Fasting and growth hormone

Fasting to build muscle?

Could fasting help build muscle and unleash anti-aging properties, via the release of human growth hormone?

The physiology of fasting is fascinating. The power of fasting lies not in the mere reduction of calories, but the beneficial hormonal changes. One of the main benefits comes from reducing insulin, but there are also increases in nor-adrenalin, cortisol, and growth hormone.

Collectively, these are known as the counter-regulatory hormones, since they all serve to increase blood glucose at a time that the body is not getting glucose from food. Here we focus on human growth hormone (HGH).

Human growth hormone

boy-growingHGH is a hormone made by the pituitary gland (the master gland), which plays a huge role in the normal development of children and adolescents as the name implies. However, it also plays a role in adults. HGH deficiency in adults leads to higher levels of body fat, lower lean body mass (sarcopenia) and decreased bone mass (osteopenia).

Once released by the pituitary gland, HGH only lasts a few minutes in the bloodstream. It goes to the liver for metabolism, where it is converted into a number of other growth factors, the most important of which is Insulin-Like Growth Factor 1 (IGF1).

This is the same IGF1 that is connected to high insulin levels and many poor health outcomes, but remember, that this brief pulse of IGF1 from HGH lasts a few minutes at most. Most hormones are secreted naturally in brief bursts to prevent the development of resistance, which usually requires both high levels and persistence of those levels (this is indeed how insulin resistance develops).

Scientists first harvested HGH from cadavers in the 1950s (eeewww), but only synthesized it in labs in the early 1980s. Soon afterwards, it became a popular performance enhancing drug. Normal levels of HGH peak in puberty (as you might expect) and gradually decrease thereafter.

Growth hormone is typically secreted during sleep and is one of the so-called counter-regulatory hormones. HGH along with cortisol and adrenalin increases blood glucose by breaking down glycogen – so it counters the effect of insulin, hence the name counter-regulatory hormone. These hormones are typically secreted in a pulse just before waking (4 am or so) during the ‘counter-regulatory surge’. This is normal and is meant to get the body ready for the upcoming day by pushing some glucose out of storage and into the blood where it is available for energy.

When people say that you ‘must’ eat breakfast to have energy for the day, they are simply wrong. Your body has already given you a big shot of the good stuff and fueled you up for the day ahead. You don’t need to eat sugary cereals and toast with jam to have energy. This is also the reason why hunger frequently is lowest first thing in the morning (8 am) even though you have not eaten for 12 hours or so.

HGH for anti-aging and building muscle

HGH typically goes down with age and abnormally low levels may lead to lower muscle and bone mass. So, what are the effects of giving HGH in older people with very low levels? This was studied in 1990 in a New England Journal of Medicine article.

untitled-2Group 1 is the HGH group and Group 2 is the control group (no HGH). Over 6 months, weight overall did not change between the two groups. But look at the lean body mass!

The HGH group packed on 3.7 kg (8.8%) more lean mass. That’s 8 pounds of lean mass! Fat mass decreased an extra 2.4 kg (5.3 pounds)! Thats a decrease of 14.2%. Even the skin thickness improved. Whoa, nelly. Loss of fat and gain of lean mass (muscle, bone and skin). That’s anti-aging, baby!

In a 2002 JAMA article, similar results were obtained in women as well. Sounds pretty great. So, why aren’t we using it for everybody? Well, there’s a little thing called side effects. Remember that this study only used it for people with very low HGH levels, not in people with normal levels.

untitled-3There was an increase in blood sugars. This makes sense, since HGH is a counter-regulatory hormone. Pre-diabetes also significantly increased. There was an increase in fluid retention as well as blood pressure, too. Over the long term, there is also a theoretical risk of increased prostate cancer and heart problems (enlarged heart). So, that’s not very good news.

So artificial injections of HGH are out. What if there is an all-natural method of increasing growth hormone? What about, say, fasting?

