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16 Feb

The Myriad Benefits of Intermittent Fasting

Calorie restriction is all the rage in anti-aging circles. A few mice and worm studies seem to show that drastic reductions in food intake over a long period of time have the effect of prolonging life – although I’m not sure I’d call it living. For one, these animals are actually restricted. There’s no ad libitum access to food. They’d prefer to eat more, but are prevented from doing so. I guarantee you they’re unhappy and, if they could put (cartoonish high-pitched) voice to physiological state, would say they’re starving.

Anyway, humans have picked up on this calorie restriction stuff. You might have seen one or two CR gurus giving TED talks; the exposed rib cages, gaunt faces, and complete lack of lean muscle mass are dead giveaways. Okay – that’s a bit unfair. CR is a legitimate way to improve many health markers. I’m just not convinced actively restricting your calorie intake through sheer will is the true path to enjoyable longevity. I’d rather be robust, vibrant, and full of zest. I want to eat big strapping meals of steak and veggies smothered in butter without counting calories. On occasion, I like to devour an entire roasted chicken (yeah, yeah, chicken has more omega-6 than most animals, but I’ll live). I want to maintain muscle mass and have enough energy to go on long hikes and have the legs to still leap for high passes (over the young guys) at the end of Ultimate games. And as I appreciate the neuroprotective and autophagy-promoting qualities of calorie restriction, I’d rather not expend the mental energy and fortitude required to maintain such a regimen day-in and day-out. Nor would I willingly subject myself to forced calorie restriction, a la the lab mice. No human-sized rat cages for me, even if they include a salt lick and a running wheel (don’t get any ideas, Blaisdell). Based on the science, I don’t think all that is necessary. I’ll just IF instead.

Fasting is one way to have your cake and eat it too. Beyond the already proven benefits of a Primal Blueprint low-carb lifestyle, fasting once in a while seems to offer many of the same benefits of calorie restriction – you know, stuff like increased longevity, neuroprotection, increased insulin sensitivity, stronger resistance to stress, some cool effects on endogenous hormone production, increased mental clarity, plus more – but without the active, agonizing restriction. You just eat Primally, focusing on meat and vegetables with plenty of animal fat, and skip meals on occasion. A sixteen-hour fast is on the low-but-still-effective end (you could easily maintain a daily sixteen hour IF regimen, giving you an eight hour eating window, a la Leangains), or you could opt for longer, more intermittent fasts – say, a full twenty-four hours once or twice a week. When you’re done with the fast, eat as much as you want (which usually isn’t an issue, once you’re PB-adapted). It essentially turns into “eat when you’re hungry,” because let’s face it: eating the types of foods we evolved eating induces powerful satiety and makes eating the right amount of food a subconscious act. Fasting becomes a whole lot easier (and intuitive) when you’ve got your food quality dialed in. And I’ll come back to that little caveat at the end here.

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


Everyone wants to live longer, but I find longevity pointless if you’re not enjoying yourself. Otherwise, life becomes dreary. Consider the plight of the vampire – he lives a long, lonely life, never really connecting with anyone, never really enjoying all the time he has at his disposal. If the suggested longevity benefits of fasting pan out, I think we can enjoy the vampire’s longevity (and perhaps even some of his incredibly immune and restorative properties) without the downsides. Oh, and we’ll eventually have to die, but we have the option of sunbathing.

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

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

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

Blood Lipids

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

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

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

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


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

The obese (read: the ones who need the most help and often have the worst time sticking to a diet) were able to “quickly adapt” to alternate day modified fasting, which meant on fasting days they’d get 26% of their normal caloric intake. They were also able to maintain physical activity despite the fasting.

