Fasting Physiology – Part II

There are many misconceptions about fasting.  It is useful to review the physiology of what happens to our body when we eat nothing.'He's conscientious about most of his religious duties, but he's slow to fast.'


Glucose and fat are the body’s main sources of energy. If glucose is not available, then the body will adjust by using fat, 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. 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 for 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.
  5. Protein conservation phase – >5 days – High levels of growth hormone maintain muscle mass and lean tissues. The energy for maintenance of basal metabolism is almost entirely met by the use of free fatty acids and ketones. Increased norepinephrine (adrenalin) levels prevent the decrease in metabolic rate.

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. So no, the body does not ‘burn muscle’ in an effort to feed itself until all the fat stores are used.

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. All foods raise insulin, so the most effective method of reducing insulin is to avoid all foods. 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 is 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.'Ha! Here's your problem. You haven't been taking the human growth hormones. Someone sold you some other kind of hormone.'

Growth 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. Secretion is known to be pulsatile, making accurate measurement difficult. 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.


Concerns about malnutrition during fasting are misplaced. Insufficient calories are not a major worry, since fat stores are quite ample. 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 .


Adrenalin 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.  Fat is food stored away in the long term, like money in the bank.  Short term food is stored as glycogen, like money in the wallet.  The problem we have, is how to access the money in the bank.  As our wallet depletes, we become nervous and go out to fill it again.  This prevents us from getting access to money in the bank.

Fat is stored away in the ‘bank’.  As our glycogen ‘wallet’ depletes, we get hungry and want to eat.  That makes us hungry, despite the fact that there is more than enough ‘food’ stored as fat in the ‘bank’.  How do we get to that fat to burn it?  Fasting provides an easy way in.

Start with Fasting Part I

Continue with Fasting part 3

Start with Calories Part I

Watch the lecture “The Fast Solution” – The Aetiology of Obesity 4/6

87 Responses

  1. When you say “regular fasting”, do you mean continuous total fasting (water only)? or do you mean alternate-day fasting, or periodic fasting? or all of the above?

    Are all of your multiple-day fasters drinking coffee and tea with coconut oil or just water?

    • There are many variations to fasting. From my studies, the most efficient may be intermittent fasting with fasting windows of 18-24 hours (if done daily it’s even more efficacious), and prolonged fasting of 30+ hours. Alternate day fasting, even though it may elicit benefits compared to regular feeding, may fall short over the long term.

  2. Re “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.” Working from memory, while insulin is necessary to draw the GLUT4 glucose transporter to the cell membrane and thus enhance uptake of glucose in skeletal muscle and adipose tissue, this is not true in the brain and several other tissues. The brain has a different GLUT that works independently of insulin.

  3. Susan Parker

    Thanks for this post Dr. Fung. I think fasting for short periods of time is safe for most people. In the linked study of the man who lost 276 lbs., it would behoove us to take into account this was a young man of 27, that he was a patient for 8 months, ( he must have mostly adhered to the diet to lose 276 lbs.), but there are two points I’d like to make, the same one I made to Cristi Vlad when he posted the same study. The subject was given a yeast supplement for the first 10 months. We don’t know how much. Two tbls. contains 8-10 grams of protein and there are up to 13 grams of carbs in 1 gram of yeast depending on brand and 1 gram of fat. If he were given lets say a cup of yeast per day, that is a lot of protein and and quite a few carbs as well. Paladac has similar. We may conclude he was injesting protein and a very few carbs, some electrolytes, minerals etc. etc. It seems as this was actually an extremely low carb and protein ketogenic diet. Since he was allowed to be an outpatient, I am also thinking it was not well controlled, although the motivation may have been the Guinness Book. ( Smile.) I also do think a colon cleanse may be wise if someone is going to undertake a long fast, or possibly even a short one. I respectfully thank you for all you are doing in the area of obesity and diabetes.

  4. Hmmm. I’m only fasting once a week for around 24 hours. Eating lower carb in between, with longer periods between meals. I’m definitely lighter.

  5. Unfortunately fasting isn’t a good option for everyone. I’m a female who suffers from chronic anxiety, and fasting significantly raises my stress levels and fatigue for many days. I’ve found the single best way for me to keep my mood stable is to eat more often.

    The following is an interesting article about cortisol levels, fasting and sleep. Since reading it a few weeks ago I’ve made sure to eat some carbs when I awake in the night. I’ve found my anxiety levels are reduced (along with my sugar cravings) since doing so.

    Taking a tablespoon of apple cider vinegar before meals also helps.

    • Interestingly, I too am a female with chronic anxiety, and fasting has been a huge help. I have been intermittent fasting (I fast for 16 – 18 hours every day) for almost a year, and my insomnia, acid reflux, and anxiety are mostly resolved. I will never go back to grazing!

