The Myriad of Benefits of Intermittent Fasting

benefits of intermittent fastingAt this point, intermittent fasting isn’t a new concept, nor is it a difficult one. You take in all of your calories for the day within a limited window of time, and the rest of the day, you stick with water, maybe a cup of coffee, or tea in the morning if you feel so inclined. The idea is that giving your body a period of time “off” from digesting food allows your cells to heal and renew in other ways.

A Practice Born Because Calorie Restriction is Unpleasant

Intermittent fasting became popular because calorie restriction was found to contribute to healthy aging. A few mouse and worm studies seem to show that drastic reductions in food intake over a long period of time could prolong your life.

The research is compelling, but I’m not convinced actively restricting your calorie intake through sheer will is the true path to enjoyable longevity. I don’t want to be thin, frail, distractible, or preoccupied with food. I’d rather be vibrant and full of zest. I want to eat big strapping meals of steak and veggies smothered in butter without worrying about calories. I want to maintain muscle mass and have enough energy to go on long hikes and have the legs to still leap for high passes (over the young guys) at the end of Ultimate games. And as I appreciate the neuroprotective and autophagy-promoting qualities of calorie restriction, I’d rather not expend the mental energy and fortitude required to maintain such a regimen day-in and day-out.

Intermittent fasting is the workaround. Pushing off breakfast for a few hours gives me all of the benefits of calorie restriction, without all the misery.

Fasting is the way to have your cake and eat it too. Beyond the already proven benefits of a Primal Blueprint low-carb lifestyle, fasting once in a while seems to offer many of the same benefits of calorie restriction – you know, stuff like increased longevity, neuroprotection, increased insulin sensitivity, stronger resistance to stress, some cool effects on endogenous hormone production, increased mental clarity, plus more – but without the active, agonizing restriction.

You just eat Primally, focusing on meat and vegetables with plenty of animal fat, and skip meals on occasion. A sixteen-hour fast is on the low-but-still-effective end, or you could opt for longer, more intermittent fasts – say, a full twenty-four hours once or twice a week. Women may need to time fasts a little differently than men. More on that here.

When you’re done with the fast, eat as much as you want (which usually isn’t an issue, once you’re keto-adapted). It essentially turns into “eat when you’re hungry,” because let’s face it: eating the types of foods we evolved eating induces powerful satiety and makes eating the right amount of food a subconscious act. Fasting becomes a whole lot easier (and intuitive) when you’ve got your food quality dialed in. And I’ll come back to that little caveat at the end here.

“Fasting” was the top search term for MDA last week, and I hadn’t done a big post on it in a while, so I thought I’d do a comprehensive rundown of all the benefits (some conclusive, others prospective) you can expect to obtain from IF.

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Intermittent Fasting and Longevity

Everyone wants to live longer, but I find longevity pointless if you’re not enjoying yourself. Otherwise, life becomes dreary.

The popular c. elegans worm enjoys increased longevity with both twenty-four and forty-eight hour IFs via signaling through a gene that we all have.1

One study (full PDF) from the 1940s found that varying amounts of twenty-four hour IFs (every other day, every fourth day, every eighth day, etc) prolonged the lifespan of rats without retarding or stunting the growth (as occurred with calorie restricting them). Female rats responded best to every eight day fasts, while males responded best to every other day fasts.

Reductions in brain insulin signaling have been shown to increase lifespan in animals, either by calorie restricting or actively knocking out brain insulin receptors.2 Fasting also reduces brain insulin signaling, at least in rats.3

Blood Lipids

Going in and pharmaceutically manhandling your cholesterol synthesizing equipment is one thing; eating real food and exercising, resulting in possible alterations to your lipid profile, is another. We don’t set out to force your blood lipids into submission, but lifestyle changes that happen to change them for “the better” are usually a good thing. Fasting brings potent changes to blood lipids in an “organic” way – you’re just letting your machinery do its thing on its own – and this is probably a very good thing.

Intermittent fasting is as effective or even more effective than calorie restriction in improving metabolic syndrome markers in overweight women, and it’s a whole lot easier to stick with.4

Alternate day fasting improved cardiovascular risk markers, including lowered triglycerides and LDL-C numbers (although it’s unclear whether the improvements were related to the weight loss alone or something unique to fasting).5

I discussed this last week, but it can’t hurt to mention that short-term alternate day fasting wrought improvements in LDL particle size and distribution in obese adults.6

Sticking with Intermittent Fasting

A dietary regimen is useless without compliance. In fact, that’s what we’ve always said about the low-fat, low-calorie diet advice we’re inundated with: sure, they might work, but they’re impossible for most people to maintain. Eating Primally solves this problem, because it’s simple, easy, delicious, and satiating (you just have to enjoy cooking, or learn to), and intermittent fasting is another compliance-breeding regimen that blends quite nicely with the Primal Blueprint. A lot of Primal eaters find that fasting just kinda happens without them setting out to do it, so it’s not even a conscious struggle.

Obese individuals were able to quickly adapt to alternate day modified fasting, which meant on fasting days they’d get 26% of their normal caloric intake. They were also able to maintain physical activity despite the fasting.7

Heck, intermittent fasting even helped cocaine addicts stick to their treatment and rehab program.8 Not bad.

Intermittent Fasting and Cancer

The notion of IF reducing cancer incidence and improving survival is compelling, but little evidence in humans exists. Ketogenic diets may also offer exciting potential for cancer patients, and both intermittent fasting diets and ketogenic diets share something: fat (either dietary or from your own adipose tissue) as primary fuel sources.

While you might not want to be in a constant state of ketosis, intermittent fasting is sustainable, simple, and can be integrated into your current diet. As of now, most of the evidence for IF’s protective effects against cancer exist in animal trials, mostly using mice. Still, fasting seems to confer so many other benefits that working it into your life for its anti-cancer potential is probably worth it. Some of the evidence:

Calorie restriction has been shown to fight cancer cell proliferation in mice, but researchers found that intermittent fasting was just as effective.9 In fact, here’s a review of most of the animal anti-cancer evidence.10 It’s quite compelling.

Some researchers are speculating, based on substantial evidence, that fasting before and during cancer treatment should result in reduced morbidity, better tolerance of chemotherapies, and higher cure rates.11 This is refreshing news. A preliminary study12 in human cancer patients found that fasting during chemotherapy reduced the negative side effects of the treatment. The authors are quick to point out that the results are in no way a prescription for fasting in chemotherapy patients and that controlled trials are needed to change official recommendations, but that doesn’t mean you – the individual – can’t experiment.

Growth Hormone

Aging humans “normally” experience reductions in growth hormone. While it’s true that unchecked growth hormone can lead to unwanted cell proliferation (like, ya know, cancer), growth hormone therapy can really help stave off the doldrums of old age. A study found that resistance training actually blunted hunger for an hour.13 I’ve found this to be the case for me. If the body “needs” food right after a workout, why would hunger be blunted? This is why I tend to hold off on the eating post-workout. Every little bit helps, especially as you age.

Neurological Health

Fasting doesn’t cause your brain tissue to waste away, contrary to what some people will tell you. It’s actually good for brain health. Any dietary restriction tends to increase neuronal plasticity and promote neurogenesis, but it was IF that had the greatest effect (with the fewest downsides).14 Another study of mice found that meal frequency impacts neuronal health.15 That is, mice who ate larger meals more infrequently saw greater increases in brain and overall bodily health. Still another study found that IF was beneficial for peripheral nerve function in mice by promoting the maintenance of the neuronal pathways responsible for locomotor performance.16 It’s almost like this stuff just puts your brain in repair, or maintenance mode.

Fasting and Autophagy

Fasting turns on autophagy (most studies nowadays treat this as common knowledge17), which is the process by which cells recycle waste material, eliminate or downregulate wasteful processes, and repair themselves. Why is autophagy so important? It’s required to maintain muscle mass18, and inhibiting it induces atrophy of adult skeletal muscle.19  It reduces the negative effects of aging20 and reduces the incidence and progression of aging-related diseases. In fact, researchers have determined that autophagy is the essential aspect of the anti-aging mechanism of fasting.21

Without the autophagy that fasting provides, you would get very few of the benefits. Fasting even increases neuronal autophagy,22 which aids in maintaining mental health and function. Short term fasting, too. No marathon thirty-six hour fast required.

Fasting and Fitness

You’ll hear that you should never exercise on an empty stomach. You’ll hear that fasted training will burn your  muscles and cause you to waste away. You’ll hear that performance will surely suffer. None of these things are necessarily true – and they are even less so if you are well-adapted adapted to a low-carb eating strategy. Fasted training can actually result in better metabolic adaptations23 (which mean better performance down the line), improved muscle protein synthesis,24 and a higher anabolic response to post-workout feeding25 (you’ll earn your meal and make more muscle out of it if you train on an empty stomach). Studies on Muslim athletes during Ramadan show no effect on performance while fasting,26 as well as a more favorable lipid profile27 in those who exercise and fast rather than just fast. When you train in a fasted state, glycogen breakdown is blunted28 and more fat is burned, leaving you more glycolytic energy in the tank for when you really need it and less body fat. Those are just a sampling of the benefits to fasted training; there are dozens more.

Mental Well-being and Clarity

A lot of health influencers will tell you that failure to eat something every few hours will cause mental fog and sluggishness, so keep a banana or a granola bar on your person at all times. Of course, this is all based on an assumption that we need to supply exogenous carbs on a regular basis to properly fuel the brain. This notion that fasting is only the province of anorexics or “caveman” has kept many people from experiencing the vast array of benefits.

I maintain that one’s comfort in handling intermittent fasting effortlessly does increase dramatically when you’ve reprogrammed those cells (and genes) to predispose your body to derive most of your day-to-day energy from fat, as opposed to constantly dipping into glycogen stores (as happens when we rely so much on refeeding carbs every few hours).

Overall, fasting just seems right. It’s like a reset button for your entire body, presumably across a large spectrum of maladies and dysfunctions. It puts your body into repair mode – at the cellular level – and it can restore normal hormonal function in the obese or overweight. Now, you don’t have to fast, but it’s definitely something to consider.

Have you tried intermittent fasting yet? Let me know how intermittent fasting has worked – or hasn’t – with your lifestyle in the comment section!

TAGS:  Aging, hormones

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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362 thoughts on “The Myriad of Benefits of Intermittent Fasting”

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  1. I suspect IF fasting combined with a paleo/LC diet is particularly helpful.

    Getting a lot of fat/protein will reduce satiety. Eating a HC ‘refeed” isn’t satisfying – you’ve got to eat plates of pasta to get the same feeling.

    So IF+paleo = less calories in.

    And there is something to be said for stomach shrinking. Is it real — or is it just brain triggers when it thinks you are full. I suspect the brain doesn’t know right away if you’ve stuffed your tummy with high-carb bread and pasta — or fat.

    1. Interesting point, re: stomach shrinking. I always assumed it had to do with insulin response overriding actual hunger cues, leading people to overeat carbs. You know, that woozy feeling after a huge plate of pancakes? But last night, I made some Primal almond flour muffins to accompany a soup, and what intrigued me was how, if I don’t eat anything remotely “bready,” my body will regulate itself and I literally cannot overeat, even if I wanted to. I feel so much more in control of my hunger cues. But with last night’s “muffins,” I felt I could go on eating them until I was uncomfortably full.
      Why would that be? Texture?

      1. I’ve always though the brain is quicker than your intestines. After all it would take 30-40 minutes for your stomach to clear.

        But the analogy is to artificial sweeteners – if your brain think it got sugar, it is going to do something.

        Carbs are low-density foods — you’d need to eat a lot of tubers as a small ancestral rodent to survive — the problem today is it is way to easy to eat them..baking them makes them taste a lot better.

        1. And the funniest thing is that people keep on saying “oh? these vegetables of yours will not sustain you for long!”…

          I think they do.
          And I don’t think their breads/pastas will either…

        2. maybe if your brain is relying on your tongue for all of it’s information…which it does not

      2. I am having the same problem of over eating when I make anything “bready”. I made almond flour biscuts last night and also could have devoured them all. Just had 2 of them with egg salad today for lunch (should have just had 1!) and now am feeling very sleepy, like I used to with carbs. I am thinking these “bready” foods are too concentrated for me and not really in the true spirit of Primal. I am going to fast now until tomorrow. I haven’t fasted in a while – thanks Mark for reminding us this is a good thing to keep doing!

        1. Check out the nutritional breakdown on almonds, they do contain carbs, if you are eating quite a large portion of these flour substitute foods you will be intaking more carbs and perhaps hence the carby after affect.

        2. If your feeling sleepy after you eat cooked food it is because u lack the enzymes in the food 2 break it down..ur food will sit in ur upper stomach 4 up 2 an hr before ur own enzymes kick in and start digesting the food…2 start the process of digesting cooked food immediately u need 2 take digestive enzymes and u will fell alot more energetic after u eat…u also dont allow the food 2 start rotting..try em

      3. Don’t feel woozy after a plate of pancakes but more like porn star Traci Lords in the film “Insatiable.”

      4. carbs. i’ve had to cut all grains AND nuts out of my diet. They’re not primal anyway…

        1. Nuts are too primal, read up on Marks books and other Paleo sites, grains and BEANs are not primal/Paleo.

      5. I think you feel this way because bread actually causes a certain chemical release in the brain that is highly addictive, similar to opiates. You may have heard about the study done on rats addicted to cocaine that were given the choice between coke and table sugar…guess what? They chose the sugar. Now bread has an even higher glycemic index than table sugar meaning it is converted into glucose in the body even faster giving you that instant chemical release and consequently that “woozy” feeling. This also means that bread is potentially more addictive than sugar and that is why when you eat bread “and when I eat bread too because I can definitely relate” you never really feel completely satisfied. This is why i excluded bread from my diet and I feel a wholw lot better.

      6. All those “bready” type foods cause serotonin to be released, which is basically a feel-good hormone. So the cycle goes like: eat bread > feel good > more please > eat bread > feel good…

    2. I have been doing IF in combination with paleolithic diet for about 2,5 years, and I highly recommend it to everyone.

    3. Yes, your stomach does shrink (and also get larger) according to how much you eat.

      When I wanted to put on some weight and bulk up, I would eat a bigger than usual dinner one night a week, followed the next day by a massive breakfast. The big dinner expands your stomach overnight, then you physically have more capacity to eat a larger breakfast the next morning.

      That’s a trick among endomorph body type bodybuilders who have trouble eating enough.

  2. I accidentally IFed for the first time this week, and felt great. No fog, no fatigue, no hunger.

    But it got me thinking…how does a morning coffee or tea fit in to IF? Is that technically breaking the fast?

    1. The consensus seems to be that coffee if anything can assist in maintaining a fast without derailing its benefits. Same goes for consuming small amounts of fats (butter, coconut oil, cream) during the fasting window. This stuff was mentioned in one of Robb Wolf’s podcasts I think.

      1. Just a quick note – Eat Stop Eat and LeanGains both follow a zero calorie fast for 24 (ESE) or 16 (LG) hours. This means no fat (all of which are high calorie)

        But, yes, coffee and tea are a godsend!

