but that seems awful risky. You would certainly have to avoid the refined processed food (as you are probably already doing)and make sure you get enough direct sunlight (for proper D levels)so that you wouldn’t fall behind on the hormones that are so important for proper fetal development. If my wife wanted to IF during pregnancy, I would ask her to be satisfied with only eating proper food and let her body be what size it needs.
      What are you trying to do with your baby? Avoiding gestational diabetes is important, but if you and the baby are exhibiting satisfactory health already, you should have a healthy baby ceteris peribus. IFing won’t make a healthy baby into a child prodigy, so avoid the risk and be satisfied.

  51. Is it weird that reading this excellent post on the benefits of fasting has made me hungry??

  52. I’m wondering about the impact of IF for those of us with compromised thyroids. I’ve read that calorie restriction can cause it to slow down even more. In my own experience, it does seem that my temp runs colder on days when I practice IF (purposefully or accidentally).

    I go through phases where I still practice it…mostly because the mental clarity IF provides is astounding! And think of all the extra time you have in a day when you’re not prepping and eating so often.

    As for the guy who lost all the weight — there’s gotta be some kind of “setpoint” where the body stops dropping, right? Back in my college days, I was anorexic (ate one meal per week). I never LOOKED it because my body just didn’t drop the weight. I imagine that if he’d kept starving himself, he would eventually stop losing weight.

  53. This made me laugh. At myself. After 17 hours of not eating I get very painful stomach cramps

  54. I’m excited to be reading about fasting. I run a small bushcraft/survival school in Maine, and fasting is part of our wilderness survival curriculum. Some of the research I read years ago left me with the understanding that fasting will extend the life of a survivor over eating minimally (ie. not meeting replacement caloric requirements). It’s a common myth that food is a priority in a short-term situation. I’m excited to be reading about it here.

    On a personal note, I fasted one day per week for a year, and have completed eight 7-day water fasts and one 11-day water fast. More intermittent these days.

  55. Fasting is definitely a superior way to lose fat.
    It’s also VERY easy to incorporate into an active lifestyle when done ‘right’.
    Nice post Mark.

  56. Very inspiring stuff. I’m keen to try to help shift my baby weight. Commonsense (or maybe CW) makes me think I should wait to try IF till I finish breastfeeding?? My baby is only 10 weeks old and I’ll definately breastfeed him for at least 13months as I did my other 2 sons. Does anyone have experience with IF and milk supply? An aside: My supply is currently excellent due to primal fare! (With previous 2 babies I had very low supply on CW diet…was hard work!)

    1. CW say you could breastfeed until the baby is 6 months old and then quit. I’ll say… listen to your body! Watch bitches!!!

      Yes, I said bitches!! Female dogs naturally get to a point when they start biting their sons when they trying to breastfeed, because… well, I think they’re done with it. Do you feel you’re done with it? Probably not, since your baby is 10 months old! But the moment after the 6 months period (just to be sure) that you feel like “I’m done with this!” is probably the best moment to start to feed the baby the wonderful world of paleo foods. Just listen to your body (and hormones),you’ll know when the time is right!

      1. Breastfeeding vs. food isn’t an all-or-nothing practice! The primal way to introduce foods would begin somewhere around the 6-9 month window, but food isn’t going to be a major source of nutrition until that baby is more than a year old. Human milk is more calorically dense and loaded with bioavailable nutrients than anything else.

        More moms see their fat stores mobilized after several months of breastfeeding than right away — give your body time to do what it knows how to do!

        1. Thanks for comments. I definately intend to introduce Primal fare around 6 months. Baby is only 10 weeks at the moment , not 10 months. Lets hope the baby fat will melt away by then 🙂

  57. Never tried this but very intrigued by the science behind it. Obviously not the best option for me at the moment since I’m not looking to lose weight. But still a very interesting read!

  58. My partner and I are debating the current health implications of IF when the soils that grow what we need to eat is deplete of nutrients. Is IF then, more harmful and would eating three squares daily and consistently but less volume, better for health?

    Please comment…thanks

  59. I’m intrigued by the idea of fasting, but in my experience (some very unhealthy attempts to lose weight in high school) sometimes after not eating enough, I would actually faint. I have low blood pressure so I’m prone to fainting often (about 5-6 times a year!) and usually I like to keep almonds in my bag and never really skip meals. So would IF be possible for someone like me?

  60. I’ve been doing PB for just the past 2 weeks now. Lost 6 lbs and can feel my muscles start to build up. I’ve actually done IF (accidentally) twice now. Fasted for 19 hours one day, 16 hours another before having a really good primal meal! I had no hunger pangs, just kept drinking water, coffee, black tea. Amazing. I’m hypoglycemic also, so I wasn’t sure how well I’d do, but I feel great!

  61. I love your timing, Mark! Just embarking on a 3 day fast/retreat starting NOW.

    I got very lean last year doing intermittent fasting (18/6) and it is SO much easier than counting calories.

  62. I run 8/16 right now, but break it with a pretty small lunch; 3 cups chopped vegetables with some olive oil/balsamic (1:1).

    Have been meaning to try extending from 16 to 24. Only have to postpone the vegetables.

    Hunger really hasn’t been an issue. Interested to see how I respond to fasted resistance training.

    1. weight training while fasting enhances the production of growth hormone. Don’t ruin that with carbohydrates and don’t eat for an hour following weight training. You will produce healthier muscles and you will be able to maintain that muscle easily.

  63. Great timing on this article for me. I’m having the hardest time getting rid of that last bit of belly fat. I think I’m going to use this as inspiration to do a fast. Thanks Mark!

  64. I started playing around with Fasting aka The Warrior Diet at the same time I dove head first into Paleo- both made tough. I never worry about a missed meal and eat pretty damn good when I do

  65. A healthy fast hurts no one. You can’t fast properly without a good healthy meal. A person with anorexia has no business trying to fast because they will do it incorrectly. A person who has a healthy psyche and understands how the body works can indeed fast and improve their health. Back in the day people did not have a corner grocery store, Mickie Dees, freezer or microwave. They ate in a manner that provided the nourishment needed to get them through several “foodless” days. They were not interested in making a fashion statement. They were interested in survival. Survival is a healthy stated to be in. It makes you understand what you need to eat, YET, in the days of “civilization” we can enjoy this ideal, survival. We can enjoy it more because we don’t have to hunt or gather. Everything is right at our fingertips and all we need is common sense. If you lack common sense then survival is out of reach.

  66. I was a little nervous about starting an Intermittent Fast because I thought the hunger would really be a distraction and unbearably uncomfortable. But from what I read, I thought it deserved an honest try. I jumped in to see if I could go a few days. It’s been 2 weeks now and I am really enjoying it. IF is so black and white, so easily defined which is perfect for the way I am. It’s not a lot of rules in between to sort out. I fast 19 hours a day from 8:00 PM til 3:00 PM the next day. During my eating phase I have 2 meals. During the fast, I am more focused and calm, not obsessing over lunch or snacks like I used to. I don’t have brain fog and fatigue during the day. I used to want a 20 minute nap in the afternoon, but not anymore. I am very excited to see where IF takes me as I have about 50 pounds to lose.

  67. I’m confused about the benefits of fasting vs. the myth of “eat less, move more.” Low-carb advocates say that our feedback systems will use various mechanisms to conserve energy when we eat less, e.g., increase our appetite, slow down metabolism. Then Mark presents evidence to the contrary. How can both arguments be right?

    1. “Low-carb advocates say that our feedback systems will use various mechanisms to conserve energy when we eat less”… I’m talking out of my ass here, but I think that would happen ONLY if you don’t have any energy storaged for the fasted times… Like if you open your wallet and see you got like a 100 dollars of fat (in 1 dollar’s bills), would you go into slowing down your money expendage because you got no 10’s???
      The whole idea of “Oh OK, I got lots of fat to burn, so now I’m fasting and I’m going to lose muscle instead” sounds so very wrong. Fat is there to support you when calories are low.

      1. Okay, lets say everyone was a hanter-gatherer. You have no concept of medical research or nuthin’ but you do know that you can stand up under bad weather, famine, drought etc, if you kept your protein intake as high as possible. By keeping your fat intake high, you will also keep ketosis high. Ketosis exists to provide energy and maintainence to our cells during famine. Hydration is also maintained even on a reduced intake of fluids because, if there’s famine then no doubt, water will be very scarce. This is why we have no appetite or even feel less thirsty when in ketosis.Fat will be burned because of ketosis…It will be used as energy and whatever else the body needs for it function optimally.

  68. I was never much of a dieter, but I was quite overweight. I decided enough was enough so started fasting 16-hours a day, with 8 hour eating windows. I also greatly up’d my Tea drinking and started eating a lot of the healthy super meat, Kangaroo Meat.

    Fast forward 6 months, I’m down almost 50 pounds and have shown no signs of regaining weight.

    Fasting is the dieting of the future.

  69. For all of you who IF, do you still eat the same amount of calories in a day (assuming it’s less than a 24 hour fast)? Do you try to cram a ton of cals into your eating window, or just have normal sized meals?

