How fasting affects your physiology and hormones

In order to fully understand fasting and its benefits, it is useful to review the physiology of what happens to our body when we eat nothing. Here’s a brief crash course.


Glucose and fat are the body’s main sources of energy. Glucose is the most easily accessible fuel source for the body. However, if glucose is not available, the body can adjust by switching to fat metabolism, without any detrimental health effects.

This is simply a natural part of life. Periods of low food availability have always been a part of human history and mechanisms have evolved to adapt to this fact of Paleolithic life. The transition from the fed state to the fasted state occurs in several stages.

  1. Feeding – During meals, insulin levels are raised. This allows uptake of glucose into tissues such as the muscle or brain to be used directly for energy. Excess glucose is stored as glycogen in the liver.
  2. The post-absorptive phase – 6-24 hours after beginning fasting. Insulin levels start to fall. Breakdown of glycogen releases glucose for energy. Glycogen stores last for roughly 24 hours.
  3. Gluconeogenesis – 24 hours to 2 days. The liver manufactures new glucose from amino acids in a process called “gluconeogenesis”. Literally, this is translated as “making new glucose”. In non-diabetic persons, glucose levels fall but stay within the normal range.
  4. Ketosis – 2-3 days after beginning fasting. The low levels of insulin reached during fasting stimulate lipolysis, the breakdown of fat for energy. The storage form of fat, known as triglycerides, is broken into the glycerol backbone and three fatty acid chains. Glycerol is used for gluconeogenesis. Fatty acids may be used directly for energy by many tissues in the body, but not the brain. Ketone bodies, capable of crossing the blood-brain barrier, are produced from fatty acids for use by the brain. After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones. The two major types of ketones produced are beta hydroxybutyrate and acetoacetate, which can increase over 70 fold during fasting.

You’ll notice that there is a period of time during gluconeogenesis (step 3) that protein is being used to produce glucose. Many have interpreted this to mean that the body is ‘burning muscle’ to provide glucose. This is not actually what happens. During this period, excess proteins are indeed broken down for glucose. But this is not necessarily muscle. There are connective tissue, skin, old cells and other junky cell parts that can be destroyed. This is the process of autophagy, whose importance is just being realized.

Further, after this breakdown period, the body will rebuild any necessary proteins that were used up. This completes the cellular renewal cycle. It’s like renovating your kitchen. You can’t simply build old cabinets on top of new ones. You first need to remove to the old junky ones. So breakdown of old protein is necessary for renewal of cells. If you only focus on the breakdown, you will miss the entire beneficial renewal cycle.
The human body has well developed mechanisms for dealing with periods of low food availability. In essence, what we are describing here is the process of switching from burning glucose (short term) to burning fat (long term). Fat is simply the body’s stored food energy. In times of low food availability, stored food is naturally released to fill the void.

Burning muscle for energy would be like storing firewood for the winter and then chopping up your sofa and burning it as soon as the temperature drops, leaving all the firewood intact. Also, consider all those aboriginal peoples, inuit, native peoples who had repeated fast/ famine cycles. If they all burned muscle, wouldn’t they all be little balls of 100% fat? It’s funny how that didn’t happen.

Hormonal Adaptation


Fasting is the most efficient and consistent strategy to decrease insulin levels. This was first noted decades ago, and widely accepted as true. It is quite simple and obvious. Almost all foods raise insulin, so the most effective method of reducing insulin is to avoid food. Blood glucose levels remain normal, as the body begins to switch over to burning fat for energy. This effect is seen with fasting periods as short as 24-36 hours. Longer duration fasts reduce insulin even more dramatically. More recently, alternate daily fasting has been studied as an acceptable technique of reducing insulin.

Regular fasting, in addition to lowering insulin levels, has also been shown to improve insulin sensitivity significantly. This may be the missing link in the weight-loss puzzle. Most diets reduce highly insulin-secreting foods, but do not address the insulin resistance issue. Weight is initially lost, but insulin resistance keeps insulin levels and Body Set Weight high. Fasting is an efficient method of reducing insulin resistance.

