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

Why Fast? Part One – Weight Loss

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Let’s go to the research:

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

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

Conventional Wisdom strikes out again.

In closing…

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

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

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

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

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

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

Thanks for reading, everyone!

Here’s the entire series for easy reference:

Why Fast? Part One – Weight Loss

Why Fast? Part Two – Cancer

Why Fast? Part Three – Longevity

Why Fast? Part Four – Brain Health

Why Fast? Part Five – Exercise

Why Fast? Part Six – Choosing a Method

Why Fast? Part Seven – Q&A

Dear Mark: Women and Intermittent Fasting

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

    Rosa wrote on June 17th, 2012
  2. To maintain my weight, I need about 2500 calories. I have 4 little kids so I have found it very hard to get exercise in as I am always running around for being a mother. Anyway, I have been intermittent fasting…18/6 for 11 days and have lost about 7 lbs. I really don’t think it is water weight as I have not been eating grains or sugar for months. I have been tracking my calories for 3 days with fit day and have come up with an average of 1300 calories a day. My carbs an average of 60 a day. My question is… at this restriction, am I losing muscle? Will my metabolism slow down or has it? I feel quite fine energy wise, my appetite is depressed so I really don’t feel that hungry and even eating 1300 calories a day I find hard to do. Though I am sure I could reverse that with a big piece of chocolate cake. thanks for the advise.

    kathy wrote on June 22nd, 2012
  3. As some others stated above if you fast your metabolism will slow down and your body will go into kind of a hibernation state which causes you to start storing calories instead of burning them.

    Zeb Mullen wrote on June 24th, 2012
  4. Wow, mind=blown haha. I will try a day of fasting every once in a while and see how it works for me. If fasting just isn’t for you I know of an extremely simple diet that works. It consists of eating slow digestive carbs like wheat bread, whole grains, and oats. Add in lean meats like chicken, fish, and turkey. Add vegetables and fruit like spinach, broccoli, apples, blueberries, and oranges. I’m bad at explaining things in txt but basically eat fresh natural foods and stay away from highly processed foods that come in containers. Oh, and try to eat smaller meals 5-6 times a day.

    John Oxnard wrote on July 9th, 2012
  5. I want to show you my secret to my weight loss

    adella wrote on July 18th, 2012
  6. >Fasting increases the catecholamines,
    >both adrenaline (epinephrine) and
    >noradrenaline (norepinephrine).

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

    Anders Emil wrote on July 19th, 2012
  7. Interesting… from the article about 1 meal/day study:

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

    Anders Emil wrote on July 19th, 2012
  8. Hi Mark

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

    Shabnam wrote on July 22nd, 2012
  9. Managed a water fast for 19 days several years ago. My appetite dropped out after about three days making each day easier. Had interesting dreams about mountains of food. Have not seriously fasted since, but comments on this site make me want to try again even if only portions of the day. Have also found helpful weight loss tips at

    Anita B. wrote on July 24th, 2012
  10. No doubt fasting is one way to reduce and you have correctly dispelled some myths about it. However, I have some reservations as whether it work for diabetics. This can prove to be dangerous as sugar levels tend to drop or rise suddenly of fasting.

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

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

    I recommend you give them a fair try.

    Kishore Kowshik wrote on July 30th, 2012
  11. I don’t agree with you one bit. Isn’t it a fact that if you fast you gain weight instead of lose it? Go here to find out how you can truly lose weight best3diets dot com

    Vanessa wrote on August 18th, 2012
  12. love your post.I don’t know anything about the primal diet or IF.I am interesting in taking on this challenge.Can you provide me with more information on how to begin this journey.It sound fascinating.I am trying to loose some weight and increase my energy.