    1. I try to cram a ton of protein and fats in the eating window(three meals). I listen to my body. I stop if it doesn’t feel right. Eggs are harder to metabolize for me.

  70. I do not restrict my caloric intake but I do not count calories…I make every meal, which is usually once a day, a meal of champions. Today I had leftover pork roast, collard greens, pumpkin walnut casserole and a sliced avocado.I used butter and coconut oil. I tell ya, it was delicious.I had one serving of everything. I did not overeat. But I did clear my plate so that I could make it through another fast…That’s exactly the way Grok would have done it.

  71. O.K. The last time I seriously fasted was before Vatican II reported out. I’m convinced based on your writing that it can help me. I’ve been trying to stay Paleo/Primal but cheat a good deal. I’m going to try it because:

    1. I think it will make me eat really great quality Paleo food on the non-fasting days.

    2. It will save me time in the kitchen.

    3. It will save me money.

    4. I’m not overly active now, so it’s not like I have to cut back on my activity level.

    5. It will help me lose the “last ten”.

    I’ll be in touch!

  72. i like to think i’m a bit of an old hand at fasting since i’ve been doing it for a month every year since i was a kid 🙂

    it’s approximately a 12 hour window from dawn to dusk, pretty manageable once you get the hang of it. you do cut a lot of body fat when u’re fasting so it’s a good option for those who have hit a plateau and need to get just that little bit more off.

  73. I was fasting once, for a week, but not in fat loss purpose. I was trying to cleanse myself.
    The experience was good, I enjoyed sauna once and really felt benefits from it. I was ‘eating’ vegetable soups 3x a day and was doing enema (if that’s how you use the word in a sentence;) in the morning. I can tell you thet this was my favourite part of fasting – emptying my colon. It sounds weird, maybe, but it’s true. 🙂
    I am thinking on doing it again, the same way I did it before, I just have to gain some courage. I definately would say that it contributes to fat loss, if you go back to eating solid foods gradually (not like I did, on a picnic :P). When I’m fasting the next time, I’ll make sure not to repeat the same mistake again. 😉

  74. The study on obese patients linked here actually had them consuming 25% of calories on fast days, so it wasn’t a simple eat-don’t eat. They would have been counting calories presumably.

  75. Is anyone able to reassure me that I won’t kill my thyroid by doing this? When I had an eating disorder as a teen (yes, I know, not the same as just fasting…), I managed to induce hypothyroidism among other things. I’d like to know what the difference between the two situations would be in terms of hurting my poor thyroid…

    1. @

      Healthy frequent short-term fasting includes & requires getting all your nutritional needs met when you do eat – macro- & micro-nutrients; it takes some knowledge and some planning to do it right. Once you know how to get your needs met when you do eat, it becomes much more flexible, if desired.

  76. I love intermittent fasting. However this is a very sensitive issue for people with (former) eating disorders.

    I have a friend that used to be bulimic-anorectic. Now that she is overweight it is the general idea among friends, family and physicians just to let her eat anything and as much as she wants. This is way better than from the time when she was slowly dying because of not eating.

    Is there anyone here with an idea on how I could take to her about healthier food and IF as ways of staying healthy without causing a psychic breakdown.

    Balance is so hard for some people.

    1. It’s really difficult because people have different levels of what will trigger them. I wouldn’t approach the subject of IF straight away, but start with healthy foods… and not for the purpose of losing weight, but for general health…
      Everyone is different though, and had their own issues that lead to the eating disorder so it’s a tricky one.

  77. Terriffic post. I think even informed Paleo people get confused sometimes about this issue. Firstly, there is so much paranoia and fear that dropping calories will reduce muscle – clearly that isn’t too much of a problem at all. Secondly, I think a common misunderstanding ermeges in low carb cricles re calories vs carbs, and there is sometimes an idea you can eat as much as you want if you lay off the carbs and still lose fat – not true. Gary Taubes gets misquoted on this level, and a UK writer Barry Groves to an extent also. To lose weight you will, as Mark points out, need a calorie deficit – but Mark does a brilliant job here of pulling it all together with supporting references and hormonal explanations that account for enhanced mental well-being and muscle sparing under conditions of calorie deficit. As always, much appreciated, thank you Mark.

  78. I am a first year medical student and we just finished going over endocrine and gastrointestinal physiology, and for the most part, I have no argument with this article. It’s funny though, we learn all these mechanisms for dealing with different caloric sources and how the body reacts to fasting, and I feel like the students dont hear it. I have many vegetarian classmates, which is not a great lifestyle in America as it compromises mostly starches and carbs. There is an inherent disdain for fat, and even though it’s irrational and not founded in science, we don’t learn how to teach proper eating habits to patients. Fasting is a regular part of my diet, along with very low carbs and high fat intake from many sources, and I plan to teach it as an option for my patients. The body is not built to handle high carb loads. It is however built to intermittently fast. Primal eating habits have a home in modern medicine.

  79. I’ve started eating Paleo this January, and now I’m at a point where I can fast ~16+ hours before I eat my next meal. Which means I eat dinner around 5PM and eat my next meal lunch next day.
    I do however have to take medication in the morning, with Vitamin D3 and Cod Liver Oil. Does this influence the fasting negatively?
    Oh, and coffee supports fasting, which I don’t drink, what about Green Tea, and Oolong tea, which I do drinnk ?

  80. the comment that brain actually prefers to feed on macronutrients which are different than glucose and that glucose just stops that mechanism is really weird. is there scientific evidence related to this?i always had the conception that glucose was neccesary for our brain health :/ .though i do know that brain is constructed mainly by fatty acids like dha so it is kinda confusing.. anyone can help in this pls? this proves the stupidity of those that are empoyed in the research and development section..the more natural the better.it seems that our body is smarter than us ^^ we just get in its way.pissing it off 😛

    1. Glucose is necessary for brain and nervous system health. However, the liver is capable of synthesizing glucose from protein via gluconeogenesis. This makes up for any deficiencies the ketones may not be able to fulfill.

  81. I honestly think that exercise is 1000 times better than fasting. So instead of preaching starvation lets preach exercise and good food.

    1. You cannot starve by not eating for an extended period of time. Your body is not going to wait until there is absolutely nothing to eat to protect your muscles and cells. Ketosis happens long before the body is out of food so starvation is a little overstated and overused when it comes to fasting. The purpose of ketosis is to help you to survive to your next meal. Exercise,like fasting and the ketogenic diet, will induce ketosis.

    2. Using exercise to loose fat is not working. I was 245lbs when I hired a personal trainer for a year, 3 times a week, plus I went 2 time just for cardio. I did everything they told me to, and I lost 5 lbs!!! during that time. I might have gained muscle, but at that point I wanted to loose the fat.
      I’ve never been able to loose weight on any diet, but this time around with Paleo diet I’ve lost 21 lbs in two month, and I’ve never felt better. I don’t exercise yet, but I’m going to start doing some soon. Just a little bit though.

  82. Mary,

    Any link to holding off food until an hour post-workout? I’ve seen that mentioned other places, and wouldn’t mind reading about it further.


  83. I am questioning the following from the “A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults” article:

    “However, when consuming 1 meal/d, subjects had … significant increases in blood pressure and in total, LDL-, and HDL-cholesterol concentrations….”

    Is blood pressure increase and a drop in good cholesterol reason for concern?

  84. I don’t need to lose weight (120lb featherweight since high school), but fasting and ketogenic diets are much easier than I ever thought. Since going on less than 30g carb in a day I have to remind myself to eat sometimes, and I occasionally just eat nothing for a while. When I do have concentrated carbs it is hard to stop, though.

    I haven’t lost a pound, but I have gained muscle, so I guess I’m burning what little fat I have. I’m also seeing strength gains far more than I did on a SAD diet and expensive mass-gain protein powders.

  85. I’m a HUGE supporter of fasting. In honour of “spring cleaning,” I’m currently undergoing a 4-week water fast. I’m making it both a physical and spiritual transformation, called my “Body, Mind, and Soul” challenge. 🙂

  86. IF is one of the last things I haven’t embraced since I became Primal in October, but this article inspired me. Since I cannot convince my husband (who would really reap the benefits in SO many ways) to even try Primal living, I thought fasting would simply be too hard while cooking for him and our mega-calorie-burning 17 y.o. son. I have also had bad issues with fasting in the past, and did’t want to try it at work. Well–today I did. I simply decided to skip breakfast, and I normally eat a big meal in the morning. It was amazing. I got hungry, I got over it. No dizziness, no lightheadness, no gnawing at my arm–no problems. By lunch I was ready to eat, but not like a deprived maniac. I’m going to try this a few times a week, and work up to a 20-24 hour fast. I strongly suggest skipping random meals as a start to anyone who has doubts about IF. Thanks Mark–again!!!

  87. Intermittent Fasting and cutting out grains has me down 23.4lbs today. It’s so much easier to maintain than counting every single calorie of all my 6 small meals a day used to be. For those worried that this will cause disordered eating, I used to be bulimic, and conventional dieting really feeds into the obsessive and anxious nature of eating disorders. Counting calories and food timing can be real triggers for those who are prone to ritual eating behaviors. Intermittent fasting allows one to stop worrying and obsessing about food. It allows freedom on non-fasting days to eat when you’re hungry until you’re not anymore without counting calories. For me, it has absolutely busted the dieting/disordered eating cycle.