Lowering insulin rids the body of excess salt and water. Insulin causes salt and water retention in the kidney. Atkins-style diets often cause diuresis, the loss of excess water, leading to the contention that much of the initial weight loss is water. While true, diuresis is beneficial in reducing bloating, and feeling ‘lighter’. Some may also note a slightly lower blood pressure. Fasting has also been noted to have an early period of rapid weight loss. For the first five days, weight loss averages 0.9 kg/ day, far exceeding the caloric restriction and likely due to a diuresis of salt and water.

Growth hormone

'Ha! Here's your problem. You haven't been taking the human growth hormones. Someone sold you some other kind of hormone.'Growth hormone is known to increase the availability and utility of fats for fuel. It also helps to preserve muscle mass and bone density. Growth hormone secretion decreases steadily with age. One of the most potent stimuli to growth hormone secretion is fasting. Over a five-day fasting period growth hormone secretion more than doubled. The net physiologic effect is to maintain muscle and bone tissue mass over the fasting period.

It is precisely this growth hormone surge that complete the cellular renewal cycle. Long term studies of intermittent fasting prove that the fasting strategy is more than 4 times better at preserving lean mass percentage compared to caloric restriction.


Concerns about malnutrition during fasting are misplaced. Insufficient calories are not a major worry, since fat stores are quite ample unless you are severely underweight to begin with. The main concern is the development of micronutrient deficiency. However, even prolonged studies of fasting have found no evidence of malnutrition.

Potassium levels may decrease slightly, but even two months of continuous fasting does not decrease levels below 3.0 mEq/L, even without the use of supplements. This duration of fasting is far longer than generally recommended. Magnesium, calcium and phosphorus levels during fasting are stable. Presumably, this is due to the large stores of these minerals in the bones. Ninety nine percent of the calcium and phosphorus in the body is stored in the bones.

The use of a multi-vitamin supplement will provide the recommended daily allowance of micronutrients. A therapeutic fast of 382 days was maintained with only a multivitamin with no harmful effect on health. Actually, this man maintained that he had felt terrific during this entire period. The only concern may be a slight elevation in uric acid that has been described in fasting.


Noradrenaline levels are increased so that we have plenty of energy to go get more food. For example, 48 hours of fasting produces a 3.6% increase in metabolic rate, not the dreaded metabolic ‘shut-down’. In response to a 4-day fast, resting energy expenditure increased up to 14%. Rather than slowing the metabolism, instead the body revvs it up. Presumably, this is done so that we have energy to go out and find more food.

This is really quite interesting. Fasting, but not low-calorie diets, results in numerous hormonal adaptations that all appear to be highly beneficial on many levels. In essence, fasting transitions the body from burning sugar to burning fat. Resting metabolism is NOT decreased but instead increased. We are, effectively, feeding our bodies through our own fat. We are ‘eating’ our own fat. This makes total sense. Fat, in essence is stored food. In fact, studies show that the epinephrine (adrenalin) induced fat burning does not depend upon lowering blood sugar.

Recall our previous discussion of how insulin works. There are two storage systems for food energy – sugar and fat. Sugar is glucose and chains of glucose (glycogen) in the liver. This is like your refrigerator. It’s easy to put food in and take it out, but there is limited storage space. Because it’s so much simpler to use, the body uses glucose first.

Fat is food stored away long term, like a freezer. It’s harder to put food into the freezer and take it out, but there is unlimited storage space. Our problem of weight loss, is how to clear out the freezer. If our body is always accustomed to burning sugar, it will not be able to access the fat stores. We then get hungry and want to eat to replenish the ‘fridge’. We are hungry, despite the fact that there is more than enough ‘food’ stored as fat in the ‘freezer’.

One solution is to simply empty out the fridge by fasting. This allows us to easily get into the food in the freezer and burn fat. Fasting provides an easy way in.

Jason Fung


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Earlier with Dr. Jason Fung

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More with Dr. Fung

Dr. Fung has his own blog at He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code

His new book, The Complete Guide to Fasting is also available on Amazon.



