Marks Daily Apple
Serving up health and fitness insights (daily, of course) with a side of irreverence.
27 Mar

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

You want comments? We got comments:

Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

  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.

    Paul Alexander wrote on March 27th, 2012
    • 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.

      Mike wrote on March 27th, 2012
      • 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.

        Paul Alexander wrote on March 27th, 2012
        • 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.

          Mike wrote on March 28th, 2012
  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?

    leida wrote on March 27th, 2012
    • crazy trivia. (psych pathways lead to difficulty in resumption, brain size has nothing to do with brain use, etc.)

      Anna wrote on March 27th, 2012
    • 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.

      Michael wrote on March 28th, 2012
    • 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.

      Tom wrote on March 28th, 2012
    • 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.

      brett wrote on March 28th, 2012
    • 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.

      Food Scientist wrote on May 18th, 2014
  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!

    Grokitmus Primal wrote on March 27th, 2012
    • 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?

      Primal Toad wrote on March 27th, 2012
      • 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?

        Primal Texas wrote on March 27th, 2012
        • 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.

          Daniel Wallen wrote on March 27th, 2012
        • I can also eat a lot over say a 2 to 3 hour period. I did this recently and it was awesome.

          Primal Toad wrote on March 28th, 2012
      • 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

        Sagar wrote on March 27th, 2012
      • 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.

        Food Scientist wrote on May 18th, 2014
    • 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.

      John wrote on March 27th, 2012
    • 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.

      Jennifer wrote on March 27th, 2012
  4. I’m hungry 😉

    smokyjoe wrote on March 27th, 2012
  5. I have been reading about reverse t3 from very low carb diets and calorie restriction. Anybody know how intermittent fasting affects thyroid function?

    Susan wrote on March 27th, 2012
  6. Great article, thank you!

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

    RedBear wrote on March 27th, 2012
  7. 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…

    Alison Golden wrote on March 27th, 2012
    • 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. :)

      Kim wrote on March 27th, 2012
      • 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?

        Susannah wrote on March 27th, 2012
      • Joint issues and brain fog? That sounds familiar, are you a former Fibro sufferer?

        FoCo wrote on March 27th, 2012
      • 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.

        Alex wrote on March 28th, 2012
  8. 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!

    FoCo wrote on March 27th, 2012
    • 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.

      Kim wrote on March 27th, 2012
  9. 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.


    Chris wrote on March 27th, 2012
    • 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.

      buffdv wrote on March 27th, 2012
      • 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.

        Craig wrote on March 27th, 2012
    • 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.

      David Cole wrote on March 27th, 2012
    • 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.

      FoCo wrote on March 27th, 2012
    • 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.

      Matthew Caton wrote on March 28th, 2012
  10. i think it should be mentioned that with a ketogenic VLC diet a lot of the same mechanisms of fasting are at work without some of the possible muscle breakdown… Dr. Michael Eades gives a good breakdown of this at

    Jake wrote on March 27th, 2012
    • 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.

      Matthew Caton wrote on March 28th, 2012
      • 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.

        Joshua wrote on March 28th, 2012
      • 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.

        Jake wrote on March 28th, 2012
        • 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.

          Jake wrote on March 28th, 2012
    • 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. :)

      Craig wrote on March 29th, 2012
  11. 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.

    Nikhil Thomas wrote on March 27th, 2012
  12. 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.

    buffdv wrote on March 27th, 2012
    • The freedom from meal times and constantly HAVING to eat is very liberating.

      Matthew Caton wrote on March 28th, 2012
  13. 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.

    Ed Dudley wrote on March 27th, 2012
    • True that, Warrior since Sep 2010.
      Ori is waay ahead of everyone when it comes to health and fitness

      Sagar wrote on March 27th, 2012
    • 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.

      Elle W. wrote on March 27th, 2012
  14. Eating 2 meals during a 6 hour window from 1PM-7PM works for me. I try to do it 5-6 days per week.

    Steve wrote on March 27th, 2012
    • 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.

      Primal Toad wrote on March 27th, 2012
      • 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?

        Chris wrote on March 27th, 2012
        • In interested in this too.

