Why Fast? Part Three – Longevity

A time-honored and research-tested way to extend an animal’s lifespan is to restrict its caloric intake. Studies repeatedly confirm that if, say, a lab mouse normally gets two full bowls of lab chow a day, limiting that mouse to one and a half bowls of lab chow a day will make that mouse live longer than the mouse eating the full two bowls. Cool, cool, a longer life is great and all, but what about the downsides of straight calorie restriction, aside from willfully restricting your food intake, ignoring hunger pangs, relegating yourself to feeling discontent with meals, and counting calories and macronutrients obsessively? Are there any others? Sure:

Loss of muscle mass. Humans undergoing calorie restriction often suffer loss of lean muscle mass and strength, all pretty objectively negative effects (unless you really go for the gaunt “Christian Bale in The Machinist” look and use a super-strong bionic exoskeleton for all your physical tasks).

Loss of bone mineral density. Humans who calorie restrict in studies also show signs of lower bone mineral density when compared to humans who lose weight from exercise, particularly in the hip and spine – the two areas most susceptible to fall-related bone breaks. I wrote about this study some time ago here.

Oh, and there’s the fact that the act of restricting one’s calories can be mind-numbing, miserable, and difficult for a great many people, especially if it’s a lifelong pursuit. (Unless, of course, you eat according to the Primal Blueprint and are fat-adapted. It can make CR not only tolerable, but a cinch because we become so good at living off stored body fat. We don’t suffer from sugar lows when we skip meals the way most people who fast do, but I digress.) That’s kind of a biggie.

What about fasting? In previous installments of this series, I’ve explained how fasting can sometimes be described as a “short cut” to the benefits of calorie restriction, an easier (and even more effective) path to the same destination. Studies on fasting/calorie restriction and cancer find that fasting is more effective in a shorter amount of time (weeks or months versus mere days). Does the same hold true for longevity? Can fasting also extend lifespan without making us look like a calorie-restricted monkey?

1945 marks the first real study (PDF) of the effects of intermittent fasting on lifespan in animals. Beginning at day 42 of their lives, rats were either fasted one day in four, one day in three, or every other day. All fasted rats, save for the females who were fasted one day in four, lived longer than control rats on a normal schedule. Although females outlived males in general (like always), fasting had the greatest effect in males. Male rats did best on every other day fasting; female rats did best on one day in three fasting. Fasted rats weighed less than control rats, so they likely also ate less, even though feeding days were ad libitum.

In a 1982 study, mice fed every other day lived 82% longer than mice fed ad libitum every day. No word on calorie intake.

Another study from 1983 showed that rats fed every other day lived longer and had lower body weights than rats fed daily and ad libitum. Fasted rats were less active in their youth, but more active than control rats when adults. The lower body weights in the fasted rats indicate a reduced calorie intake.

In 2000, female mice who fasted for four consecutive days every two weeks lived for 64 weeks on average, while normal fed mice lived just 47 weeks. Interestingly, the fasted mice were heavier than the fed mice throughout the experiment, which indicates that calories weren’t significantly restricted.

Okay, so it looks like fasting promotes longevity, which may be mediated by a reduction in calories. Provided the faster doesn’t gorge him or herself on the feeding days so much that they make up for all the missing calories, it should be effective. In other words, fasting promotes longevity in all but the most ardent of big eaters. I don’t see this as a “gotcha” or a problem, because fasting almost invariably produces calorie restriction. (I might eat a fairly big meal after a long fast, but I definitely have never doubled my intake to make up for the fasted day.) In fact, that fasting makes calorie reduction painless and inadvertent is a highly-touted feature of the practice, and a big part as to why it’s so effective for people who have failed with traditional calorie restriction.

So, how’s it all work? Is it just the calorie restriction?

Perhaps. One potential pathway by which both fasting and CR increase lifespan is via inhibition of mammalian target of rapamycin, or mTOR. The mTOR pathway can be said to drive the aging process. It is necessary for cell growth, like muscle cells (in stable mTOR states) or certain cancer cells (in overactive mTOR states), and it is highly sensitive to nutrient availability and hormonal signaling. In mice, feeding rapamycin – a potent inhibitor of mTOR – increases lifespan. Both not eating (fasting) and lowering potential hormonal messengers (like insulin) also inhibit mTOR activity by spiking AMPK.  Indeed, in the presence of insulin, mTOR is upregulated. We know from previous posts that fasting reduces fasting insulin. In insulin resistant people, insulin is chronically elevated and mTOR is overactive. We also know that fasting reduces insulin resistance and increases insulin sensitivity, thus normalizing or inhibiting excessive mTOR activity. This kind of mTOR inhibition also works with CR, but if adherence is easier, side effects are lessened, and AMPK spiking is greater with total caloric restriction (fasting) than with partial CR, which is the more effective method?

So, it’s starting to look like the longevity benefits of fasting can be attributed to the degree of caloric restriction. That is, fasting is total caloric restriction, while CR is partial. When you fast, you’re going whole hog. You’re subjecting yourself to an acute stressor, getting the hormetic benefits, and then recovering from that stressor by eating normally thereafter (until you do it again). When you calorie restrict, you’re undergoing a chronic stressor. Day in, day out, you’re worrying about food, restricting energy and nutrient intake, and there’s really no period of recovery. You’re always residing in a partially restricted state, drifting from paltry meal to paltry meal. There is no feast. It’s like lifting heavy and sprinting a few times a week versus jogging a 10k every day. Chronic cardio versus acute, high intensity exercise.

What about “healthspan”? Can fasting compress morbidity – can it help us in our quest to live long and drop dead? I mean, let’s face it: who wants to be a frail, skinny-fat centenarian relegated to the bed or the walker or the wheelchair?

Well, we know that intermittent fasting appears to conserve more lean mass than CR. In a recent meta-analysis, one group of researchers directly compared studies on calorie restriction to studies on intermittent fasting and found that while both were good for losing weight, “intermittent CR may be more effective for the retention of lean mass.” And at the very least, I’d suggest to you that having better insulin sensitivity, less fat mass, more lean mass, a better-regulated mTOR pathway, improved blood lipids, better glucose control, and a less restricted diet makes for a better healthspan, a more enjoyable life, and a reduced risk of dying from the diseases of civilization.