  1. Apicius
    Excellent post, Dr Fung! Thank you for being bold and re introducing fasting to the world. A lost art indeed. How could we have been so stupid and let this knowledge from the past disappear? It takes courage and lots of brains to do what you are doing.
  2. Gianni
    Dr. Fung you are the best! I fast 24 a day once a week and I feel great! Mind clearity is amazing! Thank you very much for sharing your knowledge.
  3. Elle
    It's so irresponsible to post about a fast lasting 382 days with no ill effect on health! The man who performed that fast, Angus Barbieri, could only eat "with caution" for the remainder of his life, and he subsequently died very young, at only 50 years old. Have you verified that it wasn't the physiological changes following his extreme starvation that caused his early death? If not, you should not assert there was no ill effect from his fast. You can read of Angus in the Evening Telegraph article, "The Tale of Angus Barbieri Who Fasted for More Than a Year - And Lost 21 Stone."

    Furthermore, the 1973 study that published Angus's extreme weight loss, "Features of a Successful Therapeutic fast of 382 Days' Duration," noted the MANY deaths associated with this type of diet. People who did extreme fasts died both during the fast and afterward, from causes such as small bowel obstruction (on only the 13th day of the fast), heart failure during the fast (we know that starvation weakens the heart), and lactic acidosis during refeeding. Only extreme prejudice against the obese could countenance this kind of unethical risk to a patient. It's wrong for you to blog about it in such an off-the cuff way. You may influence other desperate folks to try something similar and possibly lose their lives too. Please edit your post to reflect this information.

    Replies: #4, #7, #23, #26
  4. Tim
    Heh, millions more people have died while eating than ever have from fasting...correlation is not causation.
  5. Jo
    I would really like Dr Fung to address Elle's comment above. Since learning about LCHF and fasting from this site and Dr Fung's other site. I have done a 30 day fast and a 15 day fast and was planning to do more of these. Are they dangerous Dr Fung? I really appreciate your work but Would really like this question cleared up. Thank you :)
    Replies: #6, #13
  6. Tim
    If you follow up on the research you will find the people who died weren't healthy before the fast and died from causes likely to have occurred whether fasting, or not.
  7. Apicius
    Give it a rest, Elle. Are you going to write a letter to the CEO of Coca Cola or Mazola Corn Oil or any cigarette company to stop selling life-shortening guess is no...I won't hold my breath.

    I'm deeply thankful for the ancient methodologies and cures that Dr Fung has uncovered and is teaching us. They work for me, and I feel a lot healthier, and the doctor says my bio stats are better.

    Dr Fung runs a clinic in Toronto and has reversed obesity and diabetes in countless patients. He's not some random guy on the internet spewing stupidity and personal beliefs. It's based on research and actual evidence from his clinic.

    You (Elle) are some random person, nameless, and lashing out at Dr Fung. What are your credentials? Why should I believe you?

    The only thing Dr Fung is guilty of is that the fasting technique he teaches does not increase revenue (not for Big Ag, not for Big Pharma, no one!) It's absolutely free!! If you want to complain about something, complain about that.

    Reply: #8
  8. bill

    Elle actually posted some references. They did
    document what Elle stated. People died while
    fasting and just after. Barbieri died at 50 years
    old. Fasting does not have safety and efficacy
    evidence from "thousands of years" as Dr. Fung
    states. Those people who died from ritual
    fasting were not around to document the failure
    of fasting. Only those who survived were around
    to document the success of fasting.
    There are many contraindications to
    the use of fasting (e.g. T1D, pregnancy, ).
    Nobody posting responses here can prove their
    credentials, but you can follow up on what they

    Replies: #9, #12
  9. Apicius
    I did follow up on what Elle wrote. And I also read the newspaper reports, and other medical reports regarding Barbieri. I saw a lot of misaligned data and info. For instance, he was 26 years old (some sources say 27) when he started the fast. Another source says he died in 1990. So, mathematically, how the heck is it possible that he was 50 years old when he died? He finished the fast in 1971.