    2. I do IF and drink an Espresso/ Latte Brevea every morning and it has not hindered my results one bit.

  3. I don’t see how you guys fast like this. Back in high school I would do the 30 hour famine, but that was a lot of hard work and I carried around a gallon jug of water with me and drank that constantly. I’ve only been primal for about 2 months. I don’t have to eat nearly as much, but I’m still very hungry after 6-8 hours.

    1. When you did the 30 hour famine, your body was likely used to the SAD and nutirent starved. As soon as you stopped eating, your body wasn’t prepared to use fat stores for energy. Your hunger signaling would have been intense. Once primal, your body can use fat reserves easily and is no likely starved for actual nutrition. People eating primaly can easily do a mini-fast by accident because you are never experiencing that massive hunger.

      1. Not so for everyone. I will be 69 this year and my appetite is still going strong………………….

    2. Just finished my weekly 36 hour fast with a can of organic coconut milk and some organic cacao nibs for lunch and honestly didn’t want them – food just wasn’t that appealing to me. I am looking forward to dinner however.

    3. Go to and or google Ghrelin. This is the hormone that is responsible for your hunger, the reason you feel hungry when you 1st start IF is because it appears during your normal eating times, once you have adapted (doesn’t take too long) you will find you can fast without any real hunger

    4. I first tried IF when I was still eating a lot of carbs, and a low-fat diet. It was nearly impossible for me. Shaky, headaches, brain fog, so starving I could barely take it.

      Since primal/paleo eating, it’s totally different. Last week, I also “accidentally” did it, and felt great. No brain fog, no headache, fine with energy. The only thing I’ve found is that I sometimes can’t get to sleep when I do it, and it’s not hunger pains keeping me awake, it’s like my whole body is vibrating.

      1. I also have insomnia problems when I start an IF cycle. I seem to fall asleep fine, but wake up about 4-5 hours later and can’t go back to sleep. I wonder if it’s related.

        1. Try to IF by having your last meal in the evening (no later than say 6 pm) the day before.

          By the time you wake up next morning 12 or more hours have already passed. Then just skip breakfast. By the time lunch comes around you’ve made it to 18 hours!

        2. Insomnia is a side effect of being in ketosis. When you are fasting your body is breaking down fat into ketones and they are in your bloodstream being used as energy. I also have insomnia when I am fasting. Kind of sucks, but it’s worth it in the long run.

    5. its because ur stomach releases grehlin based on past training of meal patterns, u have to do IF every day to get the grehlin “entrained” to the new meal pattern so it will not be released until ur meal time, and ur daytime hunger will, for the most part, go away, your body learns your habits, its like the cars that have the transmissions that learn your driving habits and adjust the shift points accordingly, pretty cool stuff.

  4. IF was something I did naturally as a kid, before I knew anything of ‘diets’. I also stumbled back into it when doing low-carb thanks to ketosis and just plain getting my blood sugars and insulin levels in check. Having other IFers help to realize I could and should IF for health helped me ignore the advice to always eat breakfast, 6 times a day, blah blah blah. Then a genius reminded me that I always eat breakfast. We always eat breakfast, because eventually the fast has to end. I just eat breakfast at 2pm, instead of 8am. Instead of ‘skipping breakfast’, I generally sacrifice lunch or dinner. lol. Yeah, it’s a word game. It works most of the time for those people who still subscribe to CW. So, yes – breakfast is the most important meal of the day. When you eat that breakfast is up to you and your health and fitness.
    Thanks for this list. I’m sharing it with everyone.

  5. I’m doing the 16-off/8-on Leangains plan at the moment (and eating Primal during the 8-on part) as a way to get back into full Primality. It’s really easy, you just skip breakfast every day.

    1. I’m doing exactly the same thing. Started about 3 weeks ago and find that it’s easy. Saves lots of time in the morning too that I spend checking on the garden and getting a few extra minutes of sleep.

      “failure to eat something every few hours will cause mental fog and sluggishness, so keep a banana or a granola bar on your person at all times”.

      Hard to believe there’s still people who thinks that’s true. But I guess it is if you’re eating SAD…

      1. I can say if your metabolism is within a lightyear of normal, you won’t become sluggish fasting for a day or three. The human race would not be here if fasts for weeks was detrimental.

        Dr. Jason Fung has fasted people for over 300 days and has taken people who were on insulin for 27+ years totally off diabetic medication. I have currently be fat fasting for over 62 hours and feel great.

      1. I’ve been experimenting with the same, 16off/8on with primal eating. Its been great! I just cannot believe how NOT hungry I am. When I ate conventionally, I was always starving by lunchtime!

      2. Ditto, it is fricken brilliant. When I first tried IFing (that sounds dirty, lol) I decided to skip night meals for some reason, that didn’t work, my body doesn’t like sleeping on an empty stomach. However, waiting until later in the day to eat my first meal feels natural, as does working out fasted.

        1. I have better workouts when I haven’t eaten for a while than when eating on a regular schedule. I feel stronger, am more energized when I am done.

  6. The question is not IF but WHEN (When Hunger Ensues Naturally). That’s when I eat. 🙂 One of your best posts yet Mark (IMHO)! I usually eat within an 8-hour window beginning around noon and ending by about 7 or 8 pm. I am rarely hungry in the morning and I don’t often feel the need to snack between my two large meals (lunch & dinner). When I’m ill, or haven’t been getting good sleep, or am overly stressed, however, I do find I’m hungry in the morning and feel the need to nibble more throughout the day. When this happens I figure my body needs what it’s asking for and I oblige, knowing that once my sleep/stress/wellness are dialed back in the IF (or WHEN) will realign naturally. This has worked well for almost two years now and I maintain high levels of energy and alertness throughout the day and a nice level of leanness. I also do my strength or sprint workouts fasted (in the morning, usually a few hours prior to lunch). I may not live forever, but I sure feel like I will!

  7. I´m doing it Leangains style,
    fasting for 16-18 (or more) hours daily and once in a week I do a easy 24 hour fast. No problem with Primal eating (I include carb refeeds 2-3 times a week pwo). It feels so good to have actually control over your own body and I´m barely hungry within the fasts although doing a lot of sports (sports student writing).

    No, you can have your coffee and don´t have to worry about your fasting window, it won´t affect anything 😉

    1. I’m also doing Leangains and so far so good. Cutting out breakfast has made me more productive at work.
      I’ve been pushing most of fasts to 20hrs or more and then hammering food when I get home…hunger wise I’m fine, but I noticed that my brain starts to lag around the 18hr mark, making me unproductive. So at that time I go to the gym to workout or just walk which seems to clear my head.

  8. I wonder if anyone else shares my experience. I see quite a few primal types who say that they engage in frequent extended fasts almost without consideration, or that they eat a very small meal and it satisfies them for hours and hours. My experience is somewhat different.

    When I am on track and eating well (as now – going on 7 weeks!) I definitely notice that the effects of hunger are blunted. I no longer get overwhelming hunger, or headaches, or fatigue, from not eating regularly. Sometimes on weekends I’ll miss a meal because of being busy or not having access to good food, no big deal.

    But I definitely always *feel* hungry at mealtimes. I *want* to eat, and food sounds good. And when I eat, I eat my full portion. If I only eat a little, I will definitely be hungry in a couple hours. So it’s not as effortless for me to IF as for some people.

    Does anyone else feel this way too? I don’t have any weight problems or anything like that, this isn’t about that, more just a question of physiology and health. As far as adherence, I have been basically 100% since the beginning of the year; maybe I overindulge in nuts or full-fat dairy a bit. But nothing compared to carb-addicted SAD.

    I’m with Mark on this – I consider eating to be one of life’s great pleasures, so I am not at all eager to skip this little hilight of each day! But I am certainly intruigued by the health benefits of IF. It’s not that it’s hard so much as irritating, something I have to very deliberately and consciously do. Anyone else feel the same?

    1. What you’re experiencing is quite normal. Lunch is my first meal of the day (after about 16 hours sans food). I am hungry at lunchtime and typically eat a large amount. It’s my largest meal of the day. Don’t count calories, don’t worry about how much food you are eating (so long as it’s primal), and eat until you are satisfied and don’t want to eat anymore. That’s how I’ve been handling it for about two years now and it works very well for me.

      1. Hi Aaron. I naturally am inclined to follow the same eating pattern, always have been. However, following CW for decades, I forced myself to eat breakfast everyday and gained weight continually wondering how this could be until finding PB. I was wondering if you find you need any longer IFs or if the daily one is enough? It is working well for me, just wondering if it is enough? Oh and I am of Blaisdell lineage as well. Are you a member of the BFNA?

        1. I haven’t tested your question in myself, but I suspect there are diminishing returns of longer fasts if you typically eat in an 8-hour window. I think if I’ve had a bad food week (e.g., over the holidays), a few longer fasts (~24 hours) thrown in during the month do have added benefit of returning to my normal, primal leanness.

          I am not a member of the BFNA.

    2. Yeah, I’m the same way. I’ve been primal for ~9 months (with no weird carb “slips” or cravings or anything, no broken metabolism, LC or VLC by accident–not design–and no weight issues) but IF for me means being hungry. I notice the fast for sure, hunger-wise.

    3. I’ve experienced that as well. For me I narrowed it down to wanting an oral fixation or just plain “mouthertainment” while sitting at my desk at work. It’s just bordom. When I’m really busy, it doens’t enter my mind.
      Like today, I’m going to the pistol range right after work for an hour, then to get a massage. I ate a noon meal, but won’t eat my dinner ’till almost 8:00PM. I know from experience now that won’t be a problem.

    4. I find that I’m hungry throughout the time that I would have been eating breakfast. Half an hour later, even though I didn’t eat breakfast, I’m no more hungry than if I had. It’s as if my stomach gives up on getting any food and quits bugging me till lunchtime 🙂

    5. Good points about being busy – if I stop to think about it, that’s the key differentiator between not eating on the weekend (when I’m outside exercising or running errands or whatever) and the weekdays (when I’m standing at my desk all day with meals as a change of pace).

      Certainly glad to see I’m not the only one. And Liz, I’ve noticed that behavior sometimes as well, I dunno if it’s just associated with becoming busy or if it’s the effect you describe (taming the unruly stomach!)

    6. Hi Kris,

      I think the fact that you are not overweight is the explanation of your regular hunger. For the most of us, we have a small pocket of fat storage which we can use and dampens our hunger.
      An alternative explanation can be that if you know you are going to eat (socially) your body starts to prepare for it. I can have the same small cravings, but if I focus on something else I forget quickly that I wanted to eat some. The desire for food is completely gone.

  9. I feel great doing IF but I love to have my coffee with coconut oil every morning. Mark is this sabotaging my fast?

  10. I have been fasting for about the last month and a half. I eat dinner around 8 or 9 at night, and fast about 15 hours until noon the next day. I avoid eating breakfast except for a cup of black coffee with about 2 tbs of organic heavy whipping cream. By eating on this schedule with at least 8 hours of sleep each night, I have realized that my energy levels have increased significantly. I am increasing in body weight but, decreasing my body fat at the same based on visual results in the mirror in comparisons with pictures taken from the weeks before. I have also gained a better appreciation for food! I tell my friends that the things I look forward to the most in the day, is my workout, and eating!They all look at me crazy but, I am okay with it! I suggest that everyone give IF a try!

  11. Very interesting! I have been considering how I might include some IF. I have been mostly primal since April, and have vastly improved health, eliminated a 6 year struggle with severe Chronic Fatique Syndrome and arthritis pain. So I have been treating my body a bit delicately, feeling my way along, but now feel ready to advance to a higher grade. Love the way you read my mind Mark!

    1. Just to follow up with my post yesterday, I did my first fast today, about 19 hours. Even though I had a very light supper last night I wasn’t even slightly hungry this morning. I have been having my main meal in the morning, but realized that lately I am kind of pushing myself to eat it. So today I just had tea in the morning and didn’t eat until after 2 this afternoon. I had no problem, did not feel hungry, was alert and fully functional, but kinda sluggish by 2. Oddly, I was having more hip pain than usual (well I don’t usually have pain much anymore since going primal), and my belly was making some embarrassing noises. All in all it was easy, I try to listen to my body, and I think my body was definately ready for this. Now on to today’s subject: sex!

  12. i do 3 days (non-consecutive) fasting of 20-24 hrs. I only feel hungry around 5:00 pm but then i start moving slowly for errands and do sprint workout followed by climbing trees, jumping around, shadow boxing and 20-30 minutes of basketball or soccer. My runs are the fastest i ever get, my body is so light that workout becomes awsome. Overall less eat, more energy, lean body, responsive body..i love it.

  13. I’ve been struggling to lose five pounds or so for years. Involuntary fasting did it for me.

    I got a terrible case of the flu about two weeks ago. I was really sick for three days, during which time I had no appetite at all.

    I’m well now, and back at work, except for some coughing; but my appetite has stayed small for some reason, and I have lost the extra five pounds. During those days when I ate almost nothing, I discovered that it wouldn’t kill me, and now I eat smaller meals every day. It’s nice to finally be down to a healthier weight.

    I also think that eliminating nuts probably helped. I was eating a lot of them.

    1. Nuts derailed my typical primal two-meal-a-day eating for a few months last fall, so I cut them out completely and got back on the cart. Nuts are my nemesis.

      1. I’ve read glimpses of this “2-meal-a-day” eating in a few blogs recently. Is this “typical” primal? I find myself eating 3 meals and 1-2 snacks. Wondering if I’m not eating enough at my meals?

        1. It sounds like you could use to increase your eats at mealtime. It’s good to give your body a break from having to digest all the time. a Primal person would not have the liberty to eat that often…

        2. There is a good deal of variation across the community but I’d say that 2 meals a day is NOT typical; that most PBers don’t fast and that 3 meals a day is most common. I’ll be writing a post next week on who fasting is and isn’t for IMO.

  14. Splendid article! Of all the primal insights, IF has been one of my favorites. I practice 20-hour fasts almost every day. No breakfast or lunch for me — it just slows me down. I can’t bear the sluggish feeling of working out with food in my system anymore. When digestion turns off, so many other systems turn on, and the magic begins!

    I would only add two pointers for those trying to make IF work for them:

    1) When hunger becomes distracting, exercise instead. This is what I do at lunch time on fast days. It’s remarkable how 30-45 minutes of mild exercise makes the hunger go away, apparently by convincing the liver to release stored nutrients and optimize blood glucose.

    2) Get loads of sunlight, preferably at the same time as the exercise. There are probably lots of mechanisms at work here, but my theory is that Vitamin D is a primary factor in altering genetic expression to achieve the results Mark discusses above. Making Vitamin D with UV light is somehow much more effective and long-lasting than merely eating it.

    Sunlight and exercise are the best lunch of all! There’s no insulin spike and it keeps me feeling warm and fuzzy all day long. It might seem crazy before you try it, but just see how you feel afterwards.

    And when you do finally break your fast, the food just tastes amazing. Far from forsaking the pleasure of eating, you will find it amplified tremendously.