Bottom line: fasting may not work by some magical pathway separate from caloric restriction. It may, but it hasn’t been established. What we do know is that fasting (whether by inadvertent, enhanced calorie restriction or whatever else) improves lifespan in lab mammals and improves various health markers associated with aging and longevity in both humans and animals. Fasting may not give you an immediate “Life + 25” boost, and there haven’t been any real lifespan and fasting studies done on humans (if only we had mice-like lifespans!), but if it makes you less likely to get obese, diabetes, heart disease, or cancer, you’re less likely to die from those things. The fewer things you have trying to kill you, the longer you generally live.

That’s it for today, folks. Thanks for reading. Any questions? Comments?

Here’s the entire series for easy reference:

Why Fast? Part One – Weight Loss

Why Fast? Part Two – Cancer

Why Fast? Part Three – Longevity

Why Fast? Part Four – Brain Health

Why Fast? Part Five – Exercise

Why Fast? Part Six – Choosing a Method

Why Fast? Part Seven – Q&A

Dear Mark: Women and Intermittent Fasting

About the Author

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

If you'd like to add an avatar to all of your comments click here!

147 thoughts on “Why Fast? Part Three – Longevity”

Leave a Reply

Your email address will not be published. Required fields are marked *

  1. Well, I wouldn’t fast for the thought of living longer.
    It just feels great sometimes to fast, especially after days in which I’ve overfed myself. 😀
    Sometimes I eat a lot, sometimes I thrive on oxygen, the way you feel is the best indicator of what/how you should eat/do.

    1. The way I “feel” like eating would include a lot of cookies, milk chocolate and baked goods, along with the good stuff. I try to avoid that.

      1. Do you feel like eating cookies or baked goods all the time? I indulge in cookies or eat spread butter on a slice of bread and it’s all good. 🙂
        If you believe something affects in a positive or negative manner, it does.
        Our BELIEFS about the world around us shape our life.

        1. Not all the time no; it comes in waves. A week or so without temptation (sometimes more), then an avalanche of food lust which takes a while to eat myself out from underneath.

  2. I have heard that chronic caloric restriction reduces brain size, and that’s apparently why people suffering from anorexia have trouble resuming healthy eating. Is that true or is it a part of the crazy trivia? Will habitual fasting cause negative impacts to brain as well as the CR?

    Also, is eating every other day preferabl to the IF, or is IF better? I find it hard to go without at least one meal a day?

    1. crazy trivia. (psych pathways lead to difficulty in resumption, brain size has nothing to do with brain use, etc.)

    2. I think the biggest goal is to find what works best for you. Some people eat just all their calories once a day. Some skip every other day. Some never skip a meal.

      Just play with it and decide how you feel best. There’s no right nor wrong answer.

    3. I usually go eat at “BurgerMcWendyShoppe’s” as a last meal for lunch, then I don’t eat for 24 hours or so.

      –Make Sure To Drink Water!–

      I find it easy this way.

    4. I dont know about anyone else, but ever since i started doing intermittent fasting, about 2 years ago, my memory and brain function has been slowing increasing. The other day in class I had an incident in which there was a computer tech trying to fix the background while the professor was teaching, I remember glancing a couple times while paying attention to the teacher, then after the tech left we were talking about mission statements, and I remembered seeing one on the back of the techs sweatshirt and was able to recite it word for word. Definitely not something I would have been able to do 2 years ago for the life of me.

    5. I know a psychiatrist that specialises in eating disorders. He told me a few years ago is that anorexics may have a defect in their cannibinoid system which controls appetite. An effective treatment is simply to feed them a liquid via a nasogastric tube until their weight returns to normal. The compliance rate is very high and the outcomes are generally favourable.

  3. I eat like a dog. One meal per day but what a grand feast it is. It improves my weight, health and frees up my day. I think if people became fat burners first and gave it a serious try then half the populous would do it. All the feeling of control without the anorexia!

    1. You don’t feel insanely stuffed after this one meal? Anytime I eat an insame amount of food, even if its pure primal, I feel stuffed and kind of tired. I’ve only done this a few times.

      Eating around 2000 calories a day just seems like A LOT!

      What do you eat for a typical meal?

      1. I, too, find it difficult to eat all of my fuel for the day in one sitting (from a standpoint of comfort). I’ve finally gotten used to the idea of only eating when hungry (instead of when society tells us to eat (“Breakfast is the most important meal of the day!”)).

        To eat one meal per day, do you have to “manually override” the any signals from your body suggesting that you stop eating?… or has your body adapted to this protocol?

        1. I tried one meal per day myself and couldn’t pull it off. I can, however, fast for about 24 hours, and consume everything within a 2 hour period. I’m not sure if it is merely a mental thing (“I can’t eat all of this in one sitting!”), or if my stomach can only handle so much at once. Possibly both.

      2. I have done this one a meal a day at an average of about 80% of days for almost two years and about 95% of days since last March. During this time, my caloric intake for that one meal has gone from about 1400 (in the beginning it was difficult to eat so much) to about 3000 these days. I can routinely eat more than 3000 Cal if I wish to, but feel no need to. I have reduced body fat(approx 20-25% to 11-13% during the two years), increased weights on my lifts and lost about 20 pounds of weight.
        I also experience a food high after my meal, a sense of intense satisfaction and an absence of stresses. I feel sleepy 2-3 hours after eating. I do not feel like I have overeaten or bloating of any kind. I started with this book:
        highly recommend it and many others written by Ori Hofmekler

      3. It just tales practice. When I was at university we had a barbecue lunch. One of my mates ate 10 large lamb chops, 12 sausages, about 30 slices of bread and six cans of beer in less than an hour. He was a serious bodybuilder and used to eating huge meals. He didn’t suffer afterwards.

    2. It doesn’t take very long for your digestive tract and metabolism to adapt to eating a once daily feast. It’s certainly much shorter than the amount of time it would take for me to adapt to eating heart healthy whole grains as the basis of my diet.