  88. I am a big fan of intermittent fasting and really learning to deal with hunger. Just because you feel hungry does not mean you have to eat. From a fat loss stand point the biggest thing is it helps restrict your calories by condesing the window of time you are consuming calories.

  89. This from the early Christian manual called the Didache:

    And let not your fastings be with the hypocrites, for they fast on the second and the fifth day of the week; Keep your fast on the fourth and the sixth day.

    Apparently the regularly fasted two days a week.

  90. From an on-line journal I set up.

    My Journey into intermittent fasting…

    One month of intermittent fasting in the books and I am down to 180lbs from 187lbs. No detectable muscle loss (arm, chest, neck and thigh are still the same measurements) and no detectable loss in strength. I am guessing my body fat is 8-9%… down from 10-11% but again that is a guess.

    My last meal is generally between 8-9pm and my first meal is between 2-3pm. I workout fasted and use my first meal as a post workout meal.

    First two weeks were tough… I got tired and hungry. Although I think it was easier for me then most because I was paleo/primal first and because of that I really don’t get any sugar cravings to begin with.

    Had an IF breakthrough on week 3. The stress of being “hungry” had gone and I had begun to notice how short of a duration the sensation really was. I broke the fast generally at 2pm (sometimes 3pm) but felt like I could have fasted all day. I ate by choice not by habit.

    Now in week 4 and there is virtually no hunger sensation. If and or when there is, it passes too quickly to notice. I actually am beginning to like the “empty” feeling I get when fasting. This has honestly been the easiest style of eating I have ever tried.

    So I have gone on and on about the fasting… let me speak a little about the feeding. My feeding window is generally 2-3pm to 8-9pm (sometimes 10pm if my wife and I get a late start cooking. Before fasting I would eat between 2000 and 2500 calories a day. With fasting the range is from 1500 and 2000 (most of the time… once in a while I get as low as 1400 and as high as 2200). I probably average 1800 calories a day and my maintenance I figure to be at around 2000 calories a day. So I am getting a nice little 200 calorie deficit average over time and the fat has been slowly and consistently coming off. Still paleo/primal monday through friday… weekends I eat whatever I want while still fasting most of the time.

    Anyhow… long winded post I know… just wanted to share.

    Here is the website that got me interested with IF…


    IF update…

    It has been about 7 months since I have been IF’ing. Weight has settled in at 177lbs. Body fat if I had to guess is about 8%, with no muscle or strength loss to speak of. Will do a skinfold with some calipers to get a working number. All in all it has been really easy and enjoyable for me…

  91. Ok, so Mark’s “10 Sure ways to sabotage weightloss” lists #5 as Skip Breakfast, and #3 as eat once a day. Isn’t skipping breakfast or eating once a day a way of fasting? Does Mark now retract those 2 items from his list or are there conditions attached? Any thoughts?

  92. Techically it is impossible to skip breakfast. Break is the meal that you eat to BREAK a FAST. That fast can be broken any time of day. On top of that, if you prefer to break your fast in the morning, go ahead and do it. Just begin your fasting window earlier. I am not an athlete but I have an extended fast everyday and do not have my first meal until after 3:00…it is becoming more regular for me to even wait until 6:00 at night. My glucose remains stable throughout the day which is impressive considering I am pre-diabetic.

  93. I’ve started eating one meal a day for the last month. I like it because for breakfast I have a black coffee. During the day if I get hungry I have a hot cup of tea. My wife cooks dinner every day. It’s not primal but it has meats, veg, and usually rice. She is a great cook and likes to see me eat. So this way I can look forward to one big meal each day. This is much easier than eating three tiny meals for me. In the first month I’ve gone from 215 and 38 inch waist to 195 and a 35 inch waist. I’ve noticed my stomach getting smaller and I get full faster.

  94. What are your thoughts on fasting while recovering from an injury? I am under the impression that your body needs nutrients going in inorder to heal/fix itself. Should someone fast while recovering?


  95. I just don’t get this. What about all the backlash against low-calorie dieting, the idea that your metabolism slows down when your body thinks you’re starving (and that’s what fasting sounds like to me), what about all the people that supposedly ruin their metabolism by fasting or cutting calories too drastically. I have read this for years, and my personal experience bears this out as well. I went on a couple of 3 1/2-day fasts a few years ago and the results were very short-lived.

    Mark, can you explain to me how and why this research is any different?

    1. From my understanding of the article and follow up comments (if you haven’t read through the comments, I suggest that you do; there’s a lot of good info there as well), the body begins to enter the state of breaking down protein for energy once it gets near the 20 hour mark of a fast. Before that, particularly between the 12th and 18 th hours, the body is using stored fat for fuel. Ideally then, a fast would be around 18 hours at the longest. This is not the type of fasting that will harm your metabolism. Following this type of fast, if you stop eating by 7 pm, you break your fast by Around 1 pm the following day, which leaves you with plenty of time to eat several meals if you wish. Others may mention doing longer fasts, but based on the data it may not be ideal. They are just sharing what they do, not necessarily a recommendation for what others should do.

      I agree with you though in terms of fasting possibly becoming detrimental if taken beyond its peak benefit.

        1. No, I make sure that my fasting meal ( last meal of the day ) is rich in protein and fats. I do throw in some veggies but the carb count in them are so low, I don’t even count them.

      1. The benfits that basically come from fasting is ketosis. To keep my keto-levels up I increase my fat consumption. The best fat consumption for ketosis is is MCT oil and coconut oil. I add MCT oil or coconut oil to my morning coffee.

      2. An excellent book for exploring the biochemical effects of fasting is _Protein Power_. The first chapter illustrates the similarities between the body’s biochemical reaction to carb-restriction and IF. The fat-burning benefits of IF come into play when the body is low on carbohydrates as fuel. The liver converts fat stores to ketones in an effort to make up the difference between what the body wants and what the system has. For systems of the body specifically requiring a certain amount of glucose to function efficiently (e.g. the brain, the nervous system), the liver can synthesize glucose via gluconeogenesis using protein within the body.

        This is one reason why it is so important to eat an adequate amount of protein(0.7-1.0 g/pound lean body mass) when on a carb-restricted diet. Part of that protein will go towards maintaining and building muscle mass, the rest will become glucose to fuel your pickier systems.

    2. I have been doing IF for 5 years straight. I now, eat 1 meal a day and I have not noticed any unusual metabolic results from it. I did not throw myself into IF like many do. I took my time and allowed my body time to make adjustments. I also consume coconut oil and MCT oil that also helps ideally with metabolism. I keep my vitamin D levels within an optimum range and I include magnesium, zinc,K1 and K2. I try to consume a lot of calories. I wouldn’t be surprised if I get in 2,000 calories a day.

  96. Silly question alert but should I be taking my vitamins when fasting even though the directions suggests supplements be taken with a meal? Such as fish oil, vitamin D, general vitamin supplement.

    1. The reason many vitamins should be taken with a meal is because of the fat content that is in a meal. If you take fish oils, mct oil or coconut oil, you can inlude other supplemental nutrients at this time. This is what I do.

  97. fasting or skipping meals are not advisable, the more that you don’t eat the more that you’re craving for it. Thus it will lead to complications rather than losing weight. Have a balance diet, exercise, and self disciplined will help you achieve your goals.

    1. Sorry but that is not an absolute for all. Maybe you are just not cut out for IF.

  98. I don’t skip meals nor fasting, I eat in time and have a balanced diet keep me fit and healthy.

  99. I’ve never tried this but am plateauing in my weight loss efforts and think I’ll give it a go. I’m nervous however, as I hate the feeling of being hungry, and I still experience hunger even on paleo. Is it something you just have to get used to? Any tricks people use to make the hunger pangs go away? I don’t have the huge mood swings and faint feeling I used to when eating a CW diet but I hate that growly, empty, lightheaded feeling.

    1. When I get hungry before it is time to eat I just brew another cup of tea. Green or oolong, no sugar or milk.

  100. I need some advice on the IF protocol. I train in the morning. I did lean gains for 2 months. 1st month went fantastic. 2nd month I started to get very lethargic and depressed. I had no energy and was cold all the time. I do BCAA’s every 2 hours after working out but they have artificial sweeteners. I found my recovery lacked and my hypoglycemia kicked in full force. Any advice on what I could alter? Training in the afternoon is not possible Monday – Friday. Thanks!

    1. make sure that you are not magnesium deficient. As a matter of fact that is my first suspect. Also make sure that you are getting enough zinc and potassium. MCT oil is an excellent source of ketogenic energy. Work up to 2 or 3 tablespoons a day.Also make sure that you are eating plenty protein and fats the night before. I don’t eat to end a fast…I eat to begin a fast.

  101. I believe in the powers of fasting it is an ancient practice that has been used on this planet in all continents, cultures and even religions. AND….

    what concerns me about this article is that it only focuses on using the fast for weight loss.