    I don't doubt people have hurt themselves doing fasts. People have hurt themselves skydiving, motocrossing, skiing, biking and ball room dancing. People have also hurt themselves taking statins and other drugs they have no business taking, except for the purpose of stuffing the pockets of big Pharma. But, for Elle to *demand* Dr Fung to edit his post is simply ridiculous. He is simply bringing information that is hidden and lost in time to the forefront. He also is a DOCTOR who runs a CLINIC with patients who have SUCCESSFULLY fasted.

    Reply: #10
  10. bill
    Clearly, Barbieri finished the fast in 1966.
    Dr. Fung has today addressed some
    of the issues with refeeding syndrome.
    Fasting is not to be taken lightly. The
    recommendation is to have adequate
    medical support to make sure one does
    not have any contraindications and
    undertakes a fast with care.
    Reply: #11
  11. Tim
    Fasting is not to be taken lightly. The
    recommendation is to have adequate
    medical support to make sure one does
    not have any contraindications and
    undertakes a fast with care.

    Ya hafta make do with what you've got...when all you have are witchdoctors who hand out pharmaceuticals and advocate eating sugars and starches, you have to make do on your own. Fasting is no more dangerous than eating...within reason. Certainly safer than eating a high carb diet.

  12. Tim
    Those people who died from ritual
    fasting were not around to document the failure
    of fasting. Only those who survived were around
    to document the success of fasting.

    Let's exchange a couple of words and see if it still makes sense...

    Think Egyptians...

    'Those people who died from ritual
    high carb diet were not around to document the failure
    of a high carb diet. Only those who survived were around
    to document the success of a high carb diet.'

  13. Peter Biörck Team Diet Doctor
    Hi Jo!

    If you are a member you can direct questions to Dr Fung here:

  14. beto
    Muito boa a analogia com o freezer.
  15. B
    Great article on authophagy. I like the idea of fasting by eating in a restricted window like 6-9 hrs daily. I find it hard to belief that magnesium, the master mineral in the body remains constant for a month. Are there some references to follow on?
  16. zeinab farhat
    hi everybody here,
    i would like to introduce the islam fasting that differs between fasting in winter and fasting summer times depending on the body 's abilities. So muslims fast mainly one month per year .but in this month they only fast from sunrise till sunset . and some muslims speeches say its preferable to fast monday and thursday to get ur body in a rest and recontrol it again . again fasting is just from sunrise to sunset .
    take a look on this link
  17. Phil B

    This website doesn't teach fasting for religious principals. Your "introduction" to "islam fasting" has no business on a diet related website. I didn't arrive at this site to learn about islam and I don't wish to be bullied to observe your teachings.

  18. Omry Grinberg

    I'm Jewish.. We fast too.. and often.

  19. Peter

    I think you are being a little over sensitive. Zeinab was making the point that the Muslim faith advocates fasting as part of its doctrine and as a consequence millions of people who follow that faith are fasting because of it. The inference being that what Dr Fung is recommending has been known and practiced for thousands of years.

    At no stage did Zeinab try to bully you or anyone else.

  20. Ralph
    Can I fast straight after major bowel surgery? My doctors say no as I will not heal and want me to take carb drinks but if fasting increases growth hormone etc will I not heal fine? (or does the post op stree response overpower any build up effects of growth hormone?). MAny thanks
  21. George Henderson
    regarding Barbieri;

    He finished the fast in 1966 and had no adverse effects from either his fast or refeeding. It is implausible that this experience, and his ability to maintain a normal weight (he was not underweight) then went on to cause his death in 1990. Even assuming his metabolic environment contributed to his death, it had not been affected by fasting or refeeding for 24 years, so we would probably be looking at some other input, such as cigarettes, influenza, or an adverse response to medication used for a minor ailment.
    It shouldn't be impossible to find his death certificate today. Find A Grave lists his death in 1990 but strangely not the year of his birth.

  22. Joy
    For a Scottish man living at the time the age that Angus died was actually perfectly in line with the national average.

    Expected average Life spans for Scottish men at the time where abismal and you can bet he would have died a lot younger had he still weighed over 400lb.