      I personally have no problem putting away 3 lbs of red meat in less than a half hour. I’m sleepy after I eat, but I’m supposed to be tired just like all of the other meat eating mammals. That’s why I eat in the evening so I can kick back and relax afterward.

    3. I was surprised (in the 1980s) at how one friend of mine (30-something guy) ate only one meal per day and yet was so muscular, despite not working out. It seemed (at the time)…so … unnatural.

  4. I have been reading about reverse t3 from very low carb diets and calorie restriction. Anybody know how intermittent fasting affects thyroid function?

  5. Great article, thank you!

    I had my first IF just a few days ago and I found it very enjoyable!

  6. I cycle with IF and love not having to think about preparing food. It’s almost as good as a vacation! But I also like to eat. And I can’t imagine getting so thin for the purpose of longer life (maybe.) Sounds a sorry state of a life…

    1. This article is so appropriate today… day 2 of my fast.

      I don’t do it for weight but for joint issues and brain fog… and just generally resetting my system.

      I also like not having to worry about food prep for a while or taking time out to eat.

      I go by how I feel. When I start feeling good, then I know I can start eating again. That may be tomorrow or the next day. I won’t know until I wake up.

      I agree about the breakfast thing too. Sometimes I just have some clear soup broth to get me started. That’s actually one of my favourite breakfasts… followed by about 3 eggs, some bacon and veggies all wrapped in lettuce around mid morning – because that’s when I get really hungry. 🙂

      1. Has it worked for you before for joint issues? My knees and sometimes toes and fingers hurt terribly. How long of a fast and how frequently do you need to do it for it to help?

      2. Joint issues and brain fog? That sounds familiar, are you a former Fibro sufferer?

      3. I’d also be curious about whether or not this is working out for you. My mom gets joint pain and brain fog due to her fibromyalgia.

        I suggested IF to her and despite the fact that she always complains that she “hates eating” she has decided to go the 6 meals a day route.

  7. I recently had surgery and used IF as part of my recovery plan. My surgeon intially warned me that most people are not able to return to work full time for 2-3 weeks due to a lack of energy/strength. I was back in 1 week and feeling great. I went hiking 10 days post-surgery. Tomorrow will be the 2 week mark and even my doc is shocked at how quickly I have healed. Primal has turned me into the Bionic Woman!

    1. This happened to me with my second c-section. They couldn’t believe I was ready to go home the next day. They kept me one more day anyway and then I walked out of the hospital upright. (39 years old)

      My first c-section, when I was still consuming loads of gluten/grains/dairy… I walked bent over for two weeks (at least) afterward. (33 years old)

      I heal SO much faster if I fast around something stressful. Glad to know it’s not just me. ;D

      I think it really just allows the body to take time out from constantly digesting, so it can do other things, like clean out the cobwebs, detox. and do a bit of extra healing if need be.

  8. I’m considering intermittent fasting, but I had a question first: About how long does it take a person conditioned to a SAD to become “fat-adapted” on a primal diet?

    I’ve been eating primal for about a month now, and I’ve seen some spectacular results, but I don’t want to jump into fasting if my body isn’t ready for it.


    1. It was about two months for me. I noticed that even if got hungry I didn’t have any of the old symptoms of “hunger”, tired, cranky, lightheaded, etc. I was just hungry and I could eat or not. I started stretching it from 8 hours to 12 to 16 to 24. Pretty cool.

      1. I find the same thing. Hunger has become more like a gas gauge; It tells me I’m low but the amount of available energy is the same. And I guess my gas tank (fat reserves) will last quite a while (haven’t tried to find out). And as you say, pretty cool.

    2. the only thing that will happen is that you will be a little hungrier than the those that are more fat adapted. just talk a couple shortish walks and drink lots of tea while you fast then break your fast with a big steak or ribs and veggies and half a yam. in fact as fasting improves insulin sensitivity I would bet that, as long as you don’t break your fast with lots of carbs, the fast itself will speed up your fat adaptation.

    3. I have also found it is much easier to fast if I just continue the natural fast that occurs overnight. No breakfast, no lunch, and a normal dinner. I am careful not to break the fast with something carby and instead eat some quality fat first.

    4. I generally advise all my clients looking to get in IF to eat a protein heavy meal a few hours before they go sleep. In the morning when you wake up you will have already have been fasting for around ten hours. Just push breakfast back about thirty minutes to an hour every other day and you’ll work your way into it in no time.

      The morning is generally the best time to fast. The natural rise in cortisol in the morning will help to keep your blood sugar stable.

      Remember that Grok didn’t have processed foods or a fridge, so breakfast was certainly uncommon for our ancestors.

      However, if you have to eat something, you have to eat something.

      Unfortunately, we live an era where metabolic disorders are the norm, so it is really unpredictable how the “average” person might react to fasting.

    1. Muscle catabolism caused by fasting is vastly overexaggerated.

      I fast for 16-24 hours every day. Check out my website.

      Do I look like I suffer from muscle catabolism? Bottom of my about me page.

      I would like to see someone following a VLC diet who is more muscular than I am. I’m 5’10, 170 lbs this morning, 7.3% bodyfat last measured 5 days ago.

      1. Ok, that’s great man. Really. But your personal anecdote doesn’t have a lot to do with my metabolism, fitness level, and current food variety. Do you have any literature or anything you can link to that would describe in more general terms when muscle catabolism would kick in? I am interested in IF, but I’d like to only go as long as I am fat-burning.

      2. no offense man, truly im not trying to attack you, but there is no shot in hell that you are at 7.3% body fat in that pic… regarding muscle catabolism, if you don’t give your body sufficient carbohydrates it will break down proteins into amino acids and make them into glucose. thats a simple fact. your body has functions that rely on glucose. however if you eat a high protein diet your body doesnt take the protein from the muscles it simply breaks down the protein your eating and converts that to the necessary glucose. so no muscle catabolism is actually occurring. when you fast for extended periods of time, not short fasts like your 16-24 hr fasts, but beyond that your body will begin to break down muscle to provide the necessary glucose. to what extent i can’t say, i imagine its different for everybody.

        1. i’m not against fasting i love it and do it regularly but with a very low carb, high protein, high fat diet you get many of the same benefits that you would get from fasting but with that high protein intake your protecting yourself from the potential muscle breakdown experienced during fasts regardless of how minimal it might be.