          SophieE wrote on March 27th, 2012
        • Im interested in this too.

          SophieE wrote on March 27th, 2012
  15. “The fewer things you have trying to kill you, the longer you generally live.”

    Quote of the day. Love it.

    Stefanie wrote on March 27th, 2012
  16. 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.

    Brian Carr wrote on March 27th, 2012
  17. 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.

    Patrick wrote on March 27th, 2012
  18. 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.

    Tammy wrote on March 27th, 2012
  19. I’d be interested to know what type of fasting is the most effective. How long should the fasting period be, ideally?

    Frygal wrote on March 27th, 2012
    • 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.

      einstein wrote on March 27th, 2012
    • If you are an athlete, you shouldn’t push a fast past 24 hours on a regular basis.

      Matthew Caton wrote on March 28th, 2012
  20. I’m 2 days into my 5 day fast so far. So far so good.

    James wrote on March 27th, 2012
    • 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.

      Kim wrote on March 27th, 2012
      • 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 :-)

        James wrote on March 28th, 2012
  21. 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.

    Happycyclegirl wrote on March 27th, 2012
  22. 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!

    Lynn wrote on March 27th, 2012
  23. 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” ( 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!

    KRP wrote on March 27th, 2012
  24. “The fewer things you have trying to kill you, the longer you generally live.”


    Ashley North wrote on March 27th, 2012
    • I agree with this wholeheartedly! I want a MDA shirt, but I am not huge on Grok. This quote with underneath would be awesome.

      Darby wrote on March 28th, 2012
  25. So what is the recommended IF schedule. Once or Twice a week for a 24 hr period?

    Perry wrote on March 27th, 2012
    • 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.

      Matthew Caton wrote on March 28th, 2012
  26. 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?

    Ron wrote on March 27th, 2012
    • 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.

      FoCo wrote on March 27th, 2012
    • 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.

      Matthew Caton wrote on March 28th, 2012
  27. 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? :)

    Erik wrote on March 27th, 2012
    • My cat crossfits and eats paleo but I can’t get her to fast. Sorry.

      Craig wrote on March 27th, 2012
      • Best comment ever.

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

        Merryish wrote on March 28th, 2012
        • I hear it’s not good to make cats fast because it could damage their liver in some way..

          Rachel wrote on March 30th, 2012
    • 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.

      Matt wrote on March 27th, 2012
  28. 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?

    Amy wrote on March 27th, 2012
    • 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.

      Daniel Wallen wrote on March 27th, 2012
      • 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.

        Carol wrote on March 27th, 2012
        • 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.

          Amy wrote on March 28th, 2012
  29. 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!

    Daniel Wallen wrote on March 27th, 2012
  30. 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.

    Lindsay wrote on March 27th, 2012
    • 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.

      TruckerLady wrote on March 27th, 2012
    • 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.”

      Daniel Wallen wrote on March 28th, 2012
      • P.S., apologies for misspelling your name there. I hate when I do that and it won’t let me edit it. :-/

        Daniel Wallen wrote on March 28th, 2012
  31. 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?

    TruckerLady wrote on March 27th, 2012
  32. 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?

    Albert wrote on March 27th, 2012
    • 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.)

      Michael wrote on March 28th, 2012
  33. 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.

    Emily wrote on March 27th, 2012
    • 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 😛

      Michael wrote on March 28th, 2012
  34. “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.

    Arty wrote on March 27th, 2012
    • 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.

      Michael wrote on March 28th, 2012
    • 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.

      Daniel Wallen wrote on March 28th, 2012
  35. 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?

    OJ wrote on March 27th, 2012
  36. 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.

    Danielle wrote on March 27th, 2012
    • 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! 😀

      Michael wrote on March 28th, 2012
  37. 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.

    Derek wrote on March 27th, 2012
  38. 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!

    Bruce Bowen wrote on March 27th, 2012
    • 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).

      Paul wrote on March 28th, 2012
    • Exercise of your fat metabolism.

      Victor Venema wrote on March 28th, 2012
  39. Nice post! I don’t think I have enough willpower to fast though

    Austin wrote on March 27th, 2012

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