    1. I’m curious whether your single meal portions are “normal” or are they oversized?

  15. I’ve done it a few times, every occasion was an accident-like I was kept busy all day and didn’t get a chance to think about where my next Primal meal was coming from. Every time this happened I definitely felt light, fit, energetic, and clear minded. I’ve been a bit afraid to try working out in a fasted state BUT I feel like it’s in my near future.
    Right now I’ve had the flu for the past 24 hours and I haven’t eaten a thing. My body refuses to want to eat anything. There must be something to that, my body is fighting off a virus and the last thing it seems to want me to do is give it ‘fuel’. That really says something!
    Great post!

  16. I’ve found since cutting the carbs out of my diet, that I’m able to skip dinner without being hungry. I don’t always do this, it just depends on whether we’ve made something for dinner, or if I feel like eating. When I do this, I typically eat lunch around 2pm, and don’t eat again until 9 or 10am the next morning.

  17. I’ve been dipping my toe in the IF waters lately. Usually on a Monday. No breakfast or lunch then a light healthy dinner. The first week was brutal, but it’s gotten easier and honestly isn’t that bad. It’s actually gotten easier once I got my nutrition dialed-in as Rx’d here at The Apple.

  18. I did daily IF for awhile, eating all my food in an 8 hour window. I have sense switched to a more random approach which I like a little better. I definitely have seen lots of benefits from IF to where I don’t foresee not practicing it.

  19. My first fast was (a complete accident) and it was a noticeably effective fat loss tool. I was stranded on the road with a busted car in the middle of nowhere with hardly any money. I was a ball of stress and had zero appetite as a result, and didn’t eat (and barely slept) for at least 36 hours. I couldn’t believe how my already slim shape was even slimmer at the end of it.

    Unfortunately, I broke the fast with beers.

    1. Unfortunately? Can’t think of a better ending, honestly…

      (don’t spear me, Grok!)

  20. I started on the Primal journey 3.5 months ago and started IFing about 2 months ago. I try to be random with the IFs. Most of the time it means skipping dinner because I just don’t feel hungry at that point in my day. Most weeks I’ll IF all day Wed (from dinner Tues night until breakfast Thurs morning). A few times I’ve thrown in a Fri IF too.

    I find that I’m rarely hungry for breakfast on the IF days. I get a little stomach gurgly midday. Not so much hunger as rumbles. I find that those moments pass within 15-20 minutes. By the end of the day, I have more energy and I’m not even thinking about food anymore.

    I think the hardest part of IF is breaking free of societal habits and norms. Remembering it’s okay not to eat 3 square meals a day. Remembering you won’t starve to death on a 1-day fast. IFing on the days when you’ll have 100% control of your meals (not on holidays or social occasions).

    The best result of IF for me: breaking through weight loss plateaus. It always gives my body that little kick start to keep losing. Best reason #2: It’s teaching me that I’m in control of my eating habits and not the other way around!

    BTW – I always drink green and herbal teas on my IF days. Keeps the gurgles to a minimum, and I find it helps me get over the mid-day hunger phase.

  21. This is my first time to post. Thanks Mark for all the interesting articles and your book. I began my primal lifestyle about 7 months ago. I started at 256 and today am at 202. I have a very set eating pattern daily. 25gr of Whey Protein every morning, then 45 minutes at the gym. Moderate movement via treadmill 3 days a week, LHT twice per week. Do sprints outdoors on weekends. Lunches is salad with Tuna and a squeeze of lemon Mon. thru Fri. Every other week I do a 24 to 36 hour IF. Question: carb refeeding. I’ve had mixed results from this. I made the mistake the first refeed of eating a baked potatoe and a piece of cheese cake for desert. Sure was hard to get back on track mentally following that! Thanks Mark!

  22. I’ve been IF’ing for about a year now along with being Primal and it helped me drop 30lb’s to the lightest and leanest I’ve been since I was a kid!

    I typically follow Brad Pilon’s Eat Stop Eat protocol of 18-24 (+/-) once or twice a week. I’m trying to get rid of the last of my pooch, so I’ve been routinely doing twice.

    The first 2 or 3 times were a little rough, but since then I’ve had no problems. I actually enjoy a day or two off. It’s like a systemic do-over.

    IF-ing also helped re-introduce me to HUNGER and SATIETY; both of which had been foreign to me since early adolescence.

    I’m feeling MUCH better now…

  23. I’ve been doing the Eat Stop Eat method of 2 24 hour fasts per week for about 6 weeks now. Its been extremely easy to implement. I’ve been “fully” Primal since November 1st. I eat dinner one night and then don’t eat again til the next night so I’m only missing two meals. I can get a lot more done during the day not having to think about food and usually go for a walk at lunch rather than sit in the lunchroom here at work with everyone else. They all think I’m crazy anyway due to my WOE. Highly recommend IFing.

  24. I’ve been doing Eat Stop Eat 2x a week and the other 5 days, I go 15-16 hours between my dinner and my breakfast the next day. I have always worked out on an empty stomach. The benefit to me is when I’m not fasting, how easy it is to go for 4 or 5 hours without eating. You completely overcome that mental pang that tells you to eat every 4 hours. If you can make 15 or 24, hours what’s 4 or 6. I feel much better, and it’s surprisingly easy.

  25. The only thing I don’t like about this whole fat burning, fasting, thing is that I’m not hungry nearly as much. I’m sure it’s all very healthy but I like being hungry because then I can EAT 🙂

  26. I liked this blog. Since I started on the Primal Blueprint I’ve been eating between 5am and 5pm on the weekdays and between the hours of 8am to 5pm on the weekends. So I have a 12 fast every night through the week and 15 hour fast on both days of the weekend. Eating the Primal Blueprint way makes it easy to fast.

  27. I’ve been doing 15-16 hour fasts with caffeine in the morning and I think the excessive amount of caff I consume on an empty stomach may be triggering a stress response hormonally.

    Have I been dumping cortisol into my bloodstream? Yikes. So I’m backing off the caff a bit.

    If today is any indication, IF-ing works just fine without it!

  28. I’ve been experimenting with IF for the past few weeks (mainly when I’m too lazy to pack lunch and don’t have access to good food) and I’ve noticed a myriad benefits. Good post, it’s inspired me to keep it up.

    Also, props for using one of my favorite words, myriad, correctly! woo!

  29. I’m sure this has been said a LOT before, but for people with a history of ED, IFing can be very “dangerous”.

    There is an element of semantics in this. I suppose I “IF” daily (16+ hours) because I eat dinner early-ish and don’t eat again til about 1pm the next day. BUT I do NOT call it IF. I just don’t eat because I am not hungry or because I cannot eat before I work out. If I called it IF even to myself, i think I could reawaken behaviours that I am happy to say being primal is helping me overcome. Perfectionism, the urge to “cut things out”, “if some is good – more is better” (including fasting).
    I see a lot of people on the forums talking about IF – quite a few seem to me to be wanting to severely calorie restrict, to not eat even if very hungry, to accelerate weight loss way beyond safe/reasonable amounts. IF can also raise the spectre of “control” issues and the possibility of “failure” with accompanying problems.

    So, IMO, f IF works for you naturally or with little effort, great. History of ED? Be VERY careful and really look at your motivation and monitor your behaviour.

    1. Thank you for commenting on this, as its something I’ve noticed as well. I feel that even new PBers now feel pressured (and are encouraged) to IF even if it could be harmful rather than beneficial in their individual cases. There’s almost a competitive undertone on the forums… Which I dislike.

      I find that skipping a meal every once in a while helps to reset my hunger cues, but if I do it too often or start over thinking it, it brings up the whole ED nightmare again. Everyone simply needs to listen to their own body and be mindful of what works for them…

    2. Thanks Denise! Eating Primally has freed me for the first time in 3 decades from the old behaviors of my ED. It started when I was 6 and though I was “cured” at 28, those obsessive behaviors have lingered. I find it much easier mentally to focus on eating when I’m hungry, eating till I’m full and not focusing on how long it’s been between meals. I can’t call it IF and I certainly would never do a planned fast for fear of overdoing:”If 16 hours is good, I’ll do 20, it’ll be better”

    3. I agree totally! As a person with a history of ED I don’t think that I could tell myself that “fasting” was good for me. I think that I would end up taking it too far. Sometimes I don’t eat a meal; typically lunch, but it’s because I am on the go and not because I am telling myself not to eat.

      1. While I agree with the warning of treading lightly with IF if you have a history of EDs, for me this way of eating has helped mine tremendously. I find that I’ve learned to be in touch with my hunger and eat when I’m truly hungry vs. every 3 hours, or at noon because it’s lunch time or breakfast because it sets the metabolic state for the entire day, etc.

        While I still have a great deal of weight to lose, I found almost immediately that eating this way has really set me free from a life-long struggle from eating emotionally and not listening to my body’s own hunger signals.

  30. Whenever I try to figure out if something makes sense I don’t listen so much to what the “experts” say now but I think to myself, okay is it realistic of humans throughout time? For example, the common opinion that we need three meals a day. Throughout history did humans wake up in the morning and have food immediately available, and then a few hours later, more food, and then a few hours later more food…and the answer is no. I don’t think we need the amount of food we are programmed to believe we do. And to those who say but I’m hungry every six hours…hunger doesn’t mean we have to give into it. We only give into it because food is readily available. Think about other needs we have. Let’s say you are at the office or in class and you suddenly find yourself very horny. Do you just go pound one out or call someone down there right away? No, you wait until it’s the right time for it. So we are capable of ignoring or postponing the signals our bodies send us until it’s the right time to listen to them.

    1. HAHA! That’s hysterical! But so true! I have been asking myself that same question of “does it make sense from an evolutionary perspective?” a lot lately. It’s funny how many of the paleo eaters I know are still stuck on the 5-6 meals per day eating plan.

  31. My husband and I used to fast often when we were both carb eaters and it was always very difficult. Since I went totally primal 6 months ago the last time I fasted i had to actually remind myself that i needed to start eating again about 30 hours into it. I love tha now if I have to work late and don’t have any food with me for dinner I am not running all over the floor looking for something to eat because i’m “starving” and when i find my self just missing dinner and working late I tend to just let it flow into an IF until lunch or dinner the next day! Great post.

  32. My experience is IF just happens. Since I reduced carb intake and cut out grains, I fill up on protein and vegetables and don’t have the “gimme something more to eat” cravings like I used to. If I slip up and get some grains (am pretty sure I’ll be able to not do the 20% of the 80/20 Primal thing soon) all bets are off. I firmly believe this has to do with my body getting cranky that it is getting ‘filler’ (hmmm, but it doesn’t actually fill me up), rather than satiating protein and filling vegetables. Just MHO.

  33. Monday thru Friday I go 22 to 23 hours in a fasted state and have a 1 or 2 hour window, usually around 6 pm….to eat a very large and satisfying primal meal. Then on the weekends I simply eat (primal) when I feel like it. It works great for me. Energy is high and it is extremely liberating, NOT having to think about ‘when’ I’m eating next. Plus, by changing it up on the weekends, your body has to adapt and fat burning is even more intensified!! Fasting is water only.

  34. I have to say, I did the whole Ramadan fasting thing in Arizona last summer. No food or water from sunrise to sundown for a month. If it weren’t for the paleo diet, I probably wouldn’t have pulled it off. Long summer days and 100F plus weather. For some reason, once I switched to paleo, I could last easily without getting as thirsty or tired during the day.

    1. Ramadan fasting is not really fasting, except for the no water during the day.

      From 6 AM to 6 PM is 12 hours just a normal dinner to breakfast break. Not unusual even in baseline American culture.

      And Muslims probably celebrate with big meals after sundown and before sunrise.

  35. Since going primal last November I’ve found I don’t have to closely monitoring when I last ate before working out. I’ve dabbled in IF but only on a meal-by-meal basis but have been contemplating expanding it to a full day now and then, possibly moving to one day a week down the road

  36. At work today someone said they had to fast for a blood test and that they wouldn’t be able to eat until 9:30am (after the test). He said how awful this was going to be and how he might faint form a lack of calories! I then said that I regularly fasted for 16-20hrs, at which point there were deep sighs and looks of pity from my colleagues. I quite enjoy fasting and find that exercising while fasted is great. I’ve started following Mark’s recent advice to try fasting after exercise as well as beforehand and didn’t feel any affects when I tried it last weekend. I guess us “crazy cavemen and cavewomen” forget what it’s like to be at the mercy of a granola bar every few hours! Grok on guys you’re all fab!!

  37. After being primal for about a year with good results (loss of 35 lb, increased health and athletic performance), I had reached what I thought was a good baseline of fitness. However I started reading about IF in the fall, and in January started experimenting with it a bit.
    Right now I’m doing a 16-18 hour fast 3 times a week (Sunday night to Monday, Tuesday night to Wednesday, and Thursday night to Friday). I have definitely become leaner while maintaining strength and mass (I have lost an inch off my waist, but only dropped 2-3 lbs). I haven’t experienced any unexpected fatigue or loss of energy when working out either. This stuff really seems to work!

  38. My wife and I have been doing fasts every Monday, which makes for a really productive start to the week. We usually do some strength training as well on that day. I personally feel like crap if I try a met-con style workout while fasted.

  39. I’m pretty skinny and am trying to gain weight (follow Paleo diet and can’t handle dairy)… don’t think fasting is for me. It feels like I’m always fasting since being Paleo. Am I doing something wrong? I have a high metabolism and am constantly playing catch-up with the amount of carbs and fats I’m eating (trying to balance it with protein). Any tips?

    1. Cara, I’m certainly not an expert but you may be one of those lucky girls who can get away with more a bit more carbs and a few more meals.

      Go enjoy a sweet potato!


      1. I have sweet potatoes 2-3x a week if not more! that’s the thing, i’m eating starchy tubers. there’s a point where you’re too skinny and i might be there… it’s uncomfortable when people think you have an eating disorder. anyway, a few tweaks here and there should do it. i’m eating fatty meats (just had pork belly tonight, delicious). thanks for your tips!

        1. Don’t stress on skinny. As long as you feel good so what. Some guys, like myself, are very attracted to skinny
          women. Best wishes.

        2. how often do you work out? some strength trainging excercises should help. If you don’t have much body fat builing muscle will help you appear a little bigger and healthier 🙂
          I deal with the same thing, so I can relate. Thanks for the post.
          ps, i make my non primal husband lots of almond flour treats, which adds to fat.

    2. I expect some others could give you some more particular thoughts,Cara, but assuming you eat a good amount of protein, how about upping your fats? How long have you been eating low carb? If not long, it might take some adjustment as your metabolism shifts to fat-burning mode. Just help it along with more fats. Slap butter on stuff 😉

      Also, I think fermented foods are great.I don’t know how much they feature in Paleo, because while I know the basics of low carb, I’m not sure on specifics. But things like sauerkraut. And I drink milk kefir as well (tho Paleo is non-dairy, is that right?) I thought of that when you mentioned not being able to do dairy, because I know from the protocol of the GAPS diet that these sort of lactose intolerance-type things aren’t an issue with kefir, in that the cultures eat the lactose…and further, the big benefits to the digestive tract can heal intolerance issues gradually as its heals the gut lining and improves digestion in general. That can’t be a bad thing to consider when you want to gain muscle.