    Patricia wrote on September 3rd, 2012
  13. In 1997 I undertook a 40-day fast for spiritual reasons. I had absolutely no concept of Primal eating or lifestyle. After the fast I returned to normal carb-centric eating and put on all the weight very quickly, about 30 lbs. I repeated the fast every year for about six years. In those days i allowed myself the ocassional juice or starbucks latte if i was feeling weak. Here’s what I experienced physically (aside from my spiritual goals):
    1. It was was relatively easy.
    2. Hunger goes away the first or second day
    3. Counterintuitively, my energy levels would rise. I found that I wanted to run everywhere. Upstairs, outside (I was not a runner by any means and was still about 30lbs overweight)
    4. I slept better
    5. There was no decrease in mental sharpness (I design electronic controls for mission-critical applications)
    I gave it up for family-sociall reasons. Then in 2005 I felt “called” to fast again. This time I allowed myself 50 days to complete a 40-day fast. That is, I allowed myself 10 “eat-days” to accommodate family and social events. I liked this lifestyle so much that I repeated it every month from March until September. I lost 50lbs and never felt better.
    I thought that I had arrived, but now in retrospect I can see that I was a carb-addict and despite my marathons, and iron-man (v70.3) triathlon racing I was still a compulsive over-eater of carbs. Weight gain is just too easy for me when there is bread, potatoes, beer or pasta involved. To gain 30 lbs in a year is not a problem for me.
    Now, having researched and investigated and gained a much more solid understanding of paleo and strength training I have begun another long-term fast. I started 5 days ago and have allowed myself only tea (sorry, one vice I cannot live without) and lemon-water. I will inject “eat-days” full of meat and veggies every 8 days or so to make sure that the family & friends social structure is not greatly impacted.
    Bottom line, I never feel better than when I am

    Bob Tonner wrote on September 4th, 2012
  14. In addition to intermittent fasting I’ve found doing yoga has been a great way to help lose weight. It relaxes your body and mind. It can help with detoxification because being overweight may be a sign your body is high in toxicity. Yoga also helps release stuck emotions because being overweight usually has an emotional component that we need to be able to address.

    Jen wrote on September 9th, 2012
  15. Wondering about the effects of fasting and injury recovery. I have been living primal for roughly a month. I am an athlete, recovering from anterior tibial stress fractures. They are healed, and have been for several months, but I flare up the fracture site very quickly with activity. I’m being more aware of sleep, and trying to get more of it because I don’t think my body is recovering as completely as it could at night. My nutrition is good, and I’m feeling body wanting to fast, and this was not the case before going primal. Any thoughts on fasting and injuries?

    Katie wrote on September 9th, 2012
    • That’s an interesting thought Katie. A few years ago I had a shoulder operation to repaid a torn major pec muscle. I was using intermittant fasting during my rehab to manage weight and stay lean as no exercise was possible. Wish I had kept a diary on the condition during these fasted periods… perhaps the reduced inflammation aids recovery

      Luke M-Davies wrote on September 10th, 2012
  16. Wouldnt it be nice if someone actually did, or cited a research to finally put to rest, what burns when we fast – fat or muscle.

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

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

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

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

    Jazz wrote on October 3rd, 2012
  17. Do you know muslims spend an entire month fasting? It’s an obligation of Islam for the health of body and soul. Intermittent fasting is great for the soul too..

    Lila wrote on October 4th, 2012
  18. This is a cheaper & healthier way of losing weight. Am willin to try it out. Thanx

    Maru wrote on October 21st, 2012
  19. Im reading Vincent Ng’s book TEN, lean and sculpted body in ten weeks (anyone familiar with it?)

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

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

    Ronny Jansen wrote on October 29th, 2012
  20. Hello! Would you mind if I share your blog with my zynga group?
    There’s a lot of people that I think would really appreciate your content. Please let me know. Thanks

    Keith wrote on October 29th, 2012
  21. Please let me know if you’re looking for a writer for your weblog. You have some really great articles and I feel I would be a good asset. If you ever want to take some of the load off, I’d love to
    write some content for your blog in exchange for a link
    back to mine. Please send me an e-mail if interested. Thanks!

    Brock wrote on October 31st, 2012
  22. Wow, 382 days without food? 24 hour fast is good enough for me. Thanks for a great article on fasting.

    Wayne wrote on November 9th, 2012
  23. See the video I recently did about the ideal time period to fast for maximum fat-loss. Hope this helps some of you out! :-)

    Dean Bocari wrote on November 19th, 2012

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