Fasting to increase growth hormone

In 1982, Kerndt et al published a study of a single patient who decided to undergo a 40-day fast for religious purposes. Glucose goes down. From 96 initially, it drops to 56. Insulin goes way, way down. Starting at 13.5, it quickly drops to 2.91 and stays down. That is almost an 80% drop! If you are concerned about a disease such as type 2 diabetes, with hyperinsulinemia, fasting is an effective way to bring down those sky high insulin levels.

But our concern here is HGH. It starts at 0.73 and peaks at 9.86. That is a 1,250% increase in growth hormone. A shorter 5 day fast gives a 300% increase. All this HGH increases without drugs.

fasting-gh2What about the potential side effects? Increased glucose? Nope. Increased blood pressure? Nope. Higher risk of cancer? Hardly.

Other studies have shown the same increase in growth hormone. In 1988, Ho KY et al studied fasting and HGH. On the control day, you can see that meals (marked M) very effectively suppress HGH secretion. This is to be expected. Like cortisol, HGH increases glucose and thus is suppressed during feeding.

Fasting is a great stimulus to HGH secretion. During fasting, there is the spike in the early morning, but there is regular secretion throughout the day as well. Hartman et al also showed a 5 fold increase in HGH in response to a 2 day fast.

This HGH likely helps in the maintenance of lean mass – both muscle and bone. One of the major concerns about fasting is the loss of lean mass. Some people claim that fasting a single day causes loss of ¼ pound of muscle. Studies prove that this does not occur. In fact, the opposite can happen. In comparing caloric reduction diets to fasting, the short term fasting was 4 times better at preserving lean mass! Think about this for a second.

Let’s imagine that we are living in Paleolithic times. During the summer of plenty, we eat lots of food and store some of that as fat on our body. Now it is winter, and there is nothing to eat. What do you suppose our body does? Should we start burning our muscle while preserving our stored food (fat)? That doesn’t make much evolutionary sense.

It’s as if you store firewood for a wood-burning oven. You pack lots of firewood away in your storage unit. In fact, you have so much, it is spilling out all over your house and you don’t even have enough room for all the wood you’ve stored. But when the time comes to start up the oven, you immediately chop up your sofa and throw that into the oven.

The logical thing to do is to start burning the stored wood. In the case of the body, we start to burn the stored food (fat stores) instead of burning muscle. Some protein is catabolized for gluconeogenesis, but the increase in HGH maintains lean mass during fasting (however, this is likely different for those without excess fat stores and they may experience a greater loss of lean body mass with fasting).

Implications for athletes

This has enormous implications for athletes. This is called ‘training in the fasted state’. Increased nor-adrenalin from fasting may pump you up to train harder. At the same time, the elevated HGH stimulated by fasting should increase muscle mass and make recovery from a workout easier and faster. This could be an important advantage in elite level athletes, and we are seeing more and more interest in doing this exact sort of protocol, although high quality studies are lacking.

Brad PilonIt is not by accident that many of the early proponents of training in the fasted state are bodybuilders. This is a sport that demands, in particular, high intensity training and extremely low body fat for definition.

For example, Brad Pilon, who wrote the book “Eat, Stop, Eat” is a bodybuilder, as is Martin Berkhan, who popularized the ‘lean gains’ method of fasting. Somehow, I don’t think that fasting for these two fellows was ‘eating’ their muscle.

Martin BerkhanSo, for all those people who thought that fasting would make you tired, or that you could not exercise during fasting, well, hopefully you now see a different perspective. Fasting will not ‘burn’ muscle when there is adequate excess fat to burn.

Rather, fasting has the potential to increase the beneficial effects of HGH without any of the problems of excessive HGH (prostate cancer, increased blood sugar, increased blood pressure). For those interested in athletic performance, the benefits are potentially even greater.

So, let’s see. When done right, intermittent fasting can help: Train harder. Lose weight. Faster recovery. Decrease insulin and insulin resistance. Decrease blood sugars. All of these benefits are achieved without drugs, supplements or cost. Yes, like all the best things in life, it’s free. So why is everybody so against it?