    2. Nora Gedgaudas (Primal Body, Primal Mind) takes it one step further by restricting protein along with carbs. You can eat all the non-starchy veggies and all the fat you want. It down regulates mTOR more than carb restriction alone.

      From what I’ve read, Dr. Rosedale’s diet is similar for many of the same reasons. It all depends on how far you want to take it. I do IF along with PB,PM. I just hope I don’t get hit by a bus tomorrow. 🙂

  9. I find by switching to primal eating and ingesting more fats that fasting is incredibly easy as well. I like to fast for 24 hours once a week, usually 2pm – 2pm or 4pm – 4pm. It’s easy to eat a nice breakfast, have a big lunch with lots of leafy vegetables, and then just stop eating. By the time I get any hunger pangs, I can go to bed and sleep through the worst of it. I wake up feeling great.

  10. I love IFing. I fast 24 hours (dinner to dinner) several times per week. Makes life easier not having to mess with breakfast or lunch and I usually feel even better afterwards, never worse.

  11. It looks like the ‘Warrior Diet’ was way ahead of it’s time. Eating one big satisfying meal a day, after fasting for
    20 or so hours is VERY liberating. Once you adapt, you will be surprised how much energy you have and how much more alert you are.

    1. True that, Warrior since Sep 2010.
      Ori is waay ahead of everyone when it comes to health and fitness

    2. I agree, Ori has been ahead of the game. I read his book first then researched more on paleo/primal lifestyle. I noticed that when I did fast, I was able to lose stubborn fat more easily. Now I routinely end my last primal meal around 7:00pm, only have a cup of coconut milk w/coffee for breakfast and can make it until 1:00pm. I’m starving by then, but don’t gorge because my body has miraculously adapted to “eat until satisfied”. Dinner is the same way, I find that I am satisfied and full with smaller portions now. The weight and stubborn belly fat is finally going away, too. I highly recommend IF.

  12. Eating 2 meals during a 6 hour window from 1PM-7PM works for me. I try to do it 5-6 days per week.

    1. Why a 6 hour window? It seems as if everyone who has an eating window goes for 8 hours. It doesn’t seem like much but its a 33% increase in the eating window.

      I sometimes enjoy a 8 hour eating window. It’s not difficult by any means. I’m sure 6 works too but I think people do 8 because at 16 hours of fasting your body begins to get energy from your muscles! This is different for everyone of course.

      1. Please correct me if I’m wrong (still learning!), but when reserve glucose runs low, isn’t there a period of gluconeogenesis where the body converts amino acids into glucose to maintain proper brain function, before it finally switches over to fat-derived ketone bodies? And from that perspective, wouldn’t longer periods of fasting be better, since you would spend less time in gluconeogenesis compared to ketosis?

  13. “The fewer things you have trying to kill you, the longer you generally live.”

    Quote of the day. Love it.

  14. After reading Mark’s first article in this series, I decided to give IF a try. I went 24 hours without food (dinner to dinner) and felt great! I didn’t feel hungry, sluggish, tired, or anything.

    This leads me to believe that a lot of my “hunger” is driven by nothing more than eating out of habit.

    I’m going to try to do IF at least twice per week now, and when I’m not purposefully doing it, I’ll be sure to ask myself before each meal or snack if I’m eating because I’m truly hungry, or if I’m just bored/following a habit.

  15. I’ve been primal for about 2 years and I recently added the Leangains approach (16 hour fast, 8 hour feed) about 5-6 days a week. I found that it made sense for me because my stomach really cannot handle anything until the afternoon anyway. I’ve gone from about 12 percent BF to 8 percent in the past couple of months. I’ve gotten much stronger with my lifting as well. I highly recommend it. I’d like to hear how frequently Mark fasts and for long.

  16. As far as the reported bone and muscle loss with CR – it seems like if you do weight bearing exercise and continue to eat your protein in between fasts you can avoid at least the muscle loss? That seems to be the case with me anyway, I can continue to lift heavier and heavier weights so I don’t think I’m losing muscle, hopefully just fat. I usually have a 4 hour eating window on most days so I always get to exercise in the fasted state.

  17. I’d be interested to know what type of fasting is the most effective. How long should the fasting period be, ideally?

    1. that is individual. try reading the book “intermittent fasting” by Dr. Barnardi John of Precision Nutrition. He did test a lot of methods on himself.

    1. I’m on day 2 also.

      I consume anything liquid, generally speaking though.

      Yesterday I did eat some kale chips, a couple of oranges on top of my regular veg. juice and beef/cabbage broth.

      This morning I had a couple of pieces of pineapple and some kale chips (homemade with Himalayan sea salt), as well as my juice and broth.

      The next bit will be water only.

      I don’t usually eat ‘thick’ food during a fast but thought I’d try something new.

      I have a chiro appointment tonight and wanted to go in really clean for it.

      How do(have) you fast(ed)? I’m always curious.

      I do this 3 or 4 times a year (a big fast). Sometimes it’s easier than others but I always feel so much better for it at the end… more in control of my food intake, clearer, etc.

      1. I’m consuming ONLY water with multivitamins on waking and sleeping. And a bit of toothpaste I guess.

        60 hours into 120 and it’s more mental hunger than physical. In fact, there is very little physical hunger. I just feel I *should* eat, which is easy to ignore. I am looking forward to Saturday though 🙂

  18. I have watched documentaries on people who have chosen severe calorie restriction in order to live longer. One guy was chronically cold and had to take a pillow everywhere with him as it hurt to sit. He ached constantly.

    Thanks but no thanks. It’s IF for me and I feel much better that way. CR is just a grind.

  19. After a four-month plateau I started IF and FINALLY my weight loss has resumed. I am becoming a fan of three times a week IF, one day of 24+ hours. I FEEL wonderful!