      Hope this isn’t out of place…I’m posting somewhat in ignorance of the details of Paleo, even tho I eat pretty close to it myself, lol.

      1. Ah, “Primal” as contrasted with Paleo(?) does some dairy, I gather.

        1. I wish I could eat primal, but notice side effects from cheese… that’s the only difference for me. I eat butter and handle that ok but no milk, cheese, yogurt.

        2. Cara – have you tried Raw Milk? Assuming you can find any in your neck of the woods… Google: Eat

      2. thank you! this is really helpful. i love fermented foods, crave them in fact, so perhaps my body’s telling me to go for it. am big on kombucha but will try kefir. sorry to stray from topic but i might be speaking for some people who feel they’re too thin eating this way!

        thanks guys, i appreciate it. great post mark!

        1. It’s a good topic to bring up! My husband is very naturally lean and he doesn’t feel good without a lot of carbs, but it’s really difficult to get a lot of carbs just from starchy tubers. He’d have to eat them all day and that’s difficult and unappealing for him.

          He’s had better results following a more “perfect health diet” route and including white rice to up his starch intake. If you’re not sensitive to rice, that might be something to consider.

  40. Great post. I have recently gotten into the idea of IF, though I have found that, like stated above, it kind of just happens on its own. There was a post about a month ago that said, “Today, you’re going to skip your next meal. I took this as a personal challenge from Mark, and never looked back. I usually don’t eat till around lunch, and usually don’t even notice it. It works wonders in shedding unwanted body fat, while really clearing my head. If you haven’t tried it yet, well, what are you waiting for.

  41. I fast on Mondays (no food between Sunday night and Tuesday morning), but I always have trouble sleeping on Monday nights. Anyone else experience this? Perhaps the problem is that I do drink some coffee (no more than other days) which may be more potent without food in my system?

  42. Ironically I just started IFing last week. I do 2 24 hour fasts. One on Mondays and one on Thursdays (I stop eating at around 6-7 the night before and then start eating again the next evening at 6-7). I was a little scared at first that I’d be overly hungry, but after going Primal a year ago it’s really not that bad. It was actually quite easy.
    At this point in my life I really don’t care about losing weight (I went from 208 last year down to 175 due to going Primal, and that was in a matter of 3 months without even working out). I care more about lowering my body fat%. My goal is to completely get rid of my post college beer gut. 🙂

  43. Eating protein, fat and low carb vegetable ONLY when one is hungery will most likely result in reduced calories compared with what most people eat. That would result in ‘calorie restriction’ without trying.

  44. I fast on a daily basis. Actually we all fast…what I do is I do not eat my first meal of the day, or my fast-breaking meal, until 3:00. Sometimes I eat later. Eating at 3:00 opens up my 4 hour “feasting” window. When I eat later than 3:00, I eat just one meal for the day. The only reason eating 3 -4 square meals a day plus in-between snacks seem necessary is that is what we’ve been taught. I am pre-diabetic and my blood glucose levels rarely go above 100. Now that being said, my glucose levels are usually in the 80s. I am not athletic but I do walk regularly while I am in my fasting mode. This is something that I do at least one hour before I break my fast. I am planning to run in a 5K dash just to see if it can be done while fasting. I am willing to wager that it can be.

    1. That sounds like a great idea all around. I might have to give it a shot – I’m quite a foodie I’m afraid, but the problem is that I can be fairly mindless about it, even being primal. Having a time window seems like a good idea.

  45. I can definitely relate to the increased mental clarity upon fasting. I find that after about six to nine hours of IF I get way more alert and rise into a state of much more relaxed awareness. Only thing is I tend to binge hard on the next actual meal I eat.

  46. don’t we all kind of IF anyway? if we eat dinner/last snack at 8.00, then eat breakfast next am at 8.00, that’s 12 hours right there. Not that far from 16 hours IF? what is so speacial with 16 hours vs 12?

  47. I eat breakfast and lunch and IF until breakfast again. From reading these comments (and others on IFing) everyone skips breakfast and eats lunch/dinner. Is there a reason that this is the pattern and not breakfast/lunch skipping dinner?

    1. I would venture to guess it’s that most people like to go to bed on a full stomach. That’s why for me anyway.

    2. i am wondering the same thing. is it really considered fasting as your sleeping?
      i skip breakfast most days, but i never considered myself an IF.

      i also have a question…i was talking with a nonprimal coworker about this and she mention some about your body going into “starvation” mode when you skip meals. I realize this is CW but I’m just wondering what your thoughts are on this and why its made to sound as a bad thing.

  48. I’m recovering from an eating disorder. I like the idea of IF but whenever I do it my eating disorder seems to re-ignite and be fueled by my hunger (which I can’t stop). I’m lean 13.5% BF 31 yrold female. Is there any way I could incorporate IFing into my healing process?

  49. I have done and still do Dr. Johnson’s Alternate day with 600 cals one day and my regular 1800 the next especially when I have inflammation issues and the diet helps with the pain and swelling. My lower back pain improved, too. It’s not a total fast but has helped me and if I am experiencing a plateau it helps break it,too. I still have 125 to lose(already lost 212) so I can afford the fasting some days.

  50. FINALLY!!!! It feels so good to have what I’ve always known validated!!People have been scolding me for years for skipping breakfast! They insist one must eat within the first hour of rising in order to “get your metabolism going”. I began skipping breakfast when I was in high school just because I wasn’t hungry in the mornings (my Mom allowed it because she was the same way). Then, a couple years ago, I started eating first thing in the morning, and continuing with the “6 small meals”. I have to say, I felt sluggish, gained weight, and have been depressed. I’m back to eating MY way (no breakfast), and have much more energy and sleep better at night.
    Back in my 20s I fasted every Monday as part of my weight-loss program. It was the best I’ve ever felt!! I’m about to try it again. Thanks for a great post!

  51. geez mark, I was so interested in this article I burnt the almonds I was roasting.

    How dare you!

    On a serious note, WHY is IF good for you? These tell me why to do it, but not exactly why it works.

    Is it, fundamentally, autophagy? If so I’d love to read an article on that specifically

  52. I see no other way to live.
    Eat like a starved animal once a day, enjoy it thoroughly and stay full until the very next day.
    Not to mention weight loss is a breeze.

    1. Are you fasting every day? Are you eating just dinner and then, dinner again the next day? You say weight loss is a breeze. Can you be more specific about what kind of IF you are following (# of hours and eating window). Thanks.

  53. Great post, Mark! I’ve been doing IF 16/8 per the leangains approach as well as throwing in one to two 24 hour fasts a week. I’ve only seen benefits and have never had problems with energy or clarity. It also helps me stay lean and I love setting myself up to be able to listen to my body when eating to find the right satiety level. I’ve always had trouble trusting myself in how much I eat and IF has really helped that. I do miss breakfast sometimes, though. I don’t break my fast until lunch at the earliest. IF is great and I recommend giving it a try!!

  54. Just love reading these posts, Mark!

    Call me a crazy caveman or cavewoman dieter if you want… but this stuff really works. I used to be one of those crazy SAD people who constantly had to have a piece of fruit or a protein bar handy… When I didn’t eat every couple of hours, I would get the shakes, couldn’t concentrate, etc.

    Did my first 16 hour IF about 3 weeks in learning to be primal. Didn’t feel any ill effects so I decided to try a 24 hour fast this week. I ate lunch, then didn’t eat until lunch the next day. I never really felt hungry until about 23 hours in. The salad I ate never tasted so good!

    Loving the primal life!

  55. It is good to read that so many people have luck with IFing. I have tried to psych myself up for IFing but I keep psyching myself out. I also used to do it naturally before college, I was just too busy to eat.

    However now that I have made the switch to primal I feel preoccupied by food trying to make the “right” choices throughout the day.

    I am excited to give it a shot. I think I might just need to “let it happen,” to get over my mental block.

    Great encouragement to read all the positive experiences!

  56. Have been IFing for 12-16 hours easily, but am now thinking of trying longer IFs to help with torso fat. Thanks for the great info Mark, as always.

    I, too, have sworn off nuts, hoping this will help with the body fat.

    Sun is shining…..rare for here, so must get outside, even if it is cold.

  57. I’ve been fasting according to the Leangains protocol for 2 months with some very favourable results. I’ve dropped a few kilograms, have started to lean out rather nicely all while gaining lean muscle. You’re right, Mark, fasting just happens now and I’m no longer bothered when in the fasted state but do enjoy the 8 hour feeding window 🙂

  58. I just started planned IFing at the beginning of last week. Prior to that, I’d accidentally IF’d several times. Last week and this week on the days when I work out in a group setting, I have skipped breakfast, resulting in a 15-16 hour fast. I feel slightly hungry right at the time I would usually eat breakfast, but then that feeling would go away till lunchtime. My workouts are at least as effective as they were before. I feel lighter, I don’t need to drink water all the time. It’s rather good.

    For years, I joked that my son seemed to play soccer better if he hadn’t eaten anything that day. I had no idea it might actually be true!!

  59. I do not necessarily notice a smaller appetite after going primal, I’ll basically clear my plate regardless of how much is on there. If I make a whole lot of food, I’ll finish every bite and be satisfied. If I just make a light salad with a hard boiled egg, still satisfied. If I fast for a day or two I def. feel it and get hungry, but in a good way -not debilitated by hunger pains.

  60. I eat however much my body feels it needs at the moment.(in reasonable way)Going hungry on purpose dosent come across as very bright,so your going to tell your body theres a famine,so your matabolism slows down.Yes ive heard of this,folks from life extension magazine are into calorie reduction.each to his own.

    1. Your body is not stupid enough to think that going less than a day without food is a “famine”, Unless your insulin response is totally out of whack from SAD. If you eat low-carb, going without food for even 24 hours at a time is pretty effortless.

    2. I’m 44 hours fasted right now (experimenting to see what it is like and to kind of hit the reset button). I’ve seen it other places but Leangains notes that your metabolism goes up initially while fasting and doesn’t start to drop till around 5 days later.

  61. As a menopausal woman I’m finding that IF is good medicine to lose weight and reduce inflammation.

    I prefer 24-hour fasts but occasionally add a couple of boiled eggs or very small LC snack, keeping it under 500 calories. Usually I get through the fasting period OK but if I do it more than twice a week I need to watch that I don’t eat too much on the eating days. I also drink coffee with cream on the fasting days but try not to overdo it so I don’t get too buzzy.

    It’s the only thing that’s helped me lose weight and really helps with arthritis and other inflammation-related problems.

    There are challenges though:

    – I do have problems with concentration and focus sometimes on the fasting days. On days where I need to get a lot of work out I find it’s best to not schedule a fast. (Going VLC or doing IF is great for my body but sometimes affects concentration and mental sharpness. I need to tweak when needed, I often need to find a balance.)

    – I also need to stay LC on the eating days and watch that I don’t psychologically feel a need to eat a lot more because I fasted the day before.

    – I also find that the longer fasting period (18 to 24 hours) works better for me than just not eating most of the day then having a large dinner, which sometimes makes me feel sluggish and bloated.

    When I was younger I felt lousy and spacey if I skipped meals and didn’t eat throughout the day; this is much easier for me to handle now at midlife.

    Also, even going VLC didn’t result in weight loss; the alternate-day fasting has helped me lose weight again and feel much better which is such a boost at midlife when (quote from one of my favorite TV shows) it feels like I have the metabolism of “a hibernating bear in menopause.”

    One thing I also pay attention to is drinking mineral-rich herbal teas (as hot infusions, letting them steep longer than regular teas) like dandelion leaf, nettle, etc. A lot of people don’t pay attention to this but you get a lot of minerals that way, especially important for hormonal changes.

    1. I am also a menopausal woman and would be interested in hearing more about the mineral-rich herbal teas you mentioned. Are there any that are supposed to specifically ease hot flashes? I’ve been experiencing flashes for about 6 weeks now, almost always in the evenings (between 7-11pm), and they’re driving me (and my husband) just a little bit crazy.

  62. IFing really works for me too. Breakfast is generally my favorite meal of the day so on my IF days I usually have lunch as my last meal around noon and then enjoy a wonderful primal breakfast at around 9 or 10 the next morning…I do this usually 1-2 times per week. Most weeks I also do a 24 hour fast as well, Mondays work best for me…great way to start the week!

  63. Awesome. Been doing Martin’s Leangains IF for well over a year and a half. Loving it.

    I’ve tried longer fasts (36-48 hours) and those are even better. I’m still trying to get my family on the fasting bandwagon, but it’s a pretty difficult task when they aren’t health geeks like me.

  64. Intermittent fasting is great I do it from time to time but just don’t get so caught up in it, it becomes the answer to all weight problems. I see people becomes so fanstatical they start fasting during the day every day.I mean everybody loves to eat.

    Bodybuilders who are ‘shredded’ you see every minute detail of their muscles and they eat 6 times a day, the longest living folk in an island in Japan don’t fast! … so I.F. is a simple way for most of us to eat less but you can also do it a number of other ways ..its another tool to use.

    1. Have you seen bodybuilders in the off-season? They are not shredded, they are covered with a nice layer of blubber. They get shredded by fasting and using diuretics and other drugs. They eat 6 times a day precisely because they know they are going to lose muscle mass while leaning out pre-contest.

      As for the Okinawans, I’m guessing that traditionally, like most people, they did IF. Farming the old fashioned way is a lot of work–just based on the fact that you have to work sun up to sun down as a traditional farmer (and that is what most Okinawans did), you’re going to not eat most of the day. Also, all those old Okinawans did go through a lot of extended fasting during WWII. I mean if you can show us that Okinawans typically eat a bunch of small meals a day, I’d love to see evidence of it.

      On a related note, I found Mark’s comment about Muslims during Ramadan particularly interesting–I remember years ago when Hakeem Olajuwon was playing in the NBA and winning championships, he would go on these tears during Ramadan. And people would be amazed he was playing so well when he hadn’t eaten in 16 hours or so. But there he was, fasting and clearly the best conditioned athlete on the court.

  65. I’m new to the primal movement, but not new to IF. I’ve used IF in religious settings and more recently for the health benefits. Overall, my experience has been nothing but positive. The benefit I enjoy the most is the awareness I’ve gained. That first meal to “break the fast” is a great time to eat mindfully and really feel the food’s effect on your body!

    I’m glad to have found a community of folks who are on a similar path!

  66. I’m a huge advocate for IF and lean gains in particular.
    I had great results with it prior to Xmas last year (results on my website).
    It’s something that’s EASY to stick with and combining that with stellar results and the ability to eat awesome meals that may in fact be bad for you on a regular basis, equals a WIN for me.

  67. What an awesome and timely article! My question is…what constitutes “fasting”? I’ll skip dinner every now and then and feel great the next morning. Reading this article and some of the linked resources, it seems that fasting means not eating for extended periods of time – as in L-O-N-G! I once fasted for 36 hours for a colonoscopy and thought I was gonna die. Maybe I’ll try a 24 hour fast and see how I handle it. It seems the health benefits are certain.

    1. In Eat Stop Eat, Polin claims that the research shows the majority of benefits from fasting come between the 16th and 32nd hours of the fast. As such, your fast should probably be at least 16 hours and no longer than 32 hours, with 24 hours right in the middle.