    Fasting is an amazing way to detox and cleanse our bodies and allow the digestive system which is always working a break.

    It also provides time for an emotional/mental cleansing. Allowing us to reflect on our lives and the way we use our time, energy and choices.

    Fasting is MUCH MORE THAN A WEIGHT LOSS technique.

    check out what Dr. Bragg (the apple cider vinegar guy) has to say about fasting, or Survival in the 21st century,

  102. A key point people need to remember about fasting is that you need to have body fat to do it. Even an anorexic is more mentally damaged than physically damaged until their body fat drops below a certain level, then they become as physically damaged as they are mentally. I hope that didn’t sound too rude.

    Starvation, as a medical term, is more a function of the amount of body fat in your system. Putting the health benefits aside, if your body doesn’t have enough stored energy to burn, the benefits become moot. So if you’re already below 10% BF or whatever is an extremely low percent for you, I absolutely do not recommend fasting under any circumstances.

    Fasting for fat burning only makes sense if you have fat to burn… (I know, no duh)

    1. That is incorrect.

      Many people who are below 10% BF (I’m 6% by the way)fast just fine.

      I fast 16/8 everyday and eat 60% fat, 30% protein, 10% carbs. Calories is approx 3000 a day.

      The secret is to eat plenty of high quality fat and protein.

  103. I have been fasting 24 to 36 (day and night) for over a year now, always the same day every week. I recently done a 60 or so hours. It gets easier past the first day. I found it best to drink water with a few granules of salt (Hymalain). Otherwise I think its great and I want to do longer ones then that.

  104. Wow – this article has given a great coverage of fasting and its benefits for us – thanks! Looking forward to Part II.

    I used to fast at least once a month and am going to go back to this healthy way of living. I found it easiest to complete an 18 hour fast by starting at 7pm at night, getting an earlier bed and then it is really just a case of passing on breakfast and having a later lunch.

    Once the fast is complete there is a potential pitfall – I used to be tempted to overeat for the first post-fasting meal (because understandably it is great to eat after 18 hours of fasting!) The best thing to do is have a large healthy salad and force yourself to wait 20 minutes to start feeling full instead of eating lots and eating fast!

    I was inspired to write an article on how fasting can really work effectively with Cardio exercise for fat loss – https://www.lmdfitness.com/training/cardio-fasting-fat-loss/

    Happy Fasting!

  105. For those who have doubts about IF, just remember this facts…

    It takes more than 72 hours (3 days) of straight fasting on water before your muscles start to break down hence the name ‘Intermittent fasting’ meaning you break the fast long before 72 hours mark.

    Many use IF for 16 hours, 24 hours etc, but it is well before the 72 hours mark.

    I have been fasting 16/8 everyday and weight-train in the morning with BCAA.

    I’ve lost fat and gain muscles so IF DOES work. 🙂

  106. IF works great for me and I don’t feel weary or worn down. What I do figure out (usually too late) is that I didn’t drink enough water and so I feel especially thirsty if I’ve gone an extended time without eating. I have a few days per week where I have a ridiculously long commute and am mostly sedentary (in meetings all day at the office) so IF is great on those days when I know my calorie burn is minimal.

  107. Has anyone experimented with 36 hour fasts? I’ve done 3 and found I have no problem with energy levels. But getting to sleep after 24 hours of fasting as proven to be difficult (for me at least).

    On benefit is that 24 fasts become the ‘easy’ option.

    1. My understanding from everything I’ve read is that a 16-18 hr. fast is optimal. The closer your body gets to 20 hrs. it switches from using stored body fat for fuel to breaking down protein for fuel, which is not ideal.

      1. I do know people who have gone several days without solid food. I also know that it takes somewhere around 3 days to digest a 6 oz piece of steak. That being the case, the body cannot begin to consume itself until dietary protein is digested and absorbed. I also know that the body needs just enough glucose that is equivalent to a teaspoon of sugar a day in order to survive. So the thought of your body consuming so much protein that you loose muscle mass is not anything to really worry about because you won’t go that long without food. But this should exemplify the importance and need for protein in the diet especially if one uses fasting as part of their diet.

        1. Muscle loss issue aside, for the body to burn primarily stored body fat, the optimal time frame for that is the 16-18 hour fast, correct? I’m looking specifically at stored body fat loss.

  108. Wonderful and informative article! How long does an IF have to be for it to count as one? (is 20 hours the minimum, etc)

    1. Try 16 hours…

      Have your last meal in the evening by 7 or 8pm and eat your first meal the next day at noon.

      While you fast throughout the morning till noon drink water, herb tea etc.

  109. I took a day trip twice for work to Indiana and New Jersey (flew to both states on different days though). Got up reeeeaaaalllll early and got home reeeaaaalllll late (around midnight). I didn’t eat all day (except when I got home – I ate ribs and eggs, ha!). I did not crash, still had energy, no dizziness, or anything really – not even heartburn! I had only been on PB for THREE DAYS! I didn’t realize how well fasting could help me until I woke the next day FULL of energy and excitement. It forced my body to burn. When we go camping this year I’m going to try it again – I think it will be epic if I do! Great post Mark!

  110. This is so crazy about the year-long fast. It’s hard to wrap my brain around it.

    I used to IF regularly once a week, then I switched to The Warrior Diet eating absolutely nothing during the day and only allowing myself a 2-hr eating window at night.
    Nothing ever worked for me as far as weight-loss, and low and behold, the Warrior Diet didn’t do anything either.

    I still think IF is very beneficial outside of weight-loss and so I’m still practicing it once a week or so, but unfortunately, no one bill fits all.

  111. I’m experimenting a lot with IF. I’ve done cleanses and diets before but all have been shortlived and have not given me any long term positives, or not that I’ve picked up on.

    However. I’ve been experimenting with IF and have found it really helps. In general I like having breakfast and dinner. I don’t tend to have lunch or snack (unless it’s a high activity day). That means I get a “mini fast” between breakfast and dinner and then have a good period of time about 15 hours (I eat a very early dinner).

    I wrote a post about my experimenting with fasting on my blog earlier today. Have linked back to your,”The Myriad Benefits of Intermittent Fasting,” hope this is ok?

  112. Great to see more articles about fasting. I have been trying every different fasting protocol the past few months experimenting. I used LG Fasting to get from 30% bodyfat down to single digits in a few months. I love to be able to eat massive post workout meals of 1,500 calories. I was never a big breakfast eater and now all my friends want me to write them fasting protocols for the week after seeing the food I eat!! Amazing stuff!!

  113. Considering this study how far fetched is this idea:

    Energy used by the body are in this order:
    Carbs, then fat, and then protein gets converted to energy once there is not enough fat. (Assuming I am correct?)

    So if a Guy/Girl wanting to build muscle and loose fat quickly, they want to fast, but also hit it hard at the gym, so they can fast by avoiding eating Carbs and Fat, but still eat protein in pure form (Lean meat, Lean fish (tuna) or only protein shakes with water).

    Am I right in saying that doing this will result in his/her body using body fat to fuel him while he fasts, but the protein will help build muscle mass and repair muscle after the workouts?

  114. I have not tried IF and while I respect Sisson, of course, I feel like a speeding of the metabolism by eating more frequent, smaller meals throughout the day is a safer approach to weight loss. First thing that bothers me is the danger of producing tendencies towards anorexia. Many, if not most, teenage girls would read this article and the only thing they’d get from it is: “DO NOT EAT”

    1. We can’t stop people for using good information in a bad way.

      From what you’ve said, it suggests that you have not read into IF enough otherwise you would not have said what you said.

      I would suggest that you read through Mark’s current articles on fasting, and the comments as well. Sometimes you learn just as much from reading through the comments. Hope that helps. 🙂

  115. Javier,

    1 – the idea of eating more frequent meals to “speed metabolism” is a myth. the reason it feels “safe” to you is that you’ve heard it constantly over the past 15 years. this website is about opening your mind to the facts and reality… and that concept is simply not true.

    2 – Anorexia is a PSYCHOLOGICAL condition that a teenage girl can not acquire from a website. it is a mental illness where people look in the mirror and see a distorted image (one much “fatter” than they actually are). trying Intermittent Fasting will not cause anorexia if you are of sound mind beforehand. It just won’t.

    3 – After a year of low-carb and not being able to drop below 260 despite trying everything in the book, reading this one entry on IF has changed my life. Since this article was posted, I have implemented the IF fast every day. I have never felt better and I have lost 10 lbs in a week and a half. I simply only eat one meal a day (dinner). And trust me, if you saw what i ate and how much, you would not consider me anorexic! haha… last night had a HUGE bowl of chili. night before? 4 pieces of chicken with fried cauliflower.

    i have never felt better, mentally or physically, and the weight is coming off at a seriously rapid clip. Mind you, I am eating roughly the same calories as before… around 1200. But getting those calories in a 2-hour window and not consuming anything other than coffee the rest of the time has completely changed the game for me.

    i am living proof that this works. past 6 months at 1200 calories (hi fat, med prot, low carb) spread throughout the day resulted in NO weight loss. same 1200 calories at once, MASSIVE weight loss.

    i will come back here and give another update in a month or so, but i am shocked at how much impact this simple trick has had. i suggest everyone battling a weight loss plateau to try it.