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


The notion of IF reducing cancer incidence and improving survival is compelling, but little evidence in humans exists. Ketogenic diets may also offer exciting potential for cancer patients, and both IF diets and ketogenic diets share something: fat (either dietary or from your own adipose tissue) as primary fuel sources. But, while ketosis isn’t exactly desirable or optimal as a lifelong dietary regimen, IF is sustainable, simple, and can be integrated into your current diet. As of now, most of the evidence for IF’s protective effects against cancer exist in animal trials, mostly using mice. Still, fasting seems to confer so many other benefits that working it into your life for its anti-cancer potential is probably worth it. Some of the evidence:

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

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

Growth Hormone

Aging humans “normally” experience reductions in growth hormone. While it’s true that unchecked growth hormone can lead to unwanted cell proliferation (like, ya know, cancer), growth hormone therapy can really help stave off the doldrums of old age. Luckily, short-term fasting induces growth hormone secretion in “normal men.” I’m not for mainlining GH or anything, but I’m all for amping up my own production. Furthermore, a recent study found that resistance training actually blunted hunger for an hour. I’ve found this to be the case for me. If the body “needs” food right after a workout, why would hunger be blunted? This is why I tend to hold off on the eating post-workout. Every little bit helps, especially as you age.

Neurological Health

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


Fasting turns on autophagy (most studies nowadays treat this as common knowledge), which is the process by which cells recycle waste material, eliminate or downregulate wasteful processes, and repair themselves. Why is autophagy so important? It’s required to maintain muscle mass, and inhibiting it induces atrophy of adult skeletal muscle.  It reduces the negative effects of aging and reduces the incidence and progression of aging-related diseases. In fact, researchers have determined that autophagy is the essential aspect of the anti-aging mechanism of fasting. Without the autophagy that fasting provides, you would get very few of the benefits. Fasting even increases neuronal autophagy, which aids in maintaining mental health and function. Short term fasting, too. No marathon thirty-six hour fast required.


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

Mental Well-being and Clarity

You’ve heard this “advice” before, probably from an expert dietitian in some copy-and-paste article on healthy living on Yahoo! Health. It’s the mantra of the high carb crowd: failure to eat something every few hours will cause mental fog and sluggishness, so keep a banana or a granola bar on your person at all times. Of course, this is all based on an assumption that we need to supply exogenous carbs on a regular basis to properly fuel the brain. This notion that fasting is only the province of anorexics or “those crazy caveman dieters” has kept many people from experiencing the myriad benefits.

I maintain that one’s comfort in handling IF effortlessly does increase dramatically when you’ve reprogrammed those cells (and genes) to predispose your body to derive most of your day-to-day energy from fat, as opposed to constantly dipping into glycogen stores (as happens when we rely so much on refeeding carbs every few hours). It’s also why I recommend that you eat according to the PB for at least three weeks before you really start incorporating IF on a regular basis. I’ll be discussing this detail more in upcoming posts on IF and what I am calling “the Metabolic Paradigm Shift”.

Overall, fasting just seems right. It’s like a reset button for your entire body, presumably across a large spectrum of maladies and dysfunctions. It puts your body into repair mode – at the cellular level – and it can restore normal hormonal function in the obese or overweight. Now, you don’t have to fast, but it’s definitely something to consider. As I said, I don’t plan my IFs – I let them happen. I prefer to fast when it’s forced upon me, like when I’m traveling or under a deadline. Once you’re acclimated to the Primal Blueprint diet, give it a shot and report back. Obviously, you won’t know if you’re fighting budding cancer cells or turning on autophagy in your brain, but if you can tolerate fasting and even derive some subjective benefits, those potential long-term benefits make it a worthwhile addition.

Have you tried IFing yet? Did you have a great, transcendent, middling, or perhaps even awful experience? Let me know how intermittent fasting has worked – or hasn’t – with your lifestyle in the comment section!

UPDATE: See this post on 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. Also:

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

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

    Suvetar wrote on April 12th, 2011
  2. I’ve been doing IF for about a week now (took the weekend off, so I guess about 6-7 days total).

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

    Then I just eat a normal-sized dinner.

    How do I feel?