      Personally, I fast from dinner to lunch ( at least 16 hours) pretty much every day, but take a dinner to dinner fast (about 24 hours) once a week. I don’t plan the 24 hour fast; I just go for it if I’m not feeling hungry around 3 pm or so.

      Once you’re used to 16 hour fasts and eating a high fat diet, 24 hour fasts will happen almost by accident.

  68. This is simply an excellent summary. I find IF is easy after only 1 or 2 days of being hungry during times you normally eat.

  69. I fast 5-6 days a week (leangains style so 16 hours fast followed by 8 hour eating window).
    My fiance even does it now (but does 14/8 due to being female ;)).

    We find it VERY easy (may take a week or so to get used to however)

  70. “Leangains” has some great info on Ghrelin (sp?) which I believe is the hormone that tells us we’re hungry just because it’s 7am and that’s when we usually eat.

    FWIW – I’ve been IF-ing for about a month now and after an initial period of adaptation I actually feel like my energy levels are more consistent with eating less often.

    FWIW II – I think one of the biggest advantages I’ve seen is in my digestion. No more cramping/bloating. It’s almost like my guts need the break each day.

    I do the 18/6 Leangains approach (shout-out to Lean Saloon as well). if you want to read some of my ramblings…

  71. I am new to Primal eating, I began about 6 weeks ago. I note the following changes so far: I am far more satisfied with the food I am eating, and consquently am eating less food with no increased hunger or cravings. I always work out for 20 min. every morning on an empty stomach, after at least 8 hours of (far better since going Primal) sleep. After working out I often don’t want breakfast for about an hour, with no ill effects whatsoever. I think that IF will come very naturally to me given more time eating Primally. I now feel that I can “hear” what my body needs and wants without all the “static” of conventional eating habits. I have given up eating unless I am hungry, and I eat when I want to rather than 3 X a day. Thank you Mark for your generosity of Knowledge and Spirit. I have learned so much. Choosing to eat Primally feels intuitively right for me, and has changed my life for the better.

  72. Are the benefits of IF the fact that the body has to dip into its fat stores for energy and this creates some kind of favourable biological environment for anti-ageing and other benefits OR when people IF are they limiting calories as in caloric restriction? I’ve heard some say they eat the same amount of calories they normally would in a day but in a shortened window. Do most get less calories when doing IF?

  73. Amazing post! Love all the research links!

    I do an intermittent fast with a 4-6 hour eating window, usually 5 hours. (I don’t stress out on the exact times as long as it’s in that range.) I eat about 80% primal.

    And I do high intensity workouts with weights every 4-6 days.

    For the last two months, my workouts have occurred 3 hours after the eating window has closed and I don’t eat again until the window starts again, as normal.

    In that time I have added 6 lbs of body weight, obviously lost bodyfat unless the mirror lies, and added 1.5 inches to my arms, chest, and legs.

    So all the panic over eating within an hour of a workout that you see or you’ll minimize your gains, I don’t buy it. Some people seem to almost go into a panic about it.

    Your body can store amino acids and use them when needed. Otherwise we’d never have survived as a species. Otherwise, vegetarians couldn’t survive. It can also wait until new food comes in the next day.

    Besides which, I’m just not hungry immediately after a workout and feel a little sick if I workout with food in my stomach.

  74. I haven’t tried IFing yet. I plan to in the future but I’m still cleaning up some of my diet. Up until a year ago, I was eating six meals a day for about 10 years. I’m almost at 3 meals a day. The six meals a day were during my high carb days. It’s nice to to be away from the mental fog.

  75. Mark or somebody, what about fasting with adrenal fatigue? Is this absolutely contraindicated as most “experts” suggest?

    1. Stephanie, some people (many?) have incurred some pretty severe metabolic damage over the years, adrenal fatigue being one manifestation of that damage. For people who are coming off serious health issues, it might be best to acclimate to the PB style for a longer period of time (longer than three weeks, say). Then start to experiment with shorter IF periods until you are confident you can handle longer fasts.

      1. Thanks so much for your reply. I actually tried my first planned IF last Saturday, but only for about sixteen or seventeen hours from Friday dinner to a late Sat. lunch. I felt a little bad (shaky) Sat. morning, but had wonderful energy Sat. night. Will keep experimenting. Just being able to go 5-6 hours without a meal or skip lunch is amazing for me. I used to get a little crazy if I missed a meal 🙂 Primal is awesome and is changing my life. Thank you for your body of work which is greatly helping my body 🙂

      2. Mark, Thanks for your response. That’s sort of what I did. I’ve been primal since late November and stopped snacking and skipped a meal here and there with ok results. I have fasted once for 17 hours and today I broke my fast after about 23. i did feel a little shaky at times but I’m curious whether this will help or hinder. I’m going to keep experimenting and see what happens. It seems like it gets easier for me over the duration of the fast and I’ve had some really productive days these last two weekends which coincided with the fasts. Hope to be a success story soon! Thanks for everything!

  76. I agree Mark. When you are eating primal and getting a good ratio of healthy fats to proteins to carbs fasting seems to just happen.

    When you are eating a primal diet you just don’t have the same cravings that force you to eat often and eat regularly.

  77. I do LeanGains 16/8 fasting (from the comments it appears I can no longer consider myself a special snowflake), sometimes up to 20 hrs. It’s the best way to take advantage of my college meal plan; I have fewer weekly meals than average, so I just eat twice a day, usually ~12:30 PM and then again around 6 or 7.

  78. Is eating clean protein (no fat or carbs) a bad idea when IFing? I’m thinking not whey protein, do to the insulin response, but why not canned tuna (packed in brine)? I hate the thought of losing precious muscle tissue if I’m not eating anything, especially protein. I’m a hardgainer, so I had to bust my hump to put on this muscle and I aim to keep it! 😉 Any thoughts on this?

    1. I do IF and I eat the same number of calories as I would if the meals were spread over 18 hours instead of 6. Assuming you’re doing that style, I don’t see how you would have a caloric difference.

      If you have bodyfat and you’re working out and building muscle, your body will burn fat instead of protein on IF. Your body prioritizes. If you aren’t using your muscles, it will burn them. That’s how I understand the process at least.

      1. Right, I’m not worried about calories as your body can burn body fat to get all the energy it needs to keep you going, but your body uses protein to build and maintain muscle, right? So where does the protein come from when IFing? The only source is muscle, isn’t it? If that’s true then IFing would eat muscle much like catabolic hormones, which would suck. Please correct me if my understanding is off here.

    2. Jarrett, the whole idea IS to spare muscle (and increase repair) through short fasts. This is not an issue for those who have become fat- and keto-adapted by eating according to the PB. That’s that three week adaptation/acclimation period. OTOH, it can be a big problem for those who are still living in the carbohydrate paradigm (like the 80-10-10 crowd)and whose muscles expect to be fed more glucose every 3-5 hours.

      1. Thanks for the reply and writing this interesting article, Mark 🙂

  79. The study presented as showing that IF “even helped cocaine addicts stick to their treatment” is actually about mice, not men.

  80. I’ve been CRON – Calorie Restriction with Optimal Nutrition (AND primal) for about 3 years (recently participated in CRONA research study at UCSF to help establish CR/anti-aging biomarkers).
    CR and Primal go hand in hand in my book – so not sure why the misunderstandings (on both sides of the table). A great majority of CRers focus on calorically dense real food, low glycemic carbs (and incorporate the various methods of IFing discussed here). Perhaps its the muscle mass issue ( and the impact of protein on IGF-1), as in CR you are trying to down regulate the mTOR pathway (and in muscle tissue at least), you are trying to up regulate it to build muscles.
    At 5″9′ BMI of 19.5 and under 15% body fat (very fit/low fat for a female+ over 50). I’m also an RD and would say CR and Primal are far more alike than different. I just advise my clients how to fine tune to emphasize anti-aging aspects – but for sure weight loss has to come first before you can worry about tweaking/optimizing further. Won’t get my telomere length data for another 10 months when the study concludes…..will know more then!

    1. deb b, I appreciate your input and comments. Of course, I’m listening through my own biased filters when I hear tales of measuring food by the gram at CR meals or of the mind games involved in overcoming hunger in between. I suspect that a higher fat, low carb CR program might alleviate some of that (much of the CR diet lit still has Conventional % breakdowns of Fat/Pro/Cho). Please let us know when your telomere data comes back.

  81. I tried IF 3 times a couple years ago but experienced some pain in the area of my gallbladder each time at around the 16-18 hour mark. At that time, I didn’t know a shorter fast was okay and was going for 24 hours, so felt discouraged. I will try to do a more strict PB for 3 weeks and then try it again. Thanks for this information.

  82. Yes, fasting just feels right. I eat all my calories between noon and 6pm. I feel great!

    I don’t tell anyone though because they roll their eyes and start giving me grief.

    1. Yeah what is that. My co-workers will ask if I want something for lunch and I’ll say no their next reply is you’re going to get sick and develop stomach cancer. I just laugh because 20 minutes after their lunch break they are good for nothing nodding off at the computer when the bossman comes around checking while I’m alert, energetic and with a clear mind.

  83. what about the 4 hour body cheat day? anyone doing that and IF?

    1. I am. I lengthen my eating period to about 8 hours and stuff myself. Most of my splurge is fruit and rice. Perhaps a bit of bread.

      The primary reason I’m doing it, along with the reasons stated in Tim’s book, is because it fits my social calendar. I typically go out with friends, visit people, do shopping, and so forth on Saturday. So it is generally hard on that day to maintain as strict a diet and fit my eating hours into the times when my friends are eating.

    2. You’re talking about a one day carb refeed, right? From my experience it can make me feel bloated on that day, but the next day my muscles have a nice fuller look and I feel somewhat stronger than than earlier in the week. I do 24-hour modified IF fasting once a week with lots of protein (whey shakes and canned tuna in brine).

  84. I have fasted a few times before and was able to pass with flying colors. The longest I have done is about 21 hours… not sure why I did not get to 24!

    It has been months since my last fast but I frequently eat 2 meals on the weekend.

    I have considered doing a planned 24 hour fast once a week or every other week but have not gotten the urge to do so.

    Fasting is very beneficial and if one can handle it then doing it while traveling may save someone – great post Mark!

  85. All this stuff about the IF sounds good and I’ll give it a try tomorrow morning before working out (although for a hardgainer like me it sounds like suicidal… I don’t really care), but let’s say I take my vitamins and fish oil in the morning with the breakfast: if I’m on IF should I pospone them too? do they count as a source of energy?

    1. I’m thinking taking vitamins would be a good idea here, although they usually say to take vitamins after a meal. I wonder if the body would readily absorb the vitamins on an empty stomach. 1 gram of fish oil is about 10 calories, similar to chewing gum, so I don’t see why you’d forgo it.

      1. I know right? I mean I understand that IF won’t get screwed up because of those ten calories in the fish oil, but since I’m not really into this I wanted to know if this kind of supplements should be avoided too (I think I didn’t read it explained in this post or the previous ones about IF)

        1. Well, if you want any benefits of the said Fish Oil, you’ll need to be ingesting more than 1 gram of it in one sitting. I personally take 8 caps a day.
          An aside from that, calorie consumption higher than this would throw you out of a ‘fasted’ state into a ‘fed’ state therefore you’d be foregoing any of the benefits you get from an actual fast and effectively ending your fast very early in the day.
          Save your fish oil + vitamins to have with your first meal when you break the fast.

    2. Thank you for your responses guys: now I understand that IF is both about nutrient and calorie restriction for a certain period of time.

  86. Know quite a few people that practice IF and we want to try. My only experience with fasting is 72 hours of organic apples and water which is a magical experience. Awful day one. Tolerable day two. Radiant day 3. Just ease back in after, because the temptation to binge after a fast is awful!

  87. Hear, Hear! I’ve been following the Primal Blueprint in conjunction with Martin’s Leangains protocol. I eat primal, but only during a feeding window from 1pm to 9pm. It’s clearly accelerated fat loss, but I can’t believe the difference it has made mentally. I had a host of stress/anxiety issues before starting the daily 16 hour IF, but there is something about the predictable and rather austere nature of the 16 hours of fasting a day that has set things right upstairs. I love it. I don’t feel hungry and I’ve been setting PRs on the big barbell movements and performing well during 1-2 hour grappling sessions, all in a fasted state – with the addition of BCAAs before and after fasted workouts). I love it, and don’t think I’ll be going back to the old (new?) way of eating any time soon.

  88. I also only IF accidentally. I miss about two meals a week. I’ll have to pay closer attention to the mental aspect of it, as I haven’t really noticed it being tied to exceptional clear-thinking states, but it might be a contributing factor.

    About 15 years ago I didn’t eat for eight days just to see what would happen. After the second day it wasn’t particularly difficult, and not all that profound either. I broke the fast with ribs at a friend’s party. If I’d known of the concept, I could have just stayed Primal after that.

  89. This post reminded me that I did something today, and several times before now, that would have been impossible a few months ago. Today, I stumbled into a mini-fast without trying.

    Before, if I didn’t eat breakfast, I’d get the shakes and pretty well shut down. Now, after about 3 months of Primal, I just kept putting off breakfast because I was busy. Then, I put it off a bit more, I was still busy. I never would have been able to do that before. And when I did eat a light lunch, I was totally satisfied with what would not amount to much.

    Just awesome how our bodies, attitudes, and hungers adjust. Love it. Thanks Mark.

  90. I’ve done a couple of IF days since starting to eat primal. My biggest problem seems to be finding a day to do it when I’m not training too much. I can do it on a light exercise day, but not a heavy one or I just get too hungry.

    I know it’s maybe not really in accordance with a primal exercise plan, but I’m putting in some multi-hour days training for my black belt in karate. Training about 15 – 17 hours a week right now (most karate, some Crossfit-style conditioning, a couple swimming and trail running). So finding a day that I can even do IF is difficult. Most days I’m ravenously hungry.

    Today was only a one-hour day as I missed my afternoon session. I haven’t eaten dinner and I’m really not hungry. I can tell a real difference in appetite between light workout days and tough ones. Yesterday was four hours and not eating was simply not optional.

  91. Great post Mark. Back when I was taking herbal classes with Dr. Rochard Schultze, he would always talk about “when you are sick, stop eating”. Examples of animals fasting in a sick state etc. would follow. He would quote…”60 % of your energy is used to digest food. Stop eating and you now have 60% MORE energy to fight the bug.” Anyone have the exact figure of how much energy we use to digest food?

    I tell my diabetes clients, “No one got diabetes from eating too little.” Nice to not feel like I needed to snack while reading your post. “STEP away from the food…Step away from the food!”

  92. I have done the Warrior Diet (if you’re familiar with it) for one year, several years ago back in college.

    Essentially, I fasted every day from 18-20 hours, worked out and ate (it was a lot of grains back then)

    Two things I’ve noticed that may help you:

    1. Athletic performance was actually improved when fasting.

    2. “hunger” was purely behavioural. I noticed that the feeling of hunger that I would get were circumstantially psychological. If it was morning, or if I ended class, or if it was the same time I ate all the time before.