  116. I am coming up on Hour 18 of my weekly 24 hour fast. I usually break the fast with some coconut milk an hour before my first meal. My first meal is about a half pound of protein, and a salad with avocado and E.V.O.O. It seems to work pretty well for me.

  117. is there a limit of coffee one can take on this diet ? as i drink 4-6 cups while fasted as i feel it kills my hunger , i can’t take green tea on an epty stomach as i don’t know why but it makes my dizzy , does this happen to anyone else ?

  118. I’m curious…is there an added benefit to IF during the day than over night. Seems silly but I would think it may be “cheating” if I account for the hours slept as part of, say my 16 hours, then going till lunch/dinner the next day for the initiation of my next 8 hours of feeding. I’m thinking maybe it’s a hormonal thing that would benefit me more to be “awake” during the IF to benefit most from it but I really don’t know and I really haven’t seen anything discussing that on here. Thanks in advance!

    1. Trust your body’s functions and cycles, which includes sleep. You do a lot of amazing fat burning and cell healing in sleep. You should search for and read some of the posts about sleeping on this site to really understand the importance that sleep has in healing and recovery of your body. Sleeping hours should most certainly be counted! (They call the first meal after waking break-fast for a reason) Your body is still going to get all the wonderful fasting benefits Mark laid out for us while sleeping. That includes fat burning and the cell recovering. Most of the IF’ers I know eat later in the day/evening with one big meal after their workout and another meal a couple hours before bed, with great results. This isn’t a rule thought, it’s just how most people I know do it. But at any rate it’s definitely not “waking” hours that you are counting, it’s all hours. In fact I’ll bet they will find out that it’s during sleep that most of the magic is done but I’m just speculating on this. Also there is the logistics of your question to consider. Let’s pretend you are going to count sleep as fasting hours…Therefore say you finish eating at 8pm and go to sleep at 10pm that’s 2 hours of fasting. Then let’s say you sleep until 6am. Well since you didn’t count the 8 hours of sleep as fasting, you need to go now until 8pm to have 16 “waking fasting hours”. That would be 8pm to 8pm – that’s called a 24 hour fast. If you want to do that OK but the dudes at leangains suggests that the hormonal benefits are best at the 16 hours range for day to day maintenance. Perhaps they are not right about this and 24 hour fasts are better for you, but even in a 24 hour fast you would certainly count the sleep hours within that time. Again it’s a possibility that 24 hour fast is better than 16 hour fast for you, but the guys at leangains seem to think 16 is best and they seem quite geeky about digging into the science-and they are pretty freaking ripped. Plus on a 24 fast you are eating once a day. Most people I know aren’t comfortable after shoveling down one 2000-2500 (or more) calories in one meal a day. So in closing, instead of counting sheep while you sleep, count hours that you are fasting.

    2. Yes your hours sleeping count towards the fast. If they didn’t, you’d never be able to eat!

      I’ve been doing this for about 3 weeks now and feel great. I eat from noon until 7:30 pm. I eat around 2000 Calories and keep the carbs under 75 grams. My performance at the gym is also at an all time high.

      1. Thanks Matt (and David with your response up above). Yes, you’re both right…it definitely makes sense to count sleep since it otherwise would essentially be a 24 hour fast. I don’t think I’m ready for a 24 hour routine but I will definitely be trying the 16-8 routine a couple days a week.

        Oddly enough when I’m 100% primal I don’t even feel the need to have breakfast in the morning and am more than capable of not eating till noon the next day. Which I know is something Mark suggests is common and normal; you eat when you’re hungry not because conventional wisdom says to do so. I’ll try that out more often. Thanks again you two.

  119. Hi! Has anyone tried Bulletproof coffee intermittent fasting? upgraded coffee is supposed to be of grea quality, and having the coffee with kerrygold butter and MCT oil in the morning s supposed to accelerate metabolism and the formation of ketones. Is there truth to this? Also, I would have thought consuming that much fat in the morning would break the fast and give my body energy to burn instead of using my own stored fat?
    I’ve been practicing IF for a few weeks now, I do just fine. I’m just wondering if the bulletproof method’s claims are true and will accelerate fat loss.
    Any thought?


  120. In my numerous experiences with fasting, the first 8-12 hours are hardest. The hunger mechanism kicks in big time and tempts you to eat, but after that, it’s very easy. No hunger at all. Coffee definitely helps.

  121. hello everybody!
    when I do IF ( mostly 16/8 style)sometimes I feel strong tingle in my skin specially in my face area and my breath feels like “hot” or something.. have anybody of you have similar experience? any idea where these symptoms may come?

  122. I subscribe to MDA and Paleo Plan’s posts, but am I missing how long the IF goes on, or how often you do it? Is this the way everyone’s eating all the time or are you doing it weekly, monthly, as needed? My interest is mostly in fat loss, but also performance and energy. I CrossFit five days per week, and usually run on the other two days. I try hard to stick to a paleo/primal diet.

  123. Fasting works! AND jump starts a ZONE based lifetime diet and exercise program …
    Thanks Mark!

  124. All I know is that when I fast from 6pm to about noon the following day, and eat two very small meals consisting of 4 oz of organic protein and a handful of vegetables for lunch and dinner and eat a fruit as a snack 2x a day, the fat melted off of my belly, love handles, arms, thighs etc.. I lost no lean muscle. I was getting sculpted, and I’m 48. I have not tried a true fast where there’s no food at all for a period of days . . . i get too weak to work out.

  125. Im muslim n we fast thirty days every year. We eat before sunrise n fast till sunset, then eat again. Islam tells us to eat 1/3 for food, 1/3 rd for water, 1/3 rd for air. We also fast voluntary fasts throughout the year, i wont go into that here.

    It keeps your body really agile and not weighed down.

  126. I fasted for 10 days on what is called
    the Master Cleanse. You can find this particular fast on the Internet or the book is on Amazon. It basically consists of fresh lemon juice, maple syrup and a pinch of cayenne in 8 oz of water (to be drunk throughout the day – as much as you want). Honestly….I was NEVER hungry and completed the 10 days. I lost 16 lbs. of FAT and never gained it back. I have gone on to lose another 11 lbs on the Paleo plan. I fast for 16 hrs and eat within the 8 hr framework and feel great! Thanks Mark for all your insights and the studies to back what you say. Fasting lets your body clean house and breaks our obsession with food. No…we’re not going to die if we’re without food for a few days.

  127. I want to try fasting, but I get a headache and light headed. Any suggestions.

    1. I would wager that you just might be magnesium deficient. Find a good absorbable magnesium, I use ionic magnesium by Eidon. Headaches can be caused by a magnesium defifincy. I had the annoying buggers and learned about magnesium deficincies when I began fasting. I began supplementing and not only did I have a marked reduction in headaches I use magnesium to stop an onset headache instead of tylenol.

  128. It’s good to see something like this that gives an alternative option to the ‘must have good breakfast’ school of thought. I always used to obey my actual hunger signals and very rarely had breakfast, sometimes going right through to early evening and never missing food…then I started following the ‘must have breakfast ‘ rule and started eating first thing even though I wasn’t hungry…funny thing is, I’m always starving by 11 am if I do this, whereas I don’t even feel hungry at this time if I don’t eat first thing.
    I would much rather be in tune with my body and give it what it appears to be asking me for, so I’m 100% behind the IF school of thought. It makes me feel more in control of my eating, as well.

  129. I am reading in the book, Optimum Nutrition, that “Carbohydrates, not fat, are the premier fuel for performance. Also, carbohydrates can be stored as glycogen, while fat cannot…this is why endurance athletes eat rice or pasta…” Are carbohydrates necessary for sustained energy? Can the body convert protein and fats, through glucogenesis, into glycogen that can be stored in the muscle? Is this a huge misconception?

  130. Can a person with adrenal fatigue, low thyroid, and Hoshimotos fast? Specifically with veggie juices, supplements and raw adrenal and thyroid supplementation?

  131. I think that the idea of fasting is great and makes complete sense for an optimal or close to optimal human- that is, one not too metabolically deranged.

    Just personally speaking, I have tried fasting and while I feel GREAT not eating breakfast because I am generally not hungry until noon I then find that it always causes compulsive eating at night around 5pm where I am insatiable and eat past fullness. My understanding is that if you have any sort of adrenal issues then basically you are raising cortisol more by fasting which, for people like me who have too low morning cortisol, it stresses the adrenals out even more causing more long term damage. I am also still healing from an eating disorder (was never underweight however) so I believe my metabolism, at least for now, needs a larger window- I usually go 12/12 with three meals and no snacking.

    That said, I no longer have hypoglycemia since switching to vlc so if I do need to skip a meal due to external constraints I have no problem. I am looking forward to the day where I can really take advantage of IF.

  132. Just to get this straight – the Scot’s Man went 382 days with out eating anything???!!!
    Only taking vitamins and minerals?
    Did I read that right. Or was he IF’ing?