  20. I’ve been doing a 16/8 fast every day since Jan. 1 of this year (it wasn’t a “resolution” — I just had time to do a little more research during my break) and I can wholeheartedly recommend it. The combination of a Primal lifestyle (LC diet + short, intense exercise) and regular fasting seems to be the “magic bullet” for achieving that Brad-Pitt-in-Fight-Club body comp. Over the past few months I’ve seen a truly dramatic (and admittedly gratifying) change in my body comp, which was not bad to begin with — but the regular fasting has markedly increased muscular definition. After looking like a powerlifter for years, I’m getting a kick out of having sharp abs and pronounced vascularity.

    I’d suggest reading over Martin Berkhan’s “top ten fasting myths” (https://www.leangains.com/2010/10/top-ten-fasting-myths-debunked.html). He does a good job using empirical research to debunk these myths (like the bodybuilding myth that if you don’t consume protein every three hours your muscles will start consuming themselves). Also, fasted training, as Berkhan suggests, is absolutely awesome for fat burning. I was worried that I would be totally spent on my lift days coming off a 15 hour fast, but there really isn’t any effect, and the workouts feel much more efficient fasted.

    Thanks for devoting more attention to this strategy, Mark!

  21. “The fewer things you have trying to kill you, the longer you generally live.”


    1. I agree with this wholeheartedly! I want a MDA shirt, but I am not huge on Grok. This quote with marksdailyapple.com underneath would be awesome.

  22. So what is the recommended IF schedule. Once or Twice a week for a 24 hr period?

    1. Some people fast every day for 14-22 hours and eat inside a window of 10-2 hours every day. You could fast and entire day every other day, of just fast 24 once, once or twice a week.

      The science won’t be conclusive on the “best” fasting periods/frequency for some time.

  23. Whether it’s a good idea or not, I do a cheat meal once every couple of weeks (something made with flour, like pizza or a couple of dinner rolls). I wonder if it’s better to do that before a fast, or after a fast?

    1. I find after a cheat the carb cravings are full force the next few days. By fasting after I cheat it helps me “reset” so I don’t have the cravings and am easily able to resume healthy eating. I always break the fast with some good fat.

    2. I have an entire protocol for my clients that involves a feast/cheat meal.

      I consume over 5,000 calories in one sitting, and I lose bodyfat everytime.

  24. My only concern is how well fasting would work, and when the best time to fast would be for those of us that crossfit frequently (3-5 times a week). I’ve been eating paleo (or to fit in with the MDA regulars ‘primal bluebrint’) for over a year so I don’t think it would be super hard to miss a meal (which happens naturally once or twice a week) but an entire day fast would likely be exceedingly difficult.

    Are there additional guidelines for people who are extra active? Any studies on fasting mice who crossfit? 🙂

    1. My cat crossfits and eats paleo but I can’t get her to fast. Sorry.

      1. Best comment ever.

        I could make my cat fast… but she can make me bleed. She wins!

        1. I hear it’s not good to make cats fast because it could damage their liver in some way..

    2. I do CF 5 days a week at 5:30 am. I typically break my fast around noon with my biggest meal. I eat again around 7:30 and am done for the day. I recently started adding some CF Endurance before my dinner meal.

      Overall, I feel great. Weight is coming off fast, feel stronger, great endurance in the gym.

  25. I feel like I have a mental block when it comes to IFing. I can do 16 hours (with a coffee plus heavy cream), but can’t seem to get beyond that. I’d like to do 24 hours once or twice a week. It’s almost as though I am afraid to be hungry. Silly, I know I won’t actually starve, but true. Any tips?

    1. When are you breaking your fast currently? The huge tip is to fast at a time/day that you are incredibly busy. If you’re focused on a task, you won’t think about food as much. Also, do you like hot tea? Adding this in the afternoon or evening could reduce temptation if you’re not already drinking it.

      1. I agree. If I’m active and wrapped up in what I’m doing, I don’t get hungry. I never take a food break when I’m jamming with a group. Watching TV or sitting at the computer is a different story.

        1. Good point. I sit in a cubicle at a computer M-F 8:30-5. Although I have busy days, I generally have a lot of time to think about food. Maybe trying a Saturday when I am more busy would work. Thanks for the tips. I’ll also try the tea.

  26. Calorie restriction sucks. I hate counting calories, too. Normally I will fast until afternoon or evening. If it’s a workout day, eat a ton of carbs and little fat. If it’s a rest day, eat a ton of fat and little carbs. Works for me!

  27. So, I started eating mostly primal 10 months ago. I initially lost 30lbs in 5 months and gained some muscle. But in the past two months I have randomly gained 10lbs back [mostly fat, I think. I am more jiggly). My diet has been steadily the same as it was. I also noticed that I am never hungry. Ever.
    So I tried IF several times over the past two weeks to see if I would be hungry at some point. Nope. Still not hungry. (And I don’t eat too much, either).
    Should I do an extended fast of several days?
    Also, at what point would IF-ing potentially help me get out of this weightloss stall?
    I’m typically super in-tune with my body but I cannot, for the life of me, figure out what’s happening the past two months.

    THANKS for any info/help.

    1. Lindsey, I am far from expert, but have you increased the intensity of your workouts? Hunger is your body’s way of asking for more fuel. I’be also discovered digestive enzymes and highly recommend as a way to break down and move out the food you eat.

    2. Lindsey–if you can share precisely what you are eating and an idea of the kind of workouts you are doing, that would help. Like TruckerLady said, I would recommend making your workouts LESS frequent and MORE intense if you haven’t already done so. I always tell clients, “Hours spent in the gym is irrelevant. All I care about is how hard you work while you’re there.”

  28. I attempted a weekend fast but could only make it 36 hours. I was hungry enough that I felt anxious and unable to concentrate on anything else. I am puzzled, though, about one side effect. I used to have horrible sciatica. Couldn’t stand or move without my leg going numb, shaking and extreme pain. All of that has largely disappeared since my 3rd fast day. I have questioned a few friends in the medical field but none can explain the reason. Anyone have any ideas?

  29. What do you think about one 17 hour fast and one 24 hour fast in a training week? Do you think one would be better?

    1. Are you saying two fasts in one week? And one “training” week, how much training?

      I’ve posted on a few other comments, but I feel depending on body composition, etc, that as long as you’re not creating too large a calorie deficit too fast, you would be fine training. (i.e. eat a larger meal after your fast or your training might suffer.)