    It takes some mental work to get to the point where you don’t feel “hungry” for upwards of 20 hours. Some people went cold turkey. Some would portion their intake and wean themselves off food.

    IF is great accept for the psychological stress it can provide.

  93. Mark,
    While you link to TED above, I cannot find any CR talks via that link. Can you please provide the link to the talk(s) you were referencing?

      1. FYI – I’ve never heard that Aubrey practices CR, altho his associate at SENS, Michael Rae does, and is very thin.

  94. Great article, Mark!!

    I always wondered why I never felt as bad as what my parents said I would whenever I accidentally went a whole day without eating. In fact, I felt great! Then, as usual, I proceeded to end the day with a million-ingredient salad and glass of wine.

    Now, I definitely don’t feel bad about accidentally not eating at all! I’m still fairly new to primal living – just a few months now, but I definitely noticed that sometimes I accidentally don’t, or “forget,” to eat for a whole day more frequently, about every week and a half to two weeks.

    I’d really like to make this a regular thing now. It makes a lot of sense and seems like a logical component to primal living.

  95. So, about a month ago, I tried to do my first fast… I was going to do a green juice fast cause I am nursing and thought I may not be able to go without calories for a whole day. So I ate my last meal at 6pm and then started the fast, intending to through the next day. The best morning I frank some of l the fresh made green juice and all he’ll broke loose. I started throwing up immediately, and then by lunch, went blind in my left eye and got my first migraine ever! What happened?! Was it the juice?! Also I am nursing, should I not do this while nursing? Gould I try again with nothing but water?

    1. If you have calories from green juice, it is not the sort of fast being discussed here. I’ve done a juice fast before. Incredibly difficult. Made me hungrier than a straight fast and I felt terrible. Some people say that’s detoxing. I’ve done without and didn’t feel that way.

      In my experience, a one day fast is a lot harder to do than an intermittent fast with an eating window. You should start by eating in an 6-8 hour window and go from there.

      Caloric intake should remain the same on an intermittent fast, you’re just shifting the hours of eating to a smaller window to give your body more time to go through its “cleansing” processes.

      The migraine isn’t necessarily connected to the fast if it only happens once, but personally, I’d be careful. From an evolutionary/biological standpoint, I don’t see how it would impact nursing. However, I am not a doctor and don’t for sure.

      If you’re going to do it, start small. If you’re not going to cut out the calories during the fast time, then I just don’t think it’s really a fast or worth doing.

      You might also be allergic to one of the juices.

      1. Most women store some fat during pregnancy for making milk, but it takes 500-1000 calories a day to make milk. This can come from food and body fat. Hence why exclusive breastfeeding is a great way to lose post pregnancy weight.i lost aLot of weight postpartum. Just make sure you don’t restrict calories too much… You need enough to maintain yourself and milk production.

    2. This doesn’t sound like it’s related to fasting. You said that the effects started right after you drank the juice in the morning. That’s not even a fast.

      I fasted on Yom Kippur while nursing and found dehydration affected my milk supply by the late afternoon. I don’t think it was the lack of food because I didn’t really get that hungry, although I don’t know for sure.

  96. I’ve been very strictly primal for 6 months. I have tried IF a couple times, with the idea that I would eat once I was hungry. The longest I’ve ever made it was 16 hours with a post-workout meal breaking my fast.

    Am I doing something wrong? I’m not overweight, so I’m not doing this to lose weight (male, 5’9″ 140, body fat probably around 8-9%). Is my weight/body composition such that I just can’t go as long without getting hungry? While I don’t want to lose weight, I’m hoping to reap some of the other benefits mentioned in this article.

    While this is much better than before going primal (when I would need to eat every 3-4 hours and turned into a total asshole after 5 hours of fasting) and now I can do a fasting workout, something I couldn’t do before w/out feeling drained, I kind of wonder about all the people commenting on this site who say they can comfortably go 24-36 hours w/out eating. I can’t imagine myself doing this.

    Again, am I doing something wrong?

  97. I naturally have been a faster and didn’t really realize it. I rarely eat breakfast and, due to the nature of my work, often do not get a lunch; usually when I do, I take it super late–like at 4 or 5 PM. I’ve loved going Primal because I seem to feel better during these forced fasting periods; I have plenty of energy, feel light and alert–quick, even–and don’t overdo the next meal. I really think my work performance has improved and fasting might be part of it. I have so far not fasted on a plan or with intention, but I am going to try to see if I can achieve a longer fast more often because I do trust the benefits described above and at Martin’s lean gains site. I am excited to see if I can induce my body to want to fast longer or more often. I am also going to keep a mental note of my eating times. I think the 8 hour window for eating works well for me and I am going to try to stick to that… maybe tweak it a bit to decrease it, even…

    As always, Mark, THANK YOU for such thoughtful blog topics. I love your site and this community!

  98. I’ve been tip-toeing slowly into a more paleo lifestyle — I’d say I’m at 50% — and already regular eating is becoming less important. As a college student I tend to wind up keeping irregular hours and if I’ve got a bunch of things to do one day, it’s nice to know that there isn’t much of a difference if I happen to miss a few meals. There’s also an odd feeling when hunger kicks in that I’m walking on air and can focus better on work. (And though this blog is not about religion, I can’t resist noting that Judaism, Christianity, Islam, Hinduism, and Buddhism all value fasting.)

  99. Personally, I find this continual reliance on abbreviations tedious, irritating, pedantic, and lazy. Write the bloody word for heaven’s sake (fhs).

    1. Agreed – it can make posts difficult to read and smacks of pseudo-science which you don’t need!

    2. It doesn’t usually bother me except that “intermittent fast” abbreviates to “IF,” which is an actual word (and a commonly used one at that), so it made this post hard to read.

    3. I mean no offense, but I think, by definition, you’re being pedantic.

  100. I find intermittent fasting pretty easy. Eating a relatively primal, high fat, low carb diet keeps one so sated that it only makes sense to miss breakfast or lunch once or twice a week and fit in some more fun or functional exercise. I quite like it.

    Oh, new funcional wow, loading and stacking pea straw bales. An hour of that and you know you’ve done something for your body and got a job done.

    1. Martin Berkhan says more carb on workout days and more fat on rest days.

      Coconut oil is your friend.

  101. Primal now for over a year and even at age 59 I’m now recomposing nicely instead of decomposing!

    People who haven’t seen me for a while invariably ask about my ‘reconstruction’… I put them off by saying “Four’Bs’: No Bread. No Beans. No Beer. No Breakfast”

    (But I don’t tell them about the weekly deadlifts or the lovely fat!)

  102. I am a 57-yr-old woman, one of 4 sisters. 1 of my sisters, age 56, has never been on a diet of any kind and has the same body she had in high school. The other 3 of us have tried every diet in the book and have all suffered from excess poundage for most of our lives. My weight-stable sister? She practiced IF even back in her teen years. It came naturally to her. She was NOT trying to lose weight, as she didn’t need to. But she would say, “I ate a lot for dinner last night and I’m just not hungry today. Besides, I’m going out with friends tomorrow, so I’ll eat plenty again then…..” Very casual. “Just not hungry….” Looking back and what she’d eaten and ahead at what she planned to eat, and then figuring there was no need to stuff herself in the absence of hunger. Smart girl then, and smart to this day.

  103. Just two months ago I could barely last four hours without eating, then hypoglycemia would set in (irritability, dizziness, nausea, cold sweats, etc.). After switching to virtually 100% primal it’s reached the point where I can easily go 8+ hours fasting without any trouble at all, even with exercising. Skipping a meal every few days now and it feels good.

  104. It sounds from Mark’s post that IF involves/leads to CR. I can easily eat my maintenance calorie level during my feeding window of 5-6 hours, so will this prevent desired weight loss? I have been following the Fast 5 protocol for a week. Still waiting to see the pounds move. I have been PB for a year. Should I be cutting calories too?

  105. I started IF leangains style (~16/~8) a month ago and it gave me terrible gas. It’s not as bad now but I still fart more than usual (and they smell more).
    Anyone else?

    1. yes i get quite gassy as well. they don’t stink or anything but that has a lot to do with what i ate the day before. if it was a weekend with wings and draught beer well…
      in general i eat as much dark greens in the morning and evening will be something low carb.

  106. I’m going to send this post to my brother who believes that he will drop down dead if he dosn’t eat a large (and I mean large) breakfast immediatley on arising in the morning. He tuts and shakes his head if any one tells him they don’t do breakfast.

    Great post.

  107. Hi everyone!
    Thanks for all the great comments, it’s been wonderful to read! One reply which I hope will settle a few people, is to remember that everyone’s bodies are very different, so try not to compare yourself to those you think are deal tasters! You just may not be programmer like them, and that’s totally fine :).
    I have a quick question I’m hoping someone can help me with; with the nature of many people’s work and social lives these days (mine included), I find it very hard to fast when I’m actually not hungry (when my body is telling me to fast) rather than planning a day (not ideal if my body is screaming for calories). I can definitely relate to having a day where I was just not hungry, and the last time this happened I was scheduled for a couple “meal socials”.
    Thanks everyone 🙂

    1. Uh, I feel your pain!
      From June to October I used to do a 20-hour fast once or twice per week.
      It was extremelly convenient to my daily routine (as I pack homemade lunch and take it to work), allowed to save money on groceries and, most important of everything, helped me to identify and control the so infamous “pseudo hunger” sensation.

      But in November I started working at an small enterprise with rigidly scheduled breaks… and the IF habit was broken. My actual cowokers are totally “common wisdom” and going out for a walk during the lunch break isn’t an option (industrial zone, large busy roads, can’t drive to anywhere else, etc.).

      Anyway, one only has to be flexible and to find a proper alternative plan, after all… there’s no excuse!
      So, I plan to fast during Saturday and maybe skip dinner (or breakfast) a couple of days during the week.

      As a ending note, for me, the most difficult part actually is to start IFing (or re-start). But once one gets used to it, it’s great, indeed.

  108. I am so motivated by all of the comments and discussion I am going to try my first fast this weekend!

  109. Primal is totally flawed because meat is highly acidic, if you want to increase your risk of diseases later in life, eat the primal way!

  110. Grok did not find much…if anything at all to eat today..OH well.
    We try again tomorrow..THATS PRIMAL/PALEO.
    I recommend you try this..It is an integral part of a primal lifestyle.
    Little or nothing days!!
    Daveman GROK ON >>>

  111. Uh, I feel your pain!
    From June to October I used to do a 20-hour fast once or twice per week.
    It was extremelly convenient to my daily routine (as I pack homemade lunch and take it to work), allowed to save money on groceries and, most important of everything, helped me to identify and control the so infamous “pseudo hunger” sensation.

    But in November I started working at an small enteprise with a rigidly scheduled breaks, and the habit was broken. My actual cowokers are totally “common wisdom” and go out for a walk during lunch break isn’t an option (industrial zone, large busy roads and I can’t drive to anywhere else).

    I want to get back to the old habit, though. One only has to be flexible and to find a proper alternative plan, there’s no excuse.
    So, I plan to fast during at Saturday and maybe skip dinner (or breakfast) a couple of days of the week.
    The most difficult part is to start IFing (or re-start); once one gets used to it, it’s great.

  112. The biggest issue I’ve encountered with fasting or in general not eating at the times I usually do is pain from previous meals. For instance if I have greek yogurt with orange juice for breakfast if I don’t eat lunch or some other food I get really acidic pain in my stomach and have to eat something.

    I’m aware that this is likely because the yogurt is acidic (i think? or is it base) and my mother always chided that I should eat something WITH yogurt, but there’s nothing I really can eat – no fresh fruit available to me right now =(

    1. If anyone replies, do it to this second comment please! Forgot to hit email notifications…

      1. I’ve heard that yogurt can cause problems for some people with acid reflux. It’s PH is usually below 5, yet for some reason certain people just can’t handle it. The orange juice you’re drinking is acidic though, and if you are drinking juice that you didn’t make yourself then there’s also probably a bunch of sugar in it. I would say to break your fast with something like a fat spinach, bacon and egg omelet and see how you feel afterwards. Also, if you incorporate a more structured regimen, such as fasting for 16 hours, then getting your daily allotment of calories within an 8 hour period, it will entrain your hormones better than sporadic meal skipping. Your body will fall into a rhythm, making your fasts much easier to handle.

        1. I do not have acid reflux, afaik. I wish I had spinach, bacon and eggs.. I’m at a school in the middle of no where with no grocery stores easily accessible and my meal plan already paid for – the food they offer is disgusting and even the meat dishes make me sick. The salad is all wilted too :/ I graduate soon so here’s to hoping I can land near a good grocery store and get my eating habits back on track.

      2. 2 things to consider:

        1) Drinking orange juice with yogurt will cause the yogurt to curdle. Not a good combination. Like a breakfast Cement Mixer. This could invariably cause stomach pain later in the day.
        2) Orange juice is liquid calories (and sugar)

        1. Both good points. I’ll keep that in mind. I’m not Primal right now, but it’s something I’ll be moving towards this summer when I graduate from college.

  113. My question is, if you’re looking to build a little lean muscle, can you do it with IF? Or is muscle maintenance the most you can expect?

    1. Yes, you can build muscle mass while fasting. I eat a somewhat modified version of the primal diet and I also fast on about two days out of every week. I’ve managed to maintain and improve my gym performance while doing this.

    2. Check out leangains. Putting on muscle isn’t a problem while using an IF program.

  114. Hi,
    I love the idea of IF and low carb but I have a question. I take maca root in the mornings because it is suppose to be a natural hormone regulator and it gives me a natural energy which I need since I work late. Does this affect the fasting period. I hope not.

  115. Leangains is amazing. The amount of fat loss I’ve experienced paired with the amount of muscle built is beyond words. I’m lighter than I’ve ever been, lifting heavier than I ever have, all while feeling absolutely unstoppable. Thanks Martin Berkhan! And thank you Mark for this post, great stuff!

  116. As a relative newbie to primal eating I’ve noticed a huge change in my hunger levels. I was diagnosed in high school as hypoglycemic and throughout my early adult years I couldn’t go more than 3 hours without eating and DEFINITELY never skip breakfast or I would get shaky. When I discovered this site and started eating low carb I realized that I just wasn’t hungry in the mornings and will often delay eating until 10:00 or later. Not sure if that is considered IF (I typically eat dinner at 7:30 PM).

    My body is starting to drop weight (yeah!) and I can go for hours between meals and often eat only twice a day. I’m not ready to do a full 24 hour fast yet (years of experiencing negative hypoglycemic reactions have me a bit scared) but I suspect that eventually as my body heals I’ll be able to do it naturally.

  117. I accidentally did IF when I was 16, I was working long shifts and only ate one meal a day. At the time I also decided to become a vegetarian. I lost 20kg in 5-8 weeks. I thought the weight loss was from the vego diet and all the walking so I stuck with the diet and slowly over 3 years gained back the weight once I started eating 3 meals a day again. I found it hard to keep the weight off when I was only eating plants and carbs. And found the more meals I ate the hungrier I got..