  133. LMAO! Its good to know that some MD sitting in some fancy lab has FINALLY come around on fasting with the tests and stats!
    Fasting has been used for thousands of years by billions of people in traditional cultures around the world. So its the time tested built in system recovery tool-fast,smart and easy as Windows.

  134. Just started IF – I find it very convenient and easy though – I am experiencing quite the mood swings – thoughts?

  135. I am 60 hours in to my fast, didn’t intend for it to be this long but it is…and I am wondering when I should break it and with what? Eggs, avocado and bacon sounds good, pork tenderloin is in the running, chicken coconut curry with cauliflower could be the one….any thoughts on what is better? Avocado/eggs/bacon too heavy?

  136. I have searched everywhere for a way to lose those extra pounds that seem to have appeared over the course of the last few years. I have tried every diet out there and nothing seemed to work. Then I discovered African Mango Supplements and Proactol and I finally started to see the weight drop. I highly recommend both these products. You can find them at https://4abetteryou.best3weightlossproducts.com.

  137. I have grown to realise that eating natural food like steak, chicken, greens, and carbs such as sweet potato, because they are all natural. What ever you do, stay away from processed foods such as white flour, white bread, white sugar, processed meats or fast takeaway foods that you wouldn’t generally eat with a knife and fork like fried foods.
    Dont feel guilty about eating healthy, because you can still lose weight providing you eat in moderation and is a bonus if you are doing exercise on a regular basis with out over doing it. What ever you do to try and lose weight,Your pancrious makes insulin and tells your body to convert access foods such as white bread into fat, so stay away from those foods and you will be on the right track to losing weight and having a healthier body.

  138. Hello! I recently starting IF, and I am very excited about it. I’m a 42 year old woman with about 50 lbs to lose. Over the past 8-9 yrs the weight has crept up… 4 or 5 lbs gained per year for several years sure do add up. Once upon a time, I was trim (not skinny), muscular and in great shape. Over the past few years, I tried losing weight several times but would fail miserably. I hate “mini” meals (what am I, a 5 year old?). I hate counting calories, weighing food (how depressing). I hate snacking on tasteless food all day. These are the reasons my weight loss attempts never worked. I can’t do these ridiculous things every day long term.

    So now I do IF and it’s awesome! I lost 10 lbs in about 4 weeks. On stupid diets I’ve tried, I’d be lucky to lose 3-4 lbs in an entire month which was discouraging. IF is encouraging! Finally something that works. I’ve also been walking, a lot which helps even more. IF is a life style that is good for me especially since I’m not much of a breakfast or dinner person.

    I do three 24-hr fasts per week on M-W-F. My fasting goes from around 6 p.m. (previous evening) to 6 p.m. on my fasting day(s). Sometimes I change the days around or do 2 fasts if I’m going away for a long weekend. I break the fast with a pretty big dinner (ie 800-1200 calories) with awesome food, often fish (big portion) or chicken, lots of veggies or a big salad. Dessert 2 hrs later, usually berries with cool whip light. On my Tuesday-Thursdays, I eat whenever I’m hungry (usually 2 snacks and a big healthy high protien low carb dinner). My favorite snack has become a whole avacado with a little lime and sea salt. On Saturdays and Sundays… party time! I have my cold light beers, burgers, sausages, pancakes, chips and dips etc. I create massive caloric deficit during the week that I can do this and still easily drop about 2 lbs per week. I still do not “pig out”… I stop eating when I’m full. I balance some healthy stuff in with my indulgence food. So I’ll likely have a salad with my burger or grilled veggies instead of potato salad. If I want potato salad and macaroni salad, I’ll go for grilled chicken or fish instead og a hotdog or burger. But basically I enjoy indulging a bit. On Monday… back to my fast and cleaner eating for the work week. Works like a charm!

    I walk 4-5 times per week, moderate pace for 3 miles (1 hr), burning 300-400 calories. I also have an active life style. I walk on fasting days too, near the end of my fast to maximize the fat burning. I have a lot of energy, none lost whatsoever. I start feeling REAL hunger after about 20 hrs and look forward to a healthy big dinner. Now I really appreciate food. Soon I will be doing cardio kick boxing 2 days per week. So I will walk two days and kick box 2 days. My exercise along with my fasting days create massive calorie deficit so the weight is coming off rather quickly, yay!

    And this life style still allows me to enjoy “naughty” foods and adult beverages. Imagine going all Summer with out some of these enjoyments? That would be complete misery!

    For those of you thinking about it, do it!!! You won’t regret it. I should also mention that on my fasting days I drink water all day, and a few cups of black coffee in the morning. On non-fasting days, I have light cream and sugar or splenda in my coffee which I prefer. I’m not thrilled about black coffee but I’ve gotten use to it and it’s not so bad 3 times per week. But on 24- hr fasts, I am strict about being certain I have absolutely no calories. If you like my routine, and you create something similar… you will shed some serious body fat.

    I am so looking foward to this journey 🙂

    1. Hi Melissa – glad you are finding IF to be an effective way of staying healthy.

      3 x 24 hour fasts per week sounds quite tough to me and I’ve never gone to that level myself – perhaps consider shortening one of those fasts if you do start feeling any weakness etc…

      Other than that, I like the ‘clean’ feeling I get following a fasted period and I also use cardio fasting to maximise the effect – https://www.lmdfitness.com/training/cardio-fasting-fat-loss/

  139. Ooops, in my last post I said IF is pretty easy for me especially since I’m not a big breakfast or dinner person. But I meant to say “breakfast or lunch”. I am a big dinner person 😉 so every day I have dinner. That is one meal that is NEVER skipped. Good luck to everyone!

  140. I do my version of intermittent fasting. I eat no solid food before 3:00. I break my fast with a hearty primal meal that fills me up comfortably. Fasting improves my digestion and helps with constipation. I was not constipated before doing intermittent fasting but my regularity improved surprisingly. I fast for twenty hours and allow for a 4 hour window. Since I have grown comfortably with eating once daily, I may as well say that I eat one full meal a day. As for weightloss, I lost about 25 lbs. but regained it. I became peri-menopausal since I began intermittent fasting. I amped up my fsating and I am down 8 lbs. I like fasting. Food tastes better without having to overeat.I believe that we were built to eat fewer meals, that’s why ketosis exists. I also believe that eating frequently ( 3 meals with in-between snacks ) stresses and weakens the organs.I also believe that obesity and diabetes wouldn;t be at epidemic proportions if people would eat less often…

  141. I started fasting after reading a fasting book by Paul Bragg. I’ve done 1, 3, 5, and my longest lately has been 10 days with just pure water. The benefits are amazing! I always feel rejuvenated after the longer ones. My neck aches from an automobile accident in my younger years is almost completely gone! My skin cleared up, I crave fruits and veggies more then processed stuff. And when I do eat the processed stuff I feel crappy afterwards. I usually lose 2 pounds a day on a fast, gaining less than half back but maintaining through good food and exercise. I love fasting!

  142. To maintain my weight, I need about 2500 calories. I have 4 little kids so I have found it very hard to get exercise in as I am always running around for being a mother. Anyway, I have been intermittent fasting…18/6 for 11 days and have lost about 7 lbs. I really don’t think it is water weight as I have not been eating grains or sugar for months. I have been tracking my calories for 3 days with fit day and have come up with an average of 1300 calories a day. My carbs an average of 60 a day. My question is… at this restriction, am I losing muscle? Will my metabolism slow down or has it? I feel quite fine energy wise, my appetite is depressed so I really don’t feel that hungry and even eating 1300 calories a day I find hard to do. Though I am sure I could reverse that with a big piece of chocolate cake. thanks for the advise.

  143. As some others stated above if you fast your metabolism will slow down and your body will go into kind of a hibernation state which causes you to start storing calories instead of burning them.

  144. Wow, mind=blown haha. I will try a day of fasting every once in a while and see how it works for me. If fasting just isn’t for you I know of an extremely simple diet that works. It consists of eating slow digestive carbs like wheat bread, whole grains, and oats. Add in lean meats like chicken, fish, and turkey. Add vegetables and fruit like spinach, broccoli, apples, blueberries, and oranges. I’m bad at explaining things in txt but basically eat fresh natural foods and stay away from highly processed foods that come in containers. Oh, and try to eat smaller meals 5-6 times a day.

  145. >Fasting increases the catecholamines,
    >both adrenaline (epinephrine) and
    >noradrenaline (norepinephrine).

    I did my first 24 hrs fast yesterday with good results. No headaches, no dizziness- NOT until the 23rd hour where I finally felt a slight blurry vision, kinda like I feel if I haven’t slept all night.
    One thing I noticed tho, was that my pulse would increase a lot and very easily when I eg ran up stairs. I wonder if it’s because of the said effect on adrenaline?

  146. Interesting… from the article about 1 meal/day study:

    “[…] when consuming 1 meal/d, subjects had […] significant increases in blood pressure and in total, LDL-, and HDL-cholesterol concentrations; and a significant decrease in concentrations of cortisol.”