  23. Mahesh
    You are 100% wrong fasting is part of human culture and only modern civilization has forced to stop and forsee more diceases.
    The examples you have quoted fasting a year are forced fasting sessions by corporates just to fool people.
  24. Dawn
    I have controlled type 2 diabetes. I would like to eventually stop taking medications and injections. I have done a lot of research on intermittent fasting. I have not see any discussions about diabetic medications while fasting. I've read about the effects fasting has on the hormones - including insulin. If I'm taking a medication that helps my body use my own insulin (as I am insulin resistant) how does the change happen to ketosis? Should I quit taking Victoza while fasting? Will fasting still work while on the medications?
  25. Cecia
    Hi. I would like to know how the prolonged fasting for more than 5 days will affect my homones and menstrual siclos? I have many doubts about that topic.
    greetings and thank you very much for all your information
  26. Jeannie O.
    In response to the comment that "People who did extreme fasts died both during the fast and afterward, from causes such as .... heart failure during the fast (we know that starvation weakens the heart)" it should be made clear that fasting is not starvation. Most people don't do a fast anywhere near as long as 382 days. If someone has little-to-no body fat they are not proper candidates for fasting/extended fasting (along with Type 1 diabetics, and pregnant or breastfeeding women, among other populations) as Dr. Fung has made very clear. No one is forced to fast longer than they feel they should, either. Dr. Fung's information on fasting is fabulous for the majority of people (but not all), and the largest population that has benefited from his work are the many, many folks who are diabetic or prediabetic, who are usually also overweight or obese. Fasting greatly helps these people, myself included. We are not starving, as you would infer; we are consuming our own body fat through fasting. Dr. Fung also has a lot of free information on how to do a fast, different type of fasts, and how to break a fast. He also states that extended fasts should be done under medical supervision. Please look to this information and research what he actually says or suggests for the general population instead of targeting in on the example of one man who fasted in such a way (that he could have stopped if he had wanted to) that most people would not.
  27. Raj Kumar
    Excellent article, very well written as usual by Jason.
  28. Marian B.
    I've been following Dr. Fung's advice, under the supervision of my internist (who is flabbergasted because he really thought diabetes 2 was a progressive disease). I've lost 20 # in 2 months, and my A1C has dropped from 7.3 to 6.5 in 3 months. All of my other bloodwork has fallen to within normal ranges. My internist says he will start reducing my medications if my next checkup shows continued progress. I feel better than I have in years. I am fasting 2-3 times a week, sometimes for up to 3 days.
  29. Josseny Muniz
    I have a question, in the part where he explains about the postabsorptive phase, gluconeogenesis and ketosis, are the reported periods whole days? Example: Ketosis - 2-3 days after the onset of fasting - would be 3 direct days or, 3 days practicing a fasting protocol?
  30. Tempa Hull
    I have been researching, trying to find studies about fasting and peri-menopause. There is very little overall information regarding peri-menopause regardless, and I find there are not enough studies on this particular group, even though this is a huge part of human population. I'm curious what benefits there may be to fasting for women during peri-menopause specifically in relation to female hormonal imbalances, sleep issues, and hot flashes. Almost all of the information regarding fasting and peri- menopause relates to weight. I'm not interested in that. Do you have any answers? Do you know anyone who is currently studying this group of women? Thanks!
    Replies: #31, #33
  31. Kristin Parker Team Diet Doctor

    I have been researching, trying to find studies about fasting and peri-menopause. There is very little overall information regarding peri-menopause regardless, and I find there are not enough studies on this particular group, even though this is a huge part of human population. I'm curious what benefits there may be to fasting for women during peri-menopause specifically in relation to female hormonal imbalances, sleep issues, and hot flashes. Almost all of the information regarding fasting and peri- menopause relates to weight. I'm not interested in that. Do you have any answers? Do you know anyone who is currently studying this group of women? Thanks!

    Unfortunately I am not aware of anything specific along these lines. We do have a general guide for women 40+ you may find helpful.

  32. Zoha ali
    Just love this article❤️
  33. Angeles
    Hi, I am on the same boat with you.
    My mom currently got menopause and she is having hot flashes, sleep issues and hormones imbalance which led her to high blood pressure.. do you also have high blood pressure? I want my mom to try fasting for 36 hours and do OMAD for 2 months (22hours fasting everyday) and see how it goes.

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