  30. I’m amazed at everyone that can wait so late in the day to eat. I wake up each day starving. I have to have my eggs and sausage/bacon within an hour or so of waking or I’m ready to eat a cow and verging on getting cranky. I’ve been this way as long as I can remember. Once I’ve had breakfast I typically don’t get hungry again for another 8 hours and then eat a light dinner. (So, I’m eating at 6:30am/2:30pm/7pm). I could possibly skip dinner each night, but I can’t imagine skipping breakfast.

    1. I used to be the same way when I would get up and work out (i.e. chronic running), but then my lifestyle shifted and I’m usually pretty busy with work in the mornings so I feel less hungry than I do at night when I’m not busy and have easy access to the fridge.

      But maybe that means I just don’t notice it in the morning anymore 😛

  31. “Interestingly, the fasted mice were heavier than the fed mice throughout the experiment,…”

    THAT…I don’t like at all. I think constant STARVATION throughout life makes the body conserve fat. I’d say Fasting for 24-48 hours ONCE every month is okay, doing it every 3rd day is INSANE.

    1. Heavier in terms of fat or in terms of muscle mass? Also, I think mouse studies are lot more complicated than these papers make them out to be. I work in a research lab with mice everyday – we get what are supposed to be “genetically identical” mice and we free feed them as much food as they want. But after two months, you can have five “genetically identical” cage mates and two will be GINORMOUS while the other three are healthy and lean.

      My superiors aren’t really interested, because that’s not the focus of our research. But it’s still something I keep in mind.

    2. I have been doing a 24 hour fast on every rest day for a month now–currently, 2 or 3 days a week–with no negative consequences. I guess someone less active with lower caloric needs could experience low energy–I have no knowledge of this–but for someone building muscle mass and eating a caloric surplus regularly to do so, I think a regular “break” from food is beneficial.

  32. Great post!

    I’m actually posting this in a fasted state. It’s been about 20 hours since my last meal. Should I keep fasting till I hit the 48 hour mark? I’m not really hungry, but I’ve been doing the one meal a day thing for some time. Thoughts?

  33. I did a 24-hour (well, 22) fast the other day from evening to evening, but I think I might do better with a morning to morning fast. Morning is my “hungry time”. I can skip lunch and/or dinner easily, but skipping breakfast is considerably more of an effort of will. I do love how alert and strong I felt all day though! In my whole-grain days I never, ever thought I could go 6 hours without food, much less 24.

    1. I used to be the same way, however, now that I’m so busy at work, I find that I have an easy time being occupied at work and fasting, but then I can be ravenous when I get home and have nothing to do! 😀

  34. I think intermittent fasting is probably fine for a lot of people, but for some, it might be a little over the top in terms of their relationship with food.

    Human minds are more complex than a mouse.

  35. I think the best way to think of IF is as another form of exercise, albeit one not directly involving your muscles but other body systems. You’re exercising your liver. You’re avoiding a sedentary diet!

    1. I agree. IF as exercise. I keep a fitness log – one column is labeled “IF” in which I note the length of each fast. Usually this is the only fitness entry for that day (aside from my bike commute to/from work and daily lunch walk).

  36. I’ve tried it several times since going Primal, but I get way too hungry to reach 24 hours, rarely even 18. I notice these articles about IF always talk about the benefits, and I get that, but it talks about actually DOING IF like it ain’t no thang, and for me it’s not easy at all.

    Any tips?

    1. You will probably be hungry for a month or so after starting IF. Your body feels hungry at pretty much the same time of day, so for it to get used to your new schedule, it will take time (could be a week or a month, cant say).
      But it is important to stick to it

      1. That’s good to know – I’ve wanted to do it for a while, and I thought I was just doing something wrong (I was thinking maybe too much dairy, or not enough fat). About six hours after any meal my stomach goes all hollow and growly, and I feel shaky dizzy and sick. I know it’s not training because I barely do any (mobility problems, getting better but still preventing most forms of workout.)

        I’m not used to feeling hungry EVER (only recently have I ever given myself a chance to), so it’s been weird to get so hungry I feel like I have to eat a cow or die, right this minute. There’s no slow build to it, either – one minute I’m Dr. Jekyll, the next minute Ms. Hyde. Yesterday the switch flipped in a meeting with my boss and all I could do was stare at her tuna salad and salivate.

        I’ve read that it’s best to get your body comp sorted out before you try IF, but I’ve also read that IF is one of the best ways to get your body comp sorted out. I’ve been eating ketogenic Paleo since Feb 15 (and in ketosis) so I was hoping I could go into IF and not get the hungries. If it’s just something everybody gets and learns to endure, maybe I should just push through it?

    2. It takes practise for some people. You may have to get used to new and different sensations in your body when you first start fasting and maybe some psychological/mental attachments to food. Maybe anxiety at not eating when you usually would eat or fear may come up. If you can get past any of those(because you know from reading posts here that none of us have died trying) you may be pleasantly surprised at what you discover about yourself.
      I needed to do it a few times before I got used to the unusual and uncomfortable sensations and hunger pangs back in the days when I was a carb muncher . But wow, it was such a buzz to know that I could actually go without food for days and not collapse in a heap..
      Give it a go, maybe ease into it and stretch it out each time. I was determined to do it back then so I bit the bullet and just breathed through the hunger and lightheadedness of the first few days and by the third I felt more clear headed, energetic and generally high in a good way. It was a real challenge to reintroduce food on the 8-10th days of breaking the fast.. not saying you should try it for that long but that was how i started years ago.. now i am enjoying the 8/16 or 24 hr occassionally. Good Luck.

  37. I think the experience/signal of the fasting in itself generates, up regulates, some biologicals processes (autophagy for instance) that otherwise (including mild CR) would remIn dormant for our complete lifespan. There’s something specif to the act of fasting in itself to improve the health.
    Every known religion on earth has its prescription of fasting. Why? …

    1. That’s an interesting thing to point out. I have a mild latex allergy where I get a rash when I inadvertently come in contact with latex – i.e. I wore a respirator at work yesterday and didn’t think that the bands holding the mask to my face had latex in them in I started breaking out in hives later.