Jason Fung


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  1. Eric
    How much is achieved with 16 hour daily fasting? 5 day fasting mimicking diet?
    Combination? Ffd days 1-5 and eight hour the rest of the month.
  2. Christopher
    What is the raw material that HGH works with to build new protein to replace muscle catabolized for gluconeogenesis?
  3. Thomas Fry
    Dr. Fung,

    Wonderful article on fasting!. Dr. Fung, you are on a great roll against the unbelievable destruction by the modern scourge of Type II diabetes and the other likely related and similarly caused diseases of modern civilization, i.e., obesity, cancer, osteoporosis, Alzheimers, many neurological and behavior diseases, etc. I nominate you and Gary Taubes for the Nobel prize in medicine. Keep it up for the world is bound to eventually wake up and see that what you are saying is obvious. There are of course others saying the same thing, but you are doing it in a way that people can understand and you pull no punches toward and seem to have no fear of the deeply corrupt, nutritionally ignorant and misguided drug pushing for every problem medical establishment. God speed to you Dr. Fung and thank you for what you do.

  4. dom
    What about salt, is it safe, do you need to take salt, especially if you fast for 3 or more days?
    Reply: #5
  5. Johan

    Salt is safe:

    My experience is that I need to take some salt when fasting to avoid headaches, and dr. Fung mentions it:


  6. Tamarah
    I have been fasting daily for the whole month of October in and effort to drop body Fat. I normally do 18-6 window,, my results were a 3.5% loss of BF I was amazed at these results and very satisfied . When not fasting I eat a LC diet , drink tons of water . I work out daily doing HIIT or Lifting and walking in a fasted state .. always . I sleep like a rock,, my hair grows at a rapid pace , my energy level is the best its ever been, My moods are always positive . And ZI love the lifestyle of IF. I read Dr. Fung's articles as soon as they hit the website . I love the IF lifestyle and promote it to everyone who will listen. Thank You Dr. Fung for all your research and information you provide on this subject,, it has changed my life for sure . Please do not stop informing us
    Reply: #28
  7. Graham
    A very interesting article. I am type 2 diabetic on a LCHF diet (50-100gms). What sort of IF program would you recommend as a long term way of life?
  8. Don
    Very informative article! I'm wondering how well very low calroic intake affects HGH compared to complete fasting? For example what if I just drank wheatgrass juice in the morning. Is that considerable enough to breaking my fast?
  9. Kate
    Thank you so much for the article. I am really into this topic and also do intermittent fasting every day, eat huge amounts of veggies and no meat, dairy and gluten. So when we are on a fast our human growth hormone increases a lot because we lack glucose. HGH is known for its anti aging and anti inflammatory properties and also protects our muscles and proteins. I also feel much better when skipping breakfast and doing my work out in a fasted state... But on the other hand IGF 1 (insulin like growth factor) is produced out of HGH in the liver. IGF1 really promotes cell growth and is linked to cancer... So here is what confuses me a lot! I do not understand why so many doctors and researchers reccomend high levels of HGH for anti aging and anti cancer and stuff, but on the other hand low levels of IGF1. Is this even possible?? For me it is clear that fasting is healthy because our body does not need to put energy in metabolism and also less free radicals are produced and insulin and blood glucose is low.. But my major concern is IGF1.. It would be so great if you explained this in a scientific way.
    Thank you so so much!
    Reply: #17
  10. Fro
    Same question concerning igf1 and cancer
  11. Nick Anderson
    The key to minimized aging is a healthy balance of HGH/IGF1. Both extremes are harmful: low levels result in a generally weak body, and high levels result in increased risk for overgrowth (ie cancer) and increased wear of physiological/metabolic pathways.