  147. Hi Mark

    Great article! I fast for religious reasons, it is my second one today (will do 30 in total over a month). I cannot eat or drink anything for 19 hours. I am hoping to lose weight during this month and maybe continue doing a ten hour fast (but drinking water) after this month of religious fasting has finished. Would doing aerobic exercise such as the rowing machine be beneficial whilst fasting? Also, is there a particular time in the fasting day that would be the best time to exercise? At the moment I’m breaking my fast at 9.30ish

  148. Managed a water fast for 19 days several years ago. My appetite dropped out after about three days making each day easier. Had interesting dreams about mountains of food. Have not seriously fasted since, but comments on this site make me want to try again even if only portions of the day. Have also found helpful weight loss tips at https://thefatlossfacts.com.

  149. No doubt fasting is one way to reduce and you have correctly dispelled some myths about it. However, I have some reservations as whether it work for diabetics. This can prove to be dangerous as sugar levels tend to drop or rise suddenly of fasting.

    Recently I have come across 3 weight loss products that approach the problem differently.
    The main issue that they tackle is that you become what you eat. If only, the food that you eat could restrict the fat and flab, wouldn’t it be just great?

    Well, these three weight loss products do just that., naturally. Using natural products and a totally different way to tackle the flab, they ensure that you become slim and remain that way!!

    I recommend you give them a fair try.

  150. I don’t agree with you one bit. Isn’t it a fact that if you fast you gain weight instead of lose it? Go here to find out how you can truly lose weight best3diets dot com

  151. love your post.I don’t know anything about the primal diet or IF.I am interesting in taking on this challenge.Can you provide me with more information on how to begin this journey.It sound fascinating.I am trying to loose some weight and increase my energy.

  152. In 1997 I undertook a 40-day fast for spiritual reasons. I had absolutely no concept of Primal eating or lifestyle. After the fast I returned to normal carb-centric eating and put on all the weight very quickly, about 30 lbs. I repeated the fast every year for about six years. In those days i allowed myself the ocassional juice or starbucks latte if i was feeling weak. Here’s what I experienced physically (aside from my spiritual goals):
    1. It was was relatively easy.
    2. Hunger goes away the first or second day
    3. Counterintuitively, my energy levels would rise. I found that I wanted to run everywhere. Upstairs, outside (I was not a runner by any means and was still about 30lbs overweight)
    4. I slept better
    5. There was no decrease in mental sharpness (I design electronic controls for mission-critical applications)
    I gave it up for family-sociall reasons. Then in 2005 I felt “called” to fast again. This time I allowed myself 50 days to complete a 40-day fast. That is, I allowed myself 10 “eat-days” to accommodate family and social events. I liked this lifestyle so much that I repeated it every month from March until September. I lost 50lbs and never felt better.
    I thought that I had arrived, but now in retrospect I can see that I was a carb-addict and despite my marathons, and iron-man (v70.3) triathlon racing I was still a compulsive over-eater of carbs. Weight gain is just too easy for me when there is bread, potatoes, beer or pasta involved. To gain 30 lbs in a year is not a problem for me.
    Now, having researched and investigated and gained a much more solid understanding of paleo and strength training I have begun another long-term fast. I started 5 days ago and have allowed myself only tea (sorry, one vice I cannot live without) and lemon-water. I will inject “eat-days” full of meat and veggies every 8 days or so to make sure that the family & friends social structure is not greatly impacted.
    Bottom line, I never feel better than when I am

  153. In addition to intermittent fasting I’ve found doing yoga has been a great way to help lose weight. It relaxes your body and mind. It can help with detoxification because being overweight may be a sign your body is high in toxicity. Yoga also helps release stuck emotions because being overweight usually has an emotional component that we need to be able to address.

  154. Wondering about the effects of fasting and injury recovery. I have been living primal for roughly a month. I am an athlete, recovering from anterior tibial stress fractures. They are healed, and have been for several months, but I flare up the fracture site very quickly with activity. I’m being more aware of sleep, and trying to get more of it because I don’t think my body is recovering as completely as it could at night. My nutrition is good, and I’m feeling body wanting to fast, and this was not the case before going primal. Any thoughts on fasting and injuries?

    1. That’s an interesting thought Katie. A few years ago I had a shoulder operation to repaid a torn major pec muscle. I was using intermittant fasting during my rehab to manage weight and stay lean as no exercise was possible. Wish I had kept a diary on the condition during these fasted periods… perhaps the reduced inflammation aids recovery

  155. Wouldnt it be nice if someone actually did, or cited a research to finally put to rest, what burns when we fast – fat or muscle.

    Clearly there are two theories here with equal # of proponents – they agree up to glycogen depletion, but then there’s a fork in the road. One side thinks that fasting will take us to starvation/ hibernation mode and it will be harder to lose fat, and muscle will burn to fuel the body. Authors like Taubes have said that fat is a selfish tissue and when it sees caloric restriction it becomes bigger and bigger.

    The other side says that fasting will induce lipolysis – that is, will dig into fat cells. That forms the basis of concepts like IF and Glycogen Depletion.

    These are opposite, and contradicting theories. Body cannot be holding on to fat tissue and burning muscle, AND burning fat tissue at the same time.

    Which one is correct – please help! I hope we realize how important it is to determine that unless we’re ready to do things like Fasting and burn even more muscle. Point us to some authentic reference for this question. Thanks.

  156. Do you know muslims spend an entire month fasting? It’s an obligation of Islam for the health of body and soul. Intermittent fasting is great for the soul too..

  157. This is a cheaper & healthier way of losing weight. Am willin to try it out. Thanx

  158. Im reading Vincent Ng’s book TEN, lean and sculpted body in ten weeks (anyone familiar with it?)

    He(his research) says that one should not eat less than 1000 calories a day, since our body will switch to starvation mode, which slows down our metabolism.
    When you eat less, your body will slow down its metabolic rate, allowing you to last longer without food.
    Another critical reason why we should not allow our bodies to lapse into starvation mode is that the body will attack the lean muscle tissue as it consumes most calories. When you lose lean muscle tissue you will find it more difficult to lose body fat.

    So on the subject of fasting, how big is the effect of the above to any of the fasting methods in which we would apply?

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  161. Wow, 382 days without food? 24 hour fast is good enough for me. Thanks for a great article on fasting.

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  163. Um…. WTF? Is everyone here stupid… “Yes, AB fasted for 382 days, drinking only water and taking vitamin, potassium, and sodium supplements. All told, he lost 276 pounds, reaching his target weight of 180 pounds and maintaining the bulk of his weight loss”

    Does no one here realize, that wouldn’t work… There is a reason why you need to take food in regardless of what your body size is… I would know… I was anorexic long enough to know the effects. First of all, with out an input of essential amino acids, say good bye. Your enzymes with be recycled to a point, but you will eventually die.

    I’ve been anorexic. I know how fast you loose weight, especially if you are drinking zero calorie fluids. Now the important thing is, you can get glucose from glycogen… you can make glycogen from glucose, and other saccharides, but you can’t make them from acetyl Co-a… which is essentially the energy source from fatty acid metabolism (Beta oxidation). The fat provides a lot of energy (ATP) so your body can survive, but that doesn’t matter. Your body always needs an input of saccharides in order to make your muscles go… Muscles don’t work on FA catabolism, they work on creatine phosphate, glycogen, and glucose. This man would eventually completely deplete his stores of glycogen/glucose long before 276 days resulting in death. Without glucose, your muscles just don’t work…

  164. I find that sometimes I “automatically” fast. I’ve gone an entire day without eating simply because I am not hungry.

    I started eating primal and began hiking about 8 months ago.

    I’ve lost 50 pounds and I’m now at a very good weight for my build. It was the easiest weight loss I’ve ever experienced in my life.

  165. thank you for such an insightful article. I would like to share my experience with weight loss and fasting. After giving birth to my first child, i gained 20 kg ( was only 50 kg pre-pregnancy).. i tried all sorts of dieting methods but i could not stick with it long enough. so i fasted for a month (I am a Muslim by the way, fasting is compulsory in Ramadhan). I managed to lose 10 kg from fasting 30 days. afterwards i did gain weight, but only 1 kg. i checked at the hospital and i am healthy. so i would say that fasting really help me shed all the extra weights.

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  167. On a diet rapidly might be complicated and the outcomes you
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  168. I have fasted for 9 days (only water) to cure my pancreatitis and I’ve lost 7-8 kg. I work out and I didn’t notice any muscle mass loss. I experienced many positive effects like improved health, faster metabolism, healed wounds and my acne just disappeared. I also practice 2 or 3 days fasting and when I get sick I fast for 24 or 36 hours untill I feal well again – it works perfectly!

  169. You actually make it seem so easy with your presentation but
    I find this topic to be actually something which I think I would
    never understand. It seems too complicated
    and extremely broad for me. I am looking forward for your next post, I will try to get the
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  170. I would try a cleanse in addition to all of this.. I did a detox from sugar which was a great kickstart to the new healthy life choices I was making!

    good luck and stay strong !