      But on the interesting side of things, I’ve been on a 24 hour fast at this same time and my rash has been clearing up much faster than normal. I’ll have to try again next time and see if there is a trend.

  38. Interesting, haven’t really tried IF yet.. At the most i’ve skipped lunch a few days out of the week. Doesn’t hamper me one bit, although i’m mad hungry at night. Curious to see how a 16 hr fast might turn out.

  39. I want to try intermittent fasting (16 hr fast, 8 hr window) but i am a serious athlete that plays tennis almost everyday. Will my energy levels be lower or will they increase? Usually if i eat alot before playing, i feel sluggish and tired so hopefully fasting will change that for me.

    1. As long as you’re eating enough after the fast, you’ll be fine. For some people, it might take a few tries to get use to. When I was younger and a lot more active, I couldn’t work out without eating a full on 800+ calorie breakfast. Now, I prefer to workout on an empty stomach. But I will say that the first few times I tried, I was a crab 😛

      Just make sure to make up those restricted calories later after the fast, especially as a competitive athlete. If you’re restricting too many calories over the short term, your game will suffer because you won’t have the energy.

  40. I would like to IF but I have osteopenia and am concerned about losing bone mass. I have been gluten/dairy free for three years and paleolithic for about 3 months. Would IF be good for me?

    1. If you’re nervous, why not trying IF on a reduced basis? Like maybe once or twice a month, and seeing how that goes. If your bone density isn’t affected too much, maybe you can then try ramping things up?

  41. A recent issue of Harper’s Magazine has an article about fasting, and the recent American history of medical studies of fasting. It’s fascinating and I recommend it highly. The short: fasting seems to differ from caloric restriction over a prolonged period. Seems muscle loss occurs for only a day or two into fasting and then the body converts to ketone energy sources.

  42. My wife and I (for the 10th time in 6 months) began eating primal again about 10 days ago. Using Ketostix (not sure of their reliability) we determined we were in ketosis for the last week or so. Usually takes me 3-4 days to get in to the state. We decided yesterday afternoon after reading Mark’s 2nd post on IF to try an 18 hour fast. So our last meal was around 8pm…woke up this morning…I did feel a bit empty and nauseated in the shower. So I had a big cup of black coffee with two dollops of coconut cream and a bit of Stevia and the discomfort soon passed. Anyway we worked from our home office all day without much thought of food and went on an hour walk at a good pace around 1pm (17 hour mark) and broke our fast at 2pm. We enjoyed some bacon, boiled eggs and fresh blackberries. I felt great all day, mentally clear and had quite a bit of energy. For dinner we had scoconut curried chicken and spaghetti squash around 8pm.

    So we’ve decided to keep this routine for as long as we can…does anyone have any constructive criticism to give? Is everyday too often if we are feeling good?

    Also, was there anything in that coffee that would impact my fast?

    1. I’ll add that it is our intention to not restrict our eating during that 2pm-8pm window. If we get hungry we eat…as long as we stay primal. Thoughts?

      1. I’d say it largely depends on how much you’re eating when you break your fast and if you’re trying to lose a lot to weight or not.

        If you’re calorie restricting too much too fast, it seems you usually bounce back. However, if you’re eating enough at dinner and you’re not getting hunger pains during the day, I can’t see it hurting. I personally know several people that eat only once a day – at dinner – and then several “bird eaters”, that munch all day. I think it largely depends on how you feel and there’s no right nor wrong answer.

        Personally, my eating is all over the place. Sometimes I’m in the groove at work and will go up to 36 hours on a cup of coffee and maybe some cream. Other days, I feel like I can’t eat enough.

        I guess I just mean to say that you’ll eventually reach homeostasis; how you do it is irrelevant, just don’t be afraid to try a several different things.

        The only wrong answer is expecting that there is one hard-and-fast way to do it 😀

  43. I’m still not at the point where I’m the best fast-er in the world. A lot of days I get off to a good start, but by mid-day, I’m starved. A lot of times I’ll steal like 10 dark chocolate covered peanuts from the candy bowl, and then be able to resume “fasting” for the rest of the day until my evening meal, but I still feel like I it wasn’t much of a fast then, in the long run 🙁

    1. Oh, that is of course I’ve had copious amounts of caffeine or caffeine for the first time in a few weeks. That seems to staunch my appetite for a while.

  44. I may be mistaken here and I am far from an expert, but I was always under the impression that if you were to fast your body effectively went into ‘starvation’ mode and would retain fat at a high level from the next meal you eat.

  45. Gary-

    I used to be on the “small, frequent meals” bandwagon because of its supposed “metabolic boost” and “starvation mode.” Science doesn’t back the claim, however. Short-term fasting actually increases metabolism and the earliest evidence for a decrease in metabolic rate is at 60 hours.

    Study: https://www.ncbi.nlm.nih.gov/pubmed/3661473

    Short-term fasting actually increases metabolism. This study showed a 3.6-10% increase in 36-48 hour fasting.

  46. I’ve tried fasting from 8:00pm to 3:00 the next day, but have never made it past 10:00am – I get too hungry. Should I just push on through that? Part of me hates being hungry but I know it’s just another sensation. Have people found that drinking alcohol the night before affects how hungry they are the next day and their ability to fast?

    1. Try fasting when you haven’t had alcohol the night before and have a good protein feed as your last meal. Hunger is not easy to deal with if you are not used to it but believe me, as I said above, if you can just stretch the fast out a little bit further and breathe through the discomfort you will feel better for trying. And don’t beat yourself up if you can’t. Just don’t give up trying. Good Luck

  47. I have become a fan of every other day calorie restriction; Dr Johnsons Alternate Day….

    I started prolonged fasting in the ’70’s (vegetarians seem to need it) but cannot handle it now.
    My stomach is in pain if I go too long.

    Enter alternate day CR.

    After I got the hang of it, it became easy and I still get the break from thinking about food.

  48. Oh, and if you are consuming and food/juice, that is not fasting….it is calorie restriction.

  49. Okay, so this fasting thing is looking more and more appealing. I am still confused on how to do it though. I was thinking the “eating window” type of fast looked the easiest… so do you have a restricted eating window everyday forever, or do you have it 1-2X a week? Also, how does this work for an exclusively breastfeeding mama?