    I recommend reading here for a deeper analysis of this question:

  12. Dom
    If you workout during your eating window while doing intermittent fasting, does the HGH increase the next day support the muscle repair from the previous day's workout?
  13. Angela
    I did a 30-day water only fast at a water fasting resort, and I lost so much muscle and overall weight. I was thinner but very flabby and with loose skin. I was also extremely exhausted and slept a lot. Headaches, constipation, and insomnia (waking up at night) were a daily experience. How can you say you don't lose muscle mass on an extended fast? That's just not true. Everyone at the resort experienced the same as I did. When I got home after the month I had to start working out again because everything, and especially my butt, was sagging, with absolutely no muscle tone. My family said I looked thinner but unhealthy. Please explain how you think muscle mass is maintained on a water fast. And you can't work out while on a fast, with no food intake. Your cortisol would spike and your adrenals would be exhausted... Besides being too exhausted to workout to begin with.
    Reply: #44
  14. sliyah
    You should have done a real fast (dry fast) not water only "fast" which disolve everything in your body and put it out of balance.
  15. Rebecca
    I started fasting, well this is week 5. I chose to do 5/2 - 24 hours broken with a normal meal. I wanted to tap the benefits of increased growth hormone and also getting my insulin down. I knew it would take a while as I was pre-diabetic. Week 4 I felt the difference. I have lost 10 lbs. I have a scale that does body composition. My lean mass has gone up just a bit and my body fat % has gone down by 3.5%. I'm stoked. I have the metabolic syndrome and struggled because I am fodmap sensitive. I was doing low glycemic index a few years ago and did great, then fodmap killed me. I gained back all the weight. Now I have found some slowly digestible carbs I can eat, and some veges too. I am trying to minimize carbs though, doing the high fat/protein as best I can. I exercise every day already, and am back to weight lifting. I have lost about 20 inches, 3 from my waist!
  16. Gabor
    I started keto 18 months ago. In the first 9 months, I lost 17 kilos (from 95 to 78) and my fat percentage dropped from 30% to 22%. Without any change in diet, the process stopped for 5 months. Then I started fasting. Since then my weight oscillates -6kg (down to 72) during fasting and then +6 kg back. My weight is stagnating since then and my fat content is back at 24%. My BMI is still 26. Something is horribly wrong. My body composition keeps worsening, my weight is stagnating on keto + intermittent + 8 day long fastings. My keto tests are always excellent! Suggestions?
  17. Renate
    Hi Kate,
    I am curious if you would share what your daily/weekly/ monthly meals and fasting schedule look like. We are already somewhat vegetarian except that we do eat eggs, cheese and seafood - my husband eats almost no dairy but we have it mostly for our children. I would like to move away from animal proteins and we need to try gluten free. Do you have children? If so, do they participate in an intermittent fasting schedule too? Thank you in advance if you are able to share!
  18. Bunny
    I lost 70 pounds using Fung's suggestions in his books. I feel great, and look 15 years younger. I am thrilled. Best of all I haven't gained any of it back in the past year.
  19. 1 comment removed
  20. Darian S
    There is a study I hear shows a 2000% increase in growth hormone in men and 1300% increase in women after 24 hour fast. 2000% is 20 fold increase. Might it be that it rises rapidly to 2000%, then decreases and starts to rise throughout the fast? Or might it be different measuring methodologies?
  21. Michael

    In your article you state that:
    "Some people claim that fasting a single day causes loss of ¼ pound of muscle. Studies prove that this does not occur."

    However, looking at the study you yourself provided, the numbers state very clearly that while fat loss was the same in the caloric restriction (CR) group vs. the alternative day fasting group (ADF), the ADF group lost more lean mass. This does not support your claim and if anything, it disproves it.

    Fat mass lost in 8 weeks: CR (3.7kg), ADF (3.7kg)
    Lean mass lost in 8 weeks: CR (2.6kg), ADF (3.2kg)

    Before your post a study to prove a point, you should read the study and check that it actually does prove your point!

    Kind regards,
    Michael Kohlberger, B.Sc. M.Sc.

    Reply: #22
  22. Michael Kohlberger, B.Sc. M.Sc.
    Furthermore, I just did the math:

    In an 8 week alternate day fasting period, there are a total of 56 days, thus 28 days of fasting.

    Participants lost 3.2kg of lean mass in 28 days of fasting.

    3.2kg divided by 28 days makes 114g lean mass lost per day of fasting.

    This amount is equal to 0.251 pound, proving that you do indeed lose 1/4 of a pound of muscle mass during one day of fasting.