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

    Justin wrote on June 2nd, 2011
  3. I’m really happy to have stumbled onto this topic. Ever since I was in high school, I would inadvertently fast after so many days of “regular” eating. It came naturally so I guess I consider myself lucky. I wake up in the morning. Have some water and find myself not eating until 8:00pm. Other days when I fast, I wake up, have a couple of eggs with broccoli and then not eat until late that evening. I did this even before I went primal. I fasted by accident and my friends would tell me how bad it was. But when I fasted, it was because I literally didn’t feel the need for food or I was too busy to eat and didn’t feel hungry. I feel it’s a mind game. If our ancestors didn’t have a good hunt, they knew they’d have to go with what they had. I feel it’s very mental.
    My body just felt like it needed a cleanse. So I just, didn’t eat. The results are seriously remarkable. I would have many bowel movements (sorry!), I would hydrate more than usual, my body just felt like a million bucks! Then, I would eat a great meal after this respite, and swear i could literally feel my body taking in all those vitamins and yummy ‘good for you foods.’ I can’t say enough good things about fasting. We’re a society of plenty but I honestly think our bodies sometimes, need a break to process all of that :)

    Joo wrote on June 17th, 2011
  4. I’ve only recently discovered that this is how I’ve been eating almost my whole life, and most of my adult life. I didn’t know there was a word for it, though. I just don’t eat if I’m not hungry. I cannot believe this is a big deal.

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

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

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

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

    Janis wrote on July 21st, 2011
  5. I haven’t fasted in a while so thanks for highlighting the benefits. But I find it very difficult to fast for long, eating every 3-4 hours keeps me going. Maybe if I have a very heavy breakfast then I can go for 6-7 hours but then Ineed to eat.

    Suresh wrote on August 2nd, 2011
  6. Hi there all! New to The Primal Blueprint and IF…I am loosing weight and the hubby needs to, plus he is diabetic…So my question is, Mark or anyone in the know…Can anyone tell me the effects of IF on diabetics with regards to taking their medication 3 times daily with a medication

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

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

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

    Look forward to a reply and thanks in advance Yolanda

    Yolanda wrote on October 25th, 2011
  7. I like the helpful info you provide for your articles. I’ll bookmark your weblog and take a look at again right here regularly. I’m rather certain I will be told lots of new stuff proper right here! Best of luck for the following!

    diet smart wrote on October 30th, 2011
  8. Here’s my testament to the powers of eating primally (OK, I am on Perfect Health Diet not strict Paleo but it is very similar)– I fast for Yom Kippur (no food or water, about 25 hours total. Previous years I dragged myself through the final hours of the service and barely made it to the break fast. This year, I felt energetic and alive all the way to the end, and easily drove the 15 minutes to the break fast. What a difference! I attribute it to this way of eating, along with 16 hour fasts once or twice a week. I haven’t even done full day fasts (except for YK). Awesome! (Pun intended…)

    Pia wrote on November 2nd, 2011
  9. While science tries to develop a drug , we have intermittent fasting (or calorie restriction) now to accomplish this!

    deb b wrote on November 3rd, 2011
  10. Hello – thank you for your website’s information. I know there are plenty of cultures throughout the world that integrate fasting. Thought I’d try the intermittent fasting. I have just finished the first 24 hour and plan to do the alternate day IF. I am feeling much better today and am surprised that I am not hungry. Although I have been filling up on crystal light diet drinks, water. I will keep you updated. I actually feel fantastic!

    Liza wrote on November 8th, 2011
  11. What is the thoughts of IF when breastfeeding? My intuition is that it would not be helpful for milk production. I plan to breastfeed for 12-13 months as i did with my other 2 boys but I am keen to try IFing to help shift the baby fat.

    Berrylicious wrote on December 1st, 2011
    • You can get all of the fat-burning and growth hormone benefits of IF without danger to your breastfeeding by doing a cyclic ketogenic ultra-low-carb diet such as Carb Nite (the one I used).

      Naomi Most wrote on March 7th, 2012
  12. I tend to fast every few months or after I’ve had a stint of particularly poor dietary habits for the simple purpose of detoxing. If my stomach has been dodgy, if I’ve felt sluggish, I find that a 24 hour fast is beneficial to getting me back on my feet.

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

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

    Tessa wrote on January 27th, 2012

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