    This year I’m back on the IF eating plan and eating Primal I’m finding im more sated and I love only eating once or twice a day as I tend to get tired after eating and cant concentrate as well! no more afternoon crashes for me. 🙂

    I also found that if I don’t IF my weight loss stalls- no matter how many calories I eat, but as soon as I delay one or two meals a day I loose an average of 2kg a week… I have no idea why! But maybe I just wasn’t meant to eat a load of small meals throughout the day!

  118. I can honestly say that IF has allowed me to have much better control of the food I consume. I am no longer a slave to food….

  119. I like Mark’s approach of not planning for IF, but just letting it happen when need be.. which is the same as what Robb Wolf does..

    I was just listening to one of Robb’s older podcasts (I’m assuming he hasn’t changed his stance on this?), and he was saying that it’d be way more beneficial for people to get their diet and sleep dialed in first.. that people are too keen to try IF as some sort of shortcut to better results, when they’re not yet fully implementing the most important stuff..

    he also said when your body’s stressed (whose isn’t?), IF may not be beneficial.. and that if you do partake, it’s better to keep IF to a max of 18 hours (to not start digging into protein stores)..

    seemed like pretty sound advice…

  120. Only found this site last week and think it’s great. Just wanted to chip in my thoughts on fasting. I come from a background in Russian Martial Arts (Systema) which is not only a self defence system, but also a health system, so practitioners use techniques such as controlled deep breathing, dousing outside with freezing cold water all year around and fasting. The fasts are actually more extreme than here, often for up to 60 hours without food and water. I’ve done a couple of 60 hour fasts but personally I only exclude water for the first 24. I definitely felt clearer in the head and more relaxed. The idea with these longer fasts is to give the digestive system a rest and let the body use that energy to heal itself (the digestive process uses a lot of energy). During the longer fast the idea is that the body will break down the stuff it doesn’t need first for energy, such as damaged tissue, and therefore can help in clearing disease. I’m not totally convinced about this part, that the body can selectively choose between healthy and damaged tissue, but I remain open to the idea and have definitely felt the benefits myself. In systema the total dry fast(i.e no water) is also stressed to be very important, the idea being that the body begins to make it’s own water out of stored fats.

  121. One noticeable side effect for me (just started IF a couple weeks ago): I started waking up early (5:00 or 5:30 AM) without an alarm, just awake. I am already an AM person, but its like ZING, totally awake before the sun comes up….

  122. I like the mental clarity that comes with a 16 hour IF, and I like the feeling that I don’t have to eat in the morning unless I want to. Obviously, if you can be as strong as Martin Berkhan is using IF, it’s not the kiss of death for performance training. I like how you pack these posts so densely with info Mark, thanks.

  123. I follow IF leangains style and my post workout meal contains about 250 grams of carbohydrates. I eat kiddie cereal, low fat ice cream, pretzels, etc.

    My primary goal for the other 90% of my eating is that I consume lots of fruits and veggies.

    My philosophy of food is not so much “don’t eat this or that” but rather “make sure you eat this and that”.

    This and that being dark skinned fruits and dark green veggies.

  124. am new to this, still in the process of reading the book and am wondering if diet coke is permitted on the pb way of living. i know it is not good for me, but am trying to adjust everything else and not ready to give it up. does it impact the fat loss?

    1. I’m sorry you’re having this struggle. I am one of the fortunate ones who did not grow up on soft drinks so never had to cut them out; my Mom even made no sugar ice cream for us. I only know that artificial sweeteners are completely and totally bad for you, and you say you know this. Some people even say artifical sweeteners act like appetite stimulators; in that regard it probably DOES impact fat loss. Splenda was never thoroughly tested so I’m thinking it’s a no as well. Besides, splenda is slightly higher, that’s right higher, on the glycemic index than sugar so is a non-starter for getting insulin responses normal. I think that’s why they advertise the no-calorie aspect rather than the good for diabetes idea.

      All this to say, while I don’t have personal experience in letting go of soft drinks, I empathize as I also grew up on homemade bread. Wow, is it hard to smell baking bread and not WANT some.

      Keep working on it. Best wishes


  126. I’m here to say that IF regardless of diet will shrink your stomach and change your psychology. When I lost my initial 60 pounds I was eating once a day and probably just 1200 or so calories. It wasn’t on purpose, I just had an overnight job and things worked out that way. I felt great…invincible even.

    Once I resumed a normal schedule and wanted to eat 3 times a day again, I found I had ZERO desire to eat if I wasn’t truly hungry, I had no cravings, and I absolutely couldn’t eat more than two handfuls of food without feeling very “stuffed”.

    My diet was usually a late night double cheeseburger, burrito, or some sort of deli sandwich. :p

    I’ve kept the weight off (2 years) and I’m eating much better now (paleo)….

  127. Dave (and Mark…see all his info on Grok’s life), you are absolutely RIGHT ON!!!! Intermittent Fasting is just a fancy-pants way to say GROK-ON! My husband still gets nervous when I’m not hungry and simply don’t eat. I think mainly because it is a huge change from before primal. I was constantly nibbling, grazing, complaining about being hungry. With primal, …well you know the rest.

  128. Thanks so much for this. My husband and I read this last night and I decided on the spot that my 9:00 p.m. dinner would be my last meal for 16 hours. Since adapting a Paleo diet 2 months ago, I’ve found that I’ve been doing a lot of emotional eating … eating when I think I’m supposed to, not necessarily hungry. I think this is going to be a great reset for me .. a way to get in tune with my body and natural hunger cues. 20 minutes to go and I feel great (but I’m definitely looking forward to my beef stew and veggies). Thanks!

  129. I’ve done fasting Leangains style for about 3 months and it’s awesome! Paired it with the MaxFit olympic lifting program as I felt beat down when trying to metcon consistently. Still eat Paleo but with a lot more starch, mainly post workout. I’d been Paleo/CrossFit for about 3 years and had plateud, feeling beat down and slow. This kick started everything he talked about in the article. I’d been having really bad cortisol response to exercise, but I sleep a lot better now and can handle volume again. IF definitely helps clean you out, sharpen you up, and strip off stubborn body fat. My girlfriend is trying a similar set up and loves it to. I can’t say enough about IF.

  130. Enjoy If too as many have stated.
    I am noticing a pattern, for myself. When I IF, eat a meal, and follow with lifting heavy a few hours later, I am not as strong.

  131. If it works …do it. your life…ALIVE! Heres a dose of DAVEMAN strength and determination to ALL who wish to be ALIVE!
    GROK ON>>>

  132. I’ve never been a “breakfast person”, and have regularly “forgotten” to eat for whole days at a time – Just. Not. Hungry. Nice to hear, for a change, that there’s not something fundamentally wrong with me and that I haven’t been setting my body up for early, dismal decay! LOVE you blog, Mark, and all of the amazing info supplied by your listeners via their comments. Thank You all for being regular voices in my life.

    1. Just wanted to say, I did the fast 5 program for awhile, had some success, then found it was just too dismal. I am not a monk.It required you to eat during one 5 hour window every day. But it didn’t have any recommendations as to what you should eat. So it wasn’t intermittent and it wasn’t primal. It was more Zen.I have always found it easier to not eat at all than to count calories or weigh food so fasting appeals to me. I am just starting the program. It sounds like a good fit.

  133. I am a 36 y/o MALE that was diagnosed with Breast Cancer on 11/17/10. I am starting chemo this Friday 2/25/11. I have considered fasting leading up to my treatment day. My only problem is that I have to steroid pills the day before, day of and day after my treatment. I have been told these are pretty hard on the stomach. I am just considering just doing some fasts and then eating very low carb when I have to take the pills. Anyone have any ideas? Thoughts?

    1. I am so sorry you are faced with this, maybe these references will help –

      I have the full text of this article that I can email you: dbfact at comcast dot net.

      Here is link to slide from TED presentation: foods that prevent angiogenesis: (great lecture to listen to)

      Also, focus on eating cruciferous vegetables, esp. watercress.

      If you were having surgery, the “peak” of the cancer cells that may get disrupted and “stick” to the blood vessel walls and get hold/start growing is up to day 3, so I would fast post-surgery – but yours is a little different with just the chemo. I think you are on the right track to look to optimize nutrition and restrict eating.

    2. My cautious impulse would be to refer you to your own doctor for advice on this, but your presence here might mean that you are inclined to reject conventional wisdom. I can’t give you advice on this, other than to wish you the best. I’d say listen to others who’ve gone through similar experiences and listen to your doctor — in the end such a personal decision is in your own hands, though.

    3. Chemo will take it out of you..
      I have had a few run ins with this in my family…luckily not me.
      I would eat first…after chemo…no one wants to eat much.

    4. My meat supplier (he now does grass-fed beef) had stage 3 prostate cancer and healed it with a strict primal diet.

      He used to be one of the government farmers, that grow corn and other plants with chemicals year after year.
      Not only is his cancer completely gone, he is now one of the biggest grass-farmers (beef, chicken and pork) here in southern Idaho. Of course, he is doing all this on new, untouched land.

  134. And don’t forget the posts on light, sleep, and jet lag—(back ????). I frequently travel AU to Ohio- a 12 hour time shift- and I think fasting really does help re-set my internal clock.

  135. Clearly you haven’t read Twilight…Bella and Edward have LOTS to do with the rest of eternity.

    Course, they also subsist on deer blood.

  136. Be wary of IF. You do NOT get the benefits of calorie restriction with it. See:,200834,200838#msg-200838 for references.

    For that matter, recent research warns that IF may damage the heart. See:,204377,204431

    >So far, these studies seem to show that EOD is of no special benefit in the long term — but have not implied any deleterious effect, either. Surprisingly, Mattson’s most recent study ((3) — published, appropriately, in the Journal of Cardiac Failure) finds that EOD in rats actually /damages/ the heart:

    >”Four-month-old male Sprague-Dawley rats were started on ADF or continued on ad libitum diets and followed for 6 months … [A]t the end of the study, and hearts were harvested for histological assessment. The 6-month-long ADF diet resulted in a 9% reduction (P < .01) of cardiomyocyte diameter and 3-fold increase in interstitial myocardial fibrosis. Left ventricular chamber size was not affected by ADF and ejection fraction was not reduced, but left atrial diameter was increased 16%, and the ratio of early (E) and late atrial (A) waves, in Doppler-measured mitral flow was reduced (P “CONCLUSION: Chronic ADF in rats results in development of diastolic dysfunction with diminished cardiac reserve. ADF is a novel and unique experimental model of diet-induced diastolic dysfunction. [!] The deleterious effect of ADF in rats suggests that additional studies of ADF effects on cardiovascular functions in humans are warranted.”

  137. When my forced IF happened I felt like I was on a super high.
    Outragious mental clarity, focused and energetic.
    I think it was a 19 hour fast.
    Now I wish I could do it again but unless forced upon me I can’t say no to that frozen half cow in my freezer, boiled egg or big salad.

    There are definitely many benefits to IF.

  138. Also:

    I’m to the point now (after 1 year of eating primal) that I don’t even realize when I haven’t eaten.
    My husband and I get up in the morning, go grocery shopping and do other things…and it’s 3 pm by the time we get home.
    Sometimes that’s a 14 hour or more fast that I didn’t know I was doing.

    “Ah, it’s 3pm…hmm… maybe I should eat breakfast.” =P

  139. I’ve been doing IF for about a week now (took the weekend off, so I guess about 6-7 days total).

    I have coffee in the AM, but dont consume anything else until dinner.

    Then I just eat a normal-sized dinner.

    How do I feel?

    I have TONS more energy, Im thinking clearer, I feel freedom since I don’t from “haveto eat every few hours”, and I don’t feel guilty at night if I eat a few sweets along with my otherwise healthy meal.

  140. I’m really happy to have stumbled onto this topic. Ever since I was in high school, I would inadvertently fast after so many days of “regular” eating. It came naturally so I guess I consider myself lucky. I wake up in the morning. Have some water and find myself not eating until 8:00pm. Other days when I fast, I wake up, have a couple of eggs with broccoli and then not eat until late that evening. I did this even before I went primal. I fasted by accident and my friends would tell me how bad it was. But when I fasted, it was because I literally didn’t feel the need for food or I was too busy to eat and didn’t feel hungry. I feel it’s a mind game. If our ancestors didn’t have a good hunt, they knew they’d have to go with what they had. I feel it’s very mental.
    My body just felt like it needed a cleanse. So I just, didn’t eat. The results are seriously remarkable. I would have many bowel movements (sorry!), I would hydrate more than usual, my body just felt like a million bucks! Then, I would eat a great meal after this respite, and swear i could literally feel my body taking in all those vitamins and yummy ‘good for you foods.’ I can’t say enough good things about fasting. We’re a society of plenty but I honestly think our bodies sometimes, need a break to process all of that 🙂

  141. I’ve only recently discovered that this is how I’ve been eating almost my whole life, and most of my adult life. I didn’t know there was a word for it, though. I just don’t eat if I’m not hungry. I cannot believe this is a big deal.

    I mean, I like food fine — I’m Italian, I have to. 🙂 But I go to bed at 9:30 or so on Friday night, wake up next day, get busy, and I’m just not hungry until 4pm or so. Same thing Saturday night. I had no clue that this was a fast.

    I’m not sure how I feel about how spoiled and pampered most nearly-spherical first-world Americans must be to consider any period of time at all with no food in their stomach to be a Fast™. I still can’t quite believe that a mere 20 hours — less than a DAY! — without food is considered by most people to be some sort of dire emergency panic situation. GMAFB. In a world where millions of people get by on far, far less, that attitude is obscene.

    The best thing of all about eating the way I do is the free time you get. Pound a big, fresh lunch, and your free time is totally your own when you get off work. I go home, and I get three and a half hours of knitting, crocheting, piano, viola, watching operas, tatting, reading … anything I want, without wasting an hour and a half of it cooking, eating, and cleaning up.

    I’m probably jinxing myself by saying this too, but FTR I’m 45, 5’8″, and have been 128lbs since I was 18 years old. I’ve got some health issues (don’t we all), but overall, I’m on good shape. I’ve always joked that my immune system is such that, if you have a cold and stand near me, my white bloods cells will reach out and strangle YOUR viruses.

  142. I haven’t fasted in a while so thanks for highlighting the benefits. But I find it very difficult to fast for long, eating every 3-4 hours keeps me going. Maybe if I have a very heavy breakfast then I can go for 6-7 hours but then Ineed to eat.

  143. Hi there all! New to The Primal Blueprint and IF…I am loosing weight and the hubby needs to, plus he is diabetic…So my question is, Mark or anyone in the know…Can anyone tell me the effects of IF on diabetics with regards to taking their medication 3 times daily with a medication

    If the go 16 to 24 hours IF does that mean they should not take their medication of do take the medication regardless of food?

    Plus we have metered coffees impact on the blood sugar and it does go up with black coffee…does that mean the diabetic should take his medication sugar pill?

    Looking forward to all of this and 16hours is a piece of cake, no hunger pangs at all!

    Look forward to a reply and thanks in advance Yolanda

  144. I like the helpful info you provide for your articles. I’ll bookmark your weblog and take a look at again right here regularly. I’m rather certain I will be told lots of new stuff proper right here! Best of luck for the following!