    Replies: #24, #33
  23. Alan K
    I FAST THE FIRST 5 DAYS OF EVERY NEW MONTH and 10 days ever quarter of the year.
    The results have been nothing but remarkable across the board-
  24. Casey
    Good point. I think to for it to be a benefit you would have to fast for 3 days at least while working out.
  25. Carol
    Michael, Dr Fung and Zoe Harcome have explained this before. The human body is not a steam engine/toaster normal rules of number crunching don't apply. I.e. 4-2 does not always =2 when hormones get involved. I think with autophagy, the body is doing some tricky stuff that science will always struggle to breaking down the junk and building up the good stuff. The reports of People who are doing this do not match the hard data crunching. As post menopausal women in our 60's, my gym buddy and I have got stronger, healthier, more robust using Dr Fungs and other advice we find on this site. This is unheard of in the normal world. Our strategies have evolved and now we have settled mostly on one or two mealS a day. So we don't really care about your sums adding up. Go Jason!!
    Reply: #26
  26. Michael
    Hello Carol,

    Thank you for your reply. I believe that I can help you with explaining how and why this works. The reason is simple: exercise. Exercise makes all the difference between losing or gaining lean mass. The studies that were presented in this article, as far as I have understood it, were not conducted with athletes, but with average people who wanted to lose weight. This is why the numbers look like they do.

    Still, the point remains that the studies presented here showed that the claim that the author was trying to revoke was actually true. That was what I wanted to point out.

    Furthermore, even though the human body is not a toaster, rules of number crunching absolutely do apply in the long term. Even if variations are seen in the short term (think days) calories/nutrients in vs. calories/nutrients out still matters a lot in the long term (think weeks) and can be supported by relatively simple maths. Of course, this is assuming a healthy body with an intact hormonal balance.


  27. Ciro
    Un adolecente de 14 años que entrena Fútbol y Taekwondo todos los días, en plena etapa de crecimiento puede hacer ayuno de 14 a 16 hrs?
  28. susan
    I'm just starting so please forgive me if I missed something in the books. I'm wondering when you say fasting daily for a month, is that the 24 hour fast or 20 hour fast? I cant seem to make it the 24 hours without feeling sick. I'm eating 2 small Keto meals (in 4 hours ) on weekdays and doing the 16-8 fast on weekends. I am only walking but hoping to continue lifting weights soon. I had a back injury that took me out and I gained weight. UGH. Anything you can tell me to jump start my weight loss? Can I have a few drinks on the weekend or will it slow me down? Thanks for your help.
  29. Ahmad Yusuf
    I agree with you, Muslims do that in Ramadan month for all of the month and they became healthier and build a very gentle bodies. Also, they are fasting Monday and Thursday from every week as "Sunna orNaffella".Acutally, It is the greatest religion.
    I am Muslim and very happy for that.
    And YOU?
  30. Kumarmandal
    I go to university at 10 am ...and coming back home at 6 pm around =almost8/ 9 hours without eating any foods ,some biscuits + water etc. Sometimes ......I am lean and wanna grow muscle like any body builder out 4 /5 days in a week .....
    Am I right about my plans ???please send me in a mail .
    Reply: #31
  31. Kristin Parker Team Diet Doctor

    I go to university at 10 am ...and coming back home at 6 pm around =almost8/ 9 hours without eating any foods ,some biscuits + water etc. Sometimes ......I am lean and wanna grow muscle like any body builder out 4 /5 days in a week .....
    Am I right about my plans ???please send me in a mail .