  171. I have been attempting paleo for the past year. Physically and mentally I see major positive changes. The difficulty arises because I have an income of six hundred dollars a month. I have tried and simply cannot afford to live paleo. I run out of food at the end of the month and go to the food bank which is like carb nirvana. So finally I compromised and added potatoes and rice to get through, with obvious drawbacks. But the fasting might be exactly what I need to stay on track and get through the month. I appreciate the information.

  172. I have done quite a few water fasts in the past 33 days was my record before the fast I just finished which was 59 days. I studied the process in great depth and read all the research before I attempted the first one. I was well aware that this in not a new therapy. It has been done safely for thousands of years. The body know how to fast and it competent and well versed in just what to do if we stay out of the way.

    I lost 63 pounds and 14% Body Fat. I have been off the fast for 2 weeks now and have gained back some weight (bone, muscle, water) but have not gained back any body fat. I feel great and I just turned 60 years old. I know that during those 59 days my body did a lot of breaking down and recycling of old and diseased tissue, viruses, bacteria, fungus, etc. Nothing is ever wasted when a person gives the digestive system a rest. I think the key to fasting is to relax and let the body take over. Eliminate as much stress as possible and enjoy what is taking place inside your body. Feel all the good things that are going on and surround yourself with positive, supportive people. Read and study everything you can about fasting and think only positive thoughts. My fasts are always times of great peace and encouragement to me.

  173. I read the article and related comments. In my opinion the article explained clearly how fasting can help the body for fat lose, however, the article lacks more details about the best way of fasting, how long, how to start and end your fasting and so on.. Etc.

    I am fasting now… it is Ramadan and we have 30 days of fasting starting from early morning around 4 AM till sunset around 7-8 PM (this is summer time, and in winter times the day will be shorter, and therefore the fasting hours are less). During fasting period nothing will enter your mouth till the end of fasting in your day when you can eat and drink anything you want. Notwithstanding to maximize your benefits of fasting you have to be careful of your food….. its a golden chance which comes once every year to change your habits and change your life.

    in all cases, if you wish to explore more details about fasting and how it can be done for best results please contact me or visit any of websites about Ramadan and fasting.

    Thank you

  174. If anyone is interested, I’ve put together a video debunking the top 6 most common myths surrounding nutrition and getting in shape… I would love to hear feedback from this community as I plan on continuously adding on to it as I learn more, you can check it out here: wisdom-square.com/how-to-lose-weight-and-belly-fat-fast.html.

    Good luck to you all! 🙂

  175. Looking over the comments most people are either curious about IF or hav had good luck with it. Those that are skeptical seem to have a rant and point to references apparently supporting their viewpoint but say little about their experience. I would suggest that they try it and see what their direct experience is before they decide which line of evidence they wish to follow.

    I’m generally pretty clean on my diet yet fasting for 16 – 24 hours 2 days a week helped be drop some bodyfat and seems to have improved my energy levels. Just my experience

  176. Mark your article on fasting is fantastic. Is easy to read and understand,makes a lot of sense to me. On reading the comments it seems that fasting works. Great site. Have you seen this site?.lose leg fat

  177. This guy looks like he pees on peoples butts and then drinks it with a straw. Do you really want diet advice from a piss drinker?

  178. Its a no brainer really when one thinks of the cave man or Grok he caught his meal, had a big “nosh” up and then didn’t eat until he caught his next meal. Man has been doing this for quite some time until the advent of the 3 square meals a day put out by the people that need this to further their businesses. I believe that Man has had thousands of years to fine tune the Grok way until the “smartypants” came along and stuffed it all up. So for my money fasting is the way to go, this link is interesting to watch:

  179. Hahaha, I read this article a few years ago, and until today I had thought that he ate an apple a day throughout the fast… It appears the brain works in weird and wonderful ways 🙂

  180. I like to fast. After 3-4 days of adjustment I run like a well-oiled machine, usually 12-15 days is as far as I go. As long as I don’t overburn calories with excessive exercise. I sleep great! I drop fat! I don’t feel hungry! I forget food! I am pleasant and joyful!

  181. The religion called Islam requires a 29-30 day fast from dawn to sunset. the month of Ramadan. On Monday and Thursday its good to fast because the gates of paradise are open.

  182. So I’m on day 7 of nothing but coffee fast. Tell a lie, I ate a square of chocolate every other day. I haven’t weighed yet but I haven’t lost an inch, my tight jeans are still tight. Im 5 foot and 120lbs usually 105. I’m usually active but off the exercise whilst doing this. Adding minerals to reverse osmosis water and taking a probiotic. But no fat loss how?

  183. There’s so much science here that makes sense. But I want to ask about the “old” belief that “starving yourself” or fasting affected your metabolism negatively, putting your body into “survival” mode, and holding onto fat. Is that a misunderstanding?

    Thank you!

  184. Intersting to me that all through the years they’d say “breakfast is the most important meal of the day,” but I’ve always, always been averse to eating breakfast. It just never felt right to me. I wanted coffee, yes. But I’m not usually hungry until after 10am. I love that we’re finding decent science that supports skipping breakfast! (It also explains how I could stay lean all those years, even on SAD diet, which is the opposite of what many experts said should happen.) I’m Primal now, or striving to be fully primal. Fasting is definitely easier on this diet standard than the SAD.

  185. Interestingly, this recently happened to me:
    I quit my work to go abroad to work, but got trouble with my VISA – which put all my plans on hold. This led to what I now think must have been my first phase of depression in my life.

    Sleeping long nights and staying in bed until noon, meant I was not eating (nor being physical) for long parts of the day). 14 to 16 hours without food happened several times a week, some days I went even longer without food. When I finally managed to get out of bed, everyday I went for a walk to (unsuccesfully) ‘clear my mind’. Returning to my house, I had an uncontrollable hunger. I made large (and I really mean l a r g e) meals: sometimes two big burgers, salads, assorted raw vegetables jammed betweem 2×2 thick slices of high protein (13,5% protein) whole grain bread. Rest of the day, I basically ate a new meal every two hours, because for some reason I was craving food. My food intake actually went up from my previously normal eating routine. I might add that I have been on a semi-high proteine/raw vegetables diet for years, since I discovered years ago that this put an end to my craving for sweets and chocolate (16 years ago I was obese).

    Now.. after a few weeks of this accidental diet, a visitor pointed out I had lost weight. For the first time in weeks I looked myself in the mirror and noticed my body looked more defined, muscles were showing in my arms, my tendency towards man boobs were almost completely gone. I was kind of baffled. I have never been unsecure about how I look – my weight loss years ago was purely for health reasons – but I must admit that I have always been slightly annoyed about my metabolism. I have never seen my abs in my entire life – not even in periods with good workout routines. Some of my friends could eat anything, drink empty calories etc. and still stay lean – but I always had this layer of fat covering my body.

    What happened? I haven’t been working out for years, but always been in good general health. The change in my body inspired me to start working out. I used my body as my weight and did routines with my 25 pound manual. Not much.. just 20 minutes a day.. For some weird reason unknown to me (maybe because I was out of work, or maybe because my body just got used to it) I stuck to my weird diet with an 8 hour eating window.
    These days, my shoulders are broader, my arm and upper body is quite well defined and – for the first time in 34 years (since I was 8 years old!) I can see the top two cans of a sixpack! And: I am not depressed anymore!

    I must add: I know perfectly nothing about dieting or working out. Zero! Zilch! Today is literally the first time I visit a health/diet/work-out page on the internet!!

    The reason I am here is that I googled: ‘excess skin after losing weight’! Because: now I am getting a little impatient with the rest of my lower, sloppy belly… The only real excess fat on my body is on my lower belly, which now feels kind of runny and flappy to the touch/squeeze.
    I learned two things: Firstly – My belly is not suffering from excess skin, rather I have a case of “stubborn fat”. Secondly – I just realized I ‘ire-nvented’ intermittent fasting, which I find almost hilarious.

    Conclusion: I will stick to my diet and my little work out routine, and be more patient with my tummy. Thank you for all the information. Interesting read!

  186. Fasting diets include many types of fasting such as not eating at all for 24 hours, or reducing drastically the number of calories you intake for two days during the week.

    Although the effectiveness of this diet is questionable some benefits of this diet are decreasing the level of insulin, increasing human growth hormone and transforming fat into energy.
    Great article, thank you.

  187. Hey there!

    I am considering fasting again. I have about 40-50 pounds to lose. I want to be inoptimal health before we get pregnant and my husband and I are actively trying.

    Here’s the thing: I’ve done a 2 week water fast with the intention of going to one month, if I felt okay. After about 2 weeks I was fatigued and cold. I know that too little sodium can reduce blood pressure and your body will shunt blood from the appendages to maintain perfusion of the organs. This will make you feel cold. I placed salt under my tongue a few times and it helped greatly but I was not supplementing K, Mg, iron or other vitamins. I’d like to try again and jumpstart my fat loss but I want to do it correctly and not jeopardize our chance of getting pregnant. As far as I understand, fertility is not reduced until your body fat goes too low. If I have plenty of body fat it seems I should be okay.

    What are your thoughts on my situation?