  145. Here’s my testament to the powers of eating primally (OK, I am on Perfect Health Diet not strict Paleo but it is very similar)– I fast for Yom Kippur (no food or water, about 25 hours total. Previous years I dragged myself through the final hours of the service and barely made it to the break fast. This year, I felt energetic and alive all the way to the end, and easily drove the 15 minutes to the break fast. What a difference! I attribute it to this way of eating, along with 16 hour fasts once or twice a week. I haven’t even done full day fasts (except for YK). Awesome! (Pun intended…)

  146. Hello – thank you for your website’s information. I know there are plenty of cultures throughout the world that integrate fasting. Thought I’d try the intermittent fasting. I have just finished the first 24 hour and plan to do the alternate day IF. I am feeling much better today and am surprised that I am not hungry. Although I have been filling up on crystal light diet drinks, water. I will keep you updated. I actually feel fantastic!

  147. What is the thoughts of IF when breastfeeding? My intuition is that it would not be helpful for milk production. I plan to breastfeed for 12-13 months as i did with my other 2 boys but I am keen to try IFing to help shift the baby fat.

    1. You can get all of the fat-burning and growth hormone benefits of IF without danger to your breastfeeding by doing a cyclic ketogenic ultra-low-carb diet such as Carb Nite (the one I used).

  148. I tend to fast every few months or after I’ve had a stint of particularly poor dietary habits for the simple purpose of detoxing. If my stomach has been dodgy, if I’ve felt sluggish, I find that a 24 hour fast is beneficial to getting me back on my feet.

    If I start feeling like my blood sugar is getting low in these 24 hour periods I’ll drink a little organic fruit juice and it usually knocks out any shakes or faintness.

    I always feel rejuvenated and healthier after fasting and would recommend intermittent fasting to anyone.

  149. I have been intermittently fasting every other day for about three weeks. For some reason I find it easy to do. During my fast I remind myself how much I will enjoy a big breakfast on the morning of my “feast” day and that seems to pull me through. If I have a craving, I tell myself to wait one day to fulfill it. The longer I do it, the less I seem to need to “feast” on my eating day. I also feel physically strong and…content on my fast days. I am sleeping better and it has also improved my mental/emotional outlook. I feel optimistic. I just feel damn good. I don’t know if I’ve lost weight, and I don’t care. This is an exciting discovery of how I work best. Thanks.

  150. I’ve been following a 16 hour fast/8 hour feeding window for almost 5 weeks. I am so happy. Been Primal/Paleo for 19 months and still follow that when I eat. I love it! Somedays I eat right when the “window” opens and other days I go longer. Once I do eat I always eat till I am full and focus on protein, fat, then carbs. I feel my gains in my workouts have greatly improved all areas. Strength, endurance, cardio. My moods throughout the day are even better. Body fat has gone down and lean muscle remains.

  151. Hi Tracy, What is your fasting time and what is your feeding window? I have been paleo for 2 years. I am really struggling lately because I keep gaining weight and not related to what I eat but during my TOM. I’m not up 12 lbs in the last 7 months. I have IFd before for 18 hour fasts, but never dropped a pound or changed my moods or body composition as a result. I stuck with it for many months. I haven’t done it in awhile. I’m trying to eat only when hungry. I find that eating paleo I am not that hungry, except in the morning.

  152. I have tried intermittent fasting once-besides the times I unknowingly fasted to make the upcoming weigh ins during my high school wrestling days. It was one of the best experiences ever! I had no real experience in this endeavor before, and found that by eating only fruits and vegetables the day before I was well prepared for the fast. In my case, I had eaten poorly for around 4-5 years, perpetuating and often sustained by emotional distress. Although consciously I was well aware that food took tremendous energy to digest, and that if I really wanted to heal-emotionally and physically-I had to break from the constant binge, I had never successfully employed my passion. So, upon removing all grains and caffeine the day before, I began the journey into the ‘vacant’ to discover the to discover the true abundance within. First thing I noticed, on the day prior to actually fasting, was an increased sense of sound and a better memory-could be because I have deterred my gut health to being intolerant of lots and lots of foods. Now into the fasting day I definitely felt light-headed at times and experienced vertigo. I comforted myself with a dark bath, and relaxing music. Sequentially I willed myself to pick up the free weights and perform a little resistance exercise. That felt great and I noticed HUGE affects afterwords. I felt stronger than I had in a long time, cleaner than I had in a long time, smarter and more vibrant. Although, it was at this time that I finally caved to my appetite and ate an apple. An apple that all others in my lifetime pale in comparison to; so exquisite. At that point I knew that this fasting thing had some real benefit, mainly detoxifying, and rejuvenating damaged cells. Finally, I finished that night with a kale salad some beets and some bacon. To my surprise, the euphoria continued long into the next day in which I also felt more vitalized than I had in years. I’m twenty-two years old by the way, to add some context to this account. I would like to go on about the benefits of short-term deprivation, but would even-more-so like to conclude this blog-post so that I can get back to what I really find essential, motion. I would like to sum it up saying that this was a great experience, and I intend to give it another go around-to again receive the mass energizing benefits before my forthcoming vacation-tomorrow. And to that I say cheers, power to the innate human abilities.

  153. Actually, starting Eating Stop Eat caused my eating disorder (I’m serious). I don’t blame the program, I blame my brain. I do want to go back to intermittent fasting one day, but I have to fix myself mentally first. Perhaps eating PB beforehand would help too.

  154. I’m new to Primal and fell off the primal wagon over Easter. I’m back on track now and would like to try an IF. This may be a silly question, but does sleep time count? Say, if I eat dinner at 7pm, then usually eat breakfast around 10am, does that count as 15 hours? Thanks!

  155. Great article! I think that fasting may indeed be a very good way to maintain good health and maintain a healthy weight. I also like that there are so many protocols out there – there’s something for everyone (ESE, 16/8, 20/4 etc.) which makes it easier to find something which suits your lifestyle.

  156. I’m really impressed- I’m only about twelve hours into my fast (not much, but I usually eat every two hours) and…..I FEEL REALLY GOOD. Being prone to migraines (on two meds for it) I was really scared I would get one- but I actually feel really focused and clear even though I haven’t had my usual soda/coffee, just tea. I decided to this because my usually healthy vegetarian/dairy-free diet has slipped into a carb-heavy rut, so I thought I’d hit the reset button. Shooting for 24 hours, wish me luck. Glad I found this site!

  157. Just wanted to share – I have been Primal/IF for the last 6 weeks now. I alternate IF from 16 hrs to 24hrs, 7 days a week. And it is easy to do! Blood pressure down to 116/70 from 140/90. Resting heart rate is 48 bpm. Dropped from 18% body fat to 10.5% body fat.

    Here is the kicker: Tracking my weight lifting workouts, I increased my strength by 27%!!!! Who would of thought eating less meant muscle gain!!!

    This lifestyle works, period!!!! I am a caveman, and I am proud!!!!

  158. I found this website after being lectured by a nutritionist friend about how bad for my health my fasting regime was… So glad I found this website now as it supports the way I have been feeling (GREAT).

    Ive been on a primal diet and IF for the last 10 days but had no idea so many other people follow this lifestyle.

    I was under the impression that IF was a very dangerous way to loose weight fast but reading up on it, it just makes sooo much sense.

  159. Mark, I am a 35 year old female and have been doing IF. I usually do a 16/8 or a 20/5, and I feel great. I walk 4-5 miles a day (briskly) in a fasted state and lift weights several times a week. I was eating “healthy” and excercising but not losing the 15 lbs. I could never get rid of, so I tried IF and have finally been able to see results. Plus, I feel a lot calmer overall (less stress and anxiety) and realized I was obsessed with calorie counting and meal times before. IF has allowed me to be in tune with my body’s needs abd finally see results. My menstral cycle has not been affected nor has my sex drive. Thank you for your web site. It encourages me and gives me the necessary knowledge to get fit and lean plus feel better!

  160. Hi there. I am a athlete . Body really good. Strong and bodyfat 9%. What can I gain from this IF METHOD? Can anybody explain to me? I have done it now for 24 hours not food at all and I am hammered.

  161. Hi guys! I’ve been researching IFing for a while and it sounds good to me.I work out regularly(weight train 3 times per week and do cardio 2 days per week)and I’ve been following the 6 meal approach.
    The problem is that I’d really like to try it out but I’m not sure if I should-I’m 16 years old and I haven’t found anybody in my family who supports this idea.What do you suggest?

  162. I have read up on the PB within this last year and more recently eat stop eat . So this last week I started skipping breakfast and just eating lunch and supper.that puts me at 17 hrs IF at lunch time ,so far so good .I was wondering if anyone had results from a similar eating plan in the terms of weight loss they would like to share.this is all new to me.

  163. IF for me is great – I keep a strict routine of Monday Wednesdays and Fridays.. these days i dont have to worry about food and even if i feel a bit hungry it doesn’t really bother me. I break my fast in the evening usually with home made fish soup and a salad. However I do seem to have plateaued on the weight loss. I’m 44 years old 5’11” – I was 94 kilos and in 6 weeks I had lost 7 kilos or there about (fluctuates) – this hasn’t moved much in the past 6 weeks – maybe the fluctuations can take me down to 86 kilos. I cycle 120-200kms a week including fast days where i do between 20-40 kms. I mix it up with some weights and swimming.. On my non fast days i often find im not overly hungry – i guess my concern is the hibernation theory that your metabolism slows down if you maintain low food intake? Also i guess if im truthful at weekends i can consume a fair bit of alcohol – vodka mainly.. if you had any thoughts on this id appreciate it – maybe the fantastic gains i made int he first 6 weeks set my expectations to high!!

  164. I have never eaten more than 6 consecutive days for several decades now. never fast less than 30-60 hours in one stretch weekly, usually “on air” only. And it shows in absolutely superb health, particularly at this age.. So I advise this for most of my patients and those who heed the advice, reaps the benefits. When you are unwell, that what makes you unwell is in you. The only way of getting it out is fasting.

    All drugs and “supplements” only serve to enrich the manufacturers, tries to induce wellness by putting more things into you instead of getting out of you that what makes you unwell. That feeds disease and keeps the industry happy.

  165. Great post. Silly, but I hadn’t realized that Intermittent Fasting could help with longevity even though I did know about calorie restriction improving longevity studies. I’d always imagined you’d need to be in a permanent calorie restricted state for that to work, but it makes complete sense! Thanks for the info.

  166. Hi, I am new to IF, and has been going about 1 month now.

    May Dad has been going 2 months, becoming much leaner and he said he feel much better in metabolism.

    I also burnt quite a lot of fat, July 15%, September 9%, it was about 4kg fat loss and 4kg muscle gain within this period.

    I often hear that IF will cause damage to our liver, but I don’t find any research.
    What I find mostly is about healthy heart, protects us against cancer and diabetes, but not about healthy liver.

    Hope someone could help me find out about this information =)

  167. as a muslim , I sometimes do intermittent fasting n I become less sleepy . I used to think that fasting makes us tired but that is because I do not fast by following the encouraged way by making sure I eat sahoor (before the dawn
    u cn follow this way of fasting. n make sure u drink water a lot during sahoor n when breaking fast , eat dates when breaking fast for one or two dates .

  168. I think skipping breakfast is an inferior way to fast than eating breakfast then skipping lunch and dinner. Sure it’s easier. But I think it’s the cheats way. I prefer to IF twice per week from breakfast to breakfast the next day 24 hours. Sure it’s tougher, but my mental fortitude is tough enough to hack it.

    1. “I think skipping breakfast is an inferior way to fast than eating breakfast then skipping lunch and dinner. Sure it’s easier. But I think it’s the cheats way. I prefer to IF twice per week from breakfast to breakfast the next day 24 hours. Sure it’s tougher, but my mental fortitude is tough enough to hack it.”

      This makes no sense? One way is less good because you feel it’s cheating to choose one meal over another,one time period over another, due to it being easier? Breakfast to breakfast is better because it’s tougher, but hey, you can do it?

  169. It’s not because we’re all going to be open. There’s a lot that is yours dog training and becomes yours.

    I knew no one noticed her hands palmed his ribcage.
    But I called the number one favorite, for putting people against each other.

  170. I’m nearly 65 and I fast one full day every week. I usually go from a late lunch (2 p.m.) to the next day at 2 p.m. or so. I sometimes push it to the evening meal so I get 28 to 30 hours fast. No longer than that ’cause I also lift weights so I don’t want to lose any muscle by pushing past the 36 hour mark. I’ve tried 6 p.m. to 6 p.m. and 6 a.m. to 6 a.m. For me the 2 to 2 works best in my schedule and otherwise. The main benefit is I’ve got 7 to 8 hours of sleep in the middle of it. I find if I fast 6 a.m to 6 a.m. I don’t get a good night sleep. 6 p.m to 6 p.m. gives me some 12 hours of so straight while awake. I fast Saturday to Sunday because I lift Monday (arms) Tuesday (legs) and Wednesday (shoulders et cetera.) If I fast Friday to Saturday I might impinge on some muscle building from Thursday. So I wait until Saturday afternoon and make that meal high in protein. When I get up Sunday I teach from 9 to 1 so the last eight hours or so are easy because I am doing something. During the last four hours I am also walking all the time. The calorie burn is around 2,500. I don’t know the fasting effect on the body per se but I think it does contribute to mental clarity.

  171. IF has changed my life, always battled with weight, got to over 500lbs, IF has gotten me to 285, best shape in my life, pretty jacked. 3 good size meals over 8 hours. #ItsNeverTooLate

  172. Keep it Paleo. Eat a big meal after waking up and another big meal a few hours before bed. I eat a large breakfast – chicken (with skin) 1 can sardine, 4 piece broccoli and 1 banana). My dinner at night is smaller – bowl of white rice, chicken or fish and 1/4 cup green salad (no added carrots or tomatos). I eat the same thing every Monday-Friday.

  173. I tried intermittent fasting, but had to stop because I blacked out. Not sure if I was do
    ing it correctly.

  174. I’ve been doing IF for 16 hours a day every Monday through Saturday and on Sunday I’m learning how to do a longer fast 20 hours into Monday. I hope to extend that Sunday fast to 24 hours eventually. The daily 16 hour fasts are surprisingly easy to adhere to. I workout fasted on the weekends and have had zero problems. In general I’m losing weight and my body fat % has gone down.

  175. After three years of living a Paleo lifestyle I tried intermittent fasting for three months in 2013 and suffered reduced sleep quality. I tried again in January 2019 and have continued with no impairment of my sleep this time. My insulin sensitivity improved and I lost 10 pounds I was not trying to lose. The only problem I face is friends worrying that I am too thin, although if they ever see me eat, they stop worrying.

  176. I’ve been doing IF 18:6 for several years ow. I’m 58, a bit overweight with moderately high BP, cholesterol and blood sugar. Haven’t really lost any weight without modifying my diet. Started keto or low carb really, a few weeks ago. Weight slowly coming off- I mean slow. But all digestive issues and migraines are gone. I’m eating clean with two meals a day. I feel like I’m finally resetting myself and habits for good! Can’t wait to read your book!

  177. Sir, your work and person example have changed my life. Thank you so much. ~Neisha Wright