    You can read more about gaining muscle and weight here -

  32. Jamie
    Excellent article and well explained. Great to see and refreshing when compared to the usual trash on the internet.
  33. Mark
    Sure, you'll lose muscle if you fast but don't do any exercise or working out.
    Use it or lose it!
  34. Mr. Cat
    I don't see anyone mentioning the reduced stress and repairs allowed to the gastrointestinal system during fasting periods. I don't know of any body processes or structures that are engineered to run at full load 24x7- which many of us do to our guts.
    Digesting food is hard work. Giving your GI tract recovery time is a number to add to the equation... and it would seem to be a complex modifier, as a healthy and rested gut's efficacy in absorbing beneficial nutrients and overall performance could factor in to the fasting benefits like a feedback loop.
  35. Ken
    Why is hgh injections bad but fasti g induced hgh spike good?
  36. Mannpreet
    But what I would like to know is how much does GH increase at 16 hrs of a fast? This seems to be an elusive question that no one directly answers. I keep reading the same “2000% increase for men” and “1300% increase for women” for a “5 day fast” story all over the Net. Well that kinda extreme lifestyle doesn’t suit everyone, especially those with normal 5 day workweeks; for me personally, exceeding 16 hrs isn’t practical because even though I’ve done fasting for a while now and gotten used to it, still you can’t deny that during that time you still may experience headaches, lethargy, fatigue some irritability and a clouded thinking- all which are bad news when you need to be efficient, sharp and energetic at work or risk losing your job- and who the hell would jeopardize that in today’s economic condition???, and then truly fast and starve out on the street being homeless...

    Somebody please answer this question...

    Thank you,

  37. 1 comment removed
    ''All of these benefits are achieved without drugs, supplements or cost. Yes, like all the best things in life, it’s free. So why is everybody so against it?'' -Cause its free my friend, you dont need to buy something specific to eat(keto,paleo,etc). In a few words its against Capitalism!
  39. Stephanie
    I have been trying to find out how many calories it's possible to take without losing the benefits of a fast. I'd like to do a 3-day fast, but after 36 hours I start to get lightheaded. I would like to take an electrolyte drink to see if that helps, but not if it will ruin the fast.

    For IF I keep reading 50 calories is the limit, but what about extended fasts?

    Reply: #40
  40. Kerry Merritt Team Diet Doctor

    I have been trying to find out how many calories it's possible to take without losing the benefits of a fast. I'd like to do a 3-day fast, but after 36 hours I start to get lightheaded. I would like to take an electrolyte drink to see if that helps, but not if it will ruin the fast.
    For IF I keep reading 50 calories is the limit, but what about extended fasts?

    Hi, Stephanie! These links should be helpful for you!
    Being lightheaded could be a sign that you need to eat. Fasting certainly isn't required!
    Keep in mind that we don't recommend fasting for longer than 24 hours without a doctor's supervision.

  41. Stephanie
    Thank you Kerry! I appreciate your reply.
  42. Davit Mrelashvili
    I have a tricky question: Growth Hormine (GH) is a well known by far the biggest main stimulant of IGF-1 secretion.
    Fasting (including intermittent fasting), is a known stimulant of huge GH release, yet it is known to decrease IGF-1 levels.
    What am I missing - how come even though GH goes up, IGF-1 goes down? There must be another stimulant of IGF-1 control that is much stronger than GH?
    Reply: #43
  43. Dr. Bret Scher, MD Team Diet Doctor
    That is a great question Davit. While I can't give you a proven answer, I can certainly speculate. The studies looking at GH as a stimulant for IGF1 were mostly in fed animals or people. So the "easy" conclusion is that fasting introduces a new environment where the relationship is not so straight forward. Fasting can alter gene expressions, receptor levels and other effects that could alter things. While that is not a specific answer, I hope it helps you see how this could be the case. That's what you get for asking tough questions!
    Bret Scher MD FACC Medical Director,
  44. Carter Owen Steinbrecher
    He said a 5 day fast in the article when talking about extended fasting and muscle loss, you went 30 days that's not the same thing in the slightest.
  45. Leila
    I'm wondering how long does the surge of HGH last? Is it over immediately after you break your fast? Does it last for a day? What if I want elavated HGH levels all the time to reverse aging how often do I need to fast?
    Reply: #46
  46. Kristin Parker Team Diet Doctor

    I'm wondering how long does the surge of HGH last? Is it over immediately after you break your fast? Does it last for a day? What if I want elavated HGH levels all the time to reverse aging how often do I need to fast?

    I am sorry, I am not sure. Dr Fung may have more in depth information on his website

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