  50. There is some evidence of longevity in monks, in particular, Carmalite monks and nuns who only eat one meal a day as part of their routine have a longer life expectancy than the general population. hough hat may also be related to stress free lives… if getting up an regular intervals during the night is stress free!

  51. I Have been on PB for 3 months. I did my first 36 hour fast which ended yesterday with supper. Dit not feel hungry at all, didnt get any energy dips. When supper came along I was as hungry at supper as before PB when I had Lunch

    Supper was a Club steak and salad, I was Stuffed, felt like I ate to much. It feels as if it was an appetite reset for me.

    Loved it.

  52. I stumbled upon your website as I was looking for information about what exactly was a “low-carb” lifestyle. Was is 50? 100? 300? And so I discovered your site. I have to say that I am now an avid reader of your site. I have a science background, and am currently a photographer by trade. I love how your articles are often backed by real scientific research…published research. I am skeptical, and like to find out all I can about a particular topic before I jump on the bandwagon. I also need to weigh if a particular “diet”/way of living is sustainable for me. With that said, I started on a lower carb path after Thanksgiving. I had come across the book “Wheat Belly”, and decided to “give it a try”. My appetite naturally decreased, I no longer has insatiable cravings for all things carbs, and I lost 15 pounds that first month, when all of my friends inevitably gained the same amount due to the holidays. Seeing the results firsthand, I decided to give it another month. I can happily say I have been wheat-free for 4 months now. It is no longer a diet, it is just how I eat. It is normal for me. I lost another 10 pounds, and have been in a plateau for weeks. (I am 5’4″ tall woman and currently weigh 208). I started adding walking exercise and more overall activity to my daily life, and I am starting to add aspects of Primal living to my life as well. I really like this idea of fasting. Over the years I have done several fasts for Christian religious reasons. The most recent was a “Daniel Fast” that lasted 3 weeks. I felt incredible while doing it. I like the idea in this article that fasting is not about starvation, but rather a way for your body to turn to burning it’s own fat. I am trying out an ADF method for the next few weeks. I lost 3 pounds in the first 3 days that I couldn’t seem to get past. I feel great, even on fasting days, and relly think I may incorporate this as a way to gain my health back. So thank you for your informative site!

  53. A very timely post! I have a question tho’. I have been following a very low carb diet, with the intention of becoming keto adapted, aka. – a fat burner. After several weeks of consistent very low carbs, reduced levels of ketones appearing pin the KetoStix and good energy levels… I think I have made it to keto adapted!!

    So, today I tried my very first IF. As per Mark’s suggestion on the website, I decided to try a compressed eating window. So, I ate my last snack of 2 TBSP of almond butter at 8 pm… then did not eat again until 11 am the next morning.

    Just for a lark I decided to test my blood glucose to see just how ‘in the basement’ ir would be after a 14 hour total fast. I was surprised to see my BG at 5.4 mmol/l. I am from Canada, so I believe this would translate to 97.2-ish. This seems high to me for a healthy person (no diagnosis of any problems, anyway).

    Is this an indication that my body may have created glucose on it’s own? Is this a concern? Any thoughts would be appreciated!


  54. Hmm, it seems like to me that I’ve been doing this for quite sometime. So technically if you are not eating breakfast than you are fasting.

    Since breakfast is in fact meant to break your fast during the time in which you’ve been sleeping.

    I generally don’t eat to around 1:00 or 2:00 the next day sometimes even later. So I definitely on average have a 12 hour break between meals.

  55. What about fasting before you are an adult? At what point would it be good to instill this sort of habit in children? Obviously growing and energy requirements are different through our teens…should something like this wait until your 20’s…or?

  56. Would eating only one meal a day, dinner, be considered fasting? Or does it have to span over 24 hours?

    For the last 6 months, I’ve stopped having a meal for breakfast & lunch… instead I usually juice some vegetables/fruits or mix some vegetable powder (ex. new chapter berry green) with some water. Dinner and the weekends is usually the only time I’m eating.

    I’ve lost 30 lbs in 6 months, and I honestly have gotten quite used to this pattern.

    1. I have been doing the same since Sept 2010 and for the past few months, all I eat is dinner. My mind is so clear and I am very alert during the day. You should read about the Warrior Diet.
      A fast is defined as zero calorie intake. So even though fruits/vegetables break your fast, its not a big deal as long as you dont consume too many calories (undereat/overeat cycles). Since u have been doing this for 6 months, you should be able to clearly distinguish when your body is in a fasted state versus not in a fasted state. If u feel the same in the morning after eating a couple of eggs and bacon vs. eating a fruit or two, I would say you are breaking your fast by eating the fruits

  57. “Interestingly, the fasted mice were heavier than the fed mice throughout the experiment, which indicates that calories weren’t significantly restricted.”

    This “fasted mice” were heavier than the fed mice were not noted in the previous experiments. It seems to me the difference was the four consecutive fasting days. Could this induce less activities (or lower metabolism) than the other fasting schedule?

  58. How does fasting play a role with metabolism? I believe my metabolism is slow because I routinely stop eating by 8pm and don’t eat again unti around 11am or later.
    I have been told that is a big mistake as my metabolic rate goes down (starvation mode). How do I increase my metabolic rate? I do work out daily. I walk or slow jog 1 mile 3 times a day and practice karate 4 times a week 2 hours.

  59. Has anyone tried carb back loading? Everything is scientifically referenced and for someone who would constantly feel tired on a more primal diet this has made a huge difference! A lot of similarities to low carb, IF but with gaining muscle as the main goal. Just looking for anyone who might have tried it.

  60. I eat not exactly one meal a day but refrain from food until late afternoon/evening and then eat over a two hour period. I have done this for many years along with occasional 4/5 day fast.
    I am 66yrs old very fit and active and have a satisfied 36yr old girlfriend.

  61. “…restricting one’s calories can be mind-numbing, miserable…” Its obvious to me you’ve never fasted a day in your life. I fast every 24 hours and I feel like superman during the time I don’t eat. Never a dull and miserable experience for me, and I’ve been at it for 3 months now! I’d recommend try it!