      • Sandy, what is your schedule. I’m also anxiety ridden person and I want to try fasting. What do you eat in that window and do you fast like that every day? Thank you

    • Like Sandy, fasting has really improved my chronic anxiety. I have been able to start a new job that I previously was too anxious to cope with. I am amazed everyday by how I have suddenly managed to behave confidently, especially in the workplace (whereas previously I was just too anxious and worried about confrontation to put myself out there, despite being very qualified). I fast for 23 hours per day, eating only dinner and tend to finish it within a one-hour window. I eat low carb real food. I try to stay under 20 grams of carbs for my meal and almost always do. I eat heaps of natural fat and that is what keeps me full. For example, my meal yesterday was dinner at 6 pm with a large green salad with half an avocado, 75g hot smoked salmon, olive oil, 20 g cheese, a spoonful of vinegar, plus three boiled eggs mashed with homemade olive/avocado oil mayo, mixed with one briny dill pickle and I use cucumber sticks to dip into the egg mixture. Yum.

      • I also forgot to say that I could not fast if I was not in ketosis. Before I started LCHF, I would have literally passed out in a hypoglycaemic sweaty mess if I tried to skip a meal. I think LCHF is the first step to fasting for people who feel like they have blood sugar issues, just based on personal anecdotal experience. What is interesting is that I have normal blood sugar levels and do not have diabetes, despite a family history, and always struggling with hypoglycaemia pre-LCHF.

  6. Thank you for this series. I have been low carb for over 4 yrs. It took quite a bit of work to find and/or make over recipes to fit this lifestyle. I lost 20 lbs. and hoped I would slowly lose the last 10. I tried weight training ala Jonathan Bailor and Mark Sisson but lost nothing. Then I upped carbs and gained weight. When I googled how to lose the last 10lbs I stumbled upon alternate day fasting. It was magic! And my fasting glucose went from 89-94 down to 68! I am not diabetic and felt fine. So I am guessing the fasting began to heal my insulin resistance.

    • Pam, when you say your fasting glucose when from 89-94 to 68, what do you mean? Is 68 the goal? How many hours after fasting you test your blood? I normally fast from 6pm to 12pm the next day and I’m curious to know how many hours should I actually fast since I don’t want to fast to the point where my body starts burning muscle instead of fat?

      • The longer you fast the more fat you will burn. At first you use up your sugar stores so go at least 24 hours if you can to maybe 3 days and see how you feel. Dont hurt yourself because food is necessary, man cannot live on air alone – though i try sometimes. Sometimes i get light headed sometimes i simply can’t stop thinking about food. It is at these times that i break fast and have something fatty even if just a bulletproof coffee.

        If you are fat adapted it is easier to fast because your body transitions to fasted self feeding much easier.

        Blood test for ketones

  7. Hi Jason – is your ideal fasting protocol available anywhere? Im keen to get started but I’m not sure wether to do alternate day intermittent or an extended period. Im also not sure if your ideal fast includes 500 calls a day or wether to just water fast. Im ketogenic at the moment but I would like to see if I can improve my insulin sensitivity at least enough to return to glucose fuel without high bgl’s. I really wanted to have some fruit and toast for breakfast instead of eggs so it would be nice to be able to have the option of a carb meal available occasionally. Sorry to be a pest, you have given so much at great personal expense and I really appreciate and respect you for that. Ill figure it out if you don’t have time to respond

    • Sorry, you’ll have to work out a plan that works for *you*. I believe this is Dr. Fung’s position too.

      A lot is a matter of personal choice and what works in your lifestyle.

  8. Thank you for putting so much time and effort into your blog. I’ve read all your writing and watched every video presentation.

    I’m a 58 year old female who has a regular fasting schedule. Typically I fast for 36 hours once a week and 24 hours twice a week.

    After the first week or so, it becomes very easy. I like taking a break from thinking about what to eat all day.

    Like Pam, my fasting glucose has gone down. I didn’t have any health problems, I’m using fasting and a lower carb diet to prevent future complications.
    Thanks again for all you do Dr Fung. I look forward to reading your next blog post.

  9. One thing you say, Dr Fung, that none of the other people I read say, is that everything you eat leads to insulin secretion. The other people I read would say fat cannot do that. Do you agree?

    • webgrrrrl

      In certain circumstances, fat will lead to insulin secretion, Bob. This is true; it is textbook metabolism – for example, read Vasudevan, Textbook of Biochemistry, Section B, General Metabolism, chapters 9 thru 11, where the medical professors go through glucose, carb & fat metabolism in deep detail. You can find it on google books.

    • There is a wide variability in fat. Some fats cause little insulin secretion, others much more. However, most do not eat a meal of pure butter. In a mixed meal, fat tends to decrease insulin secretion, but sometimes it may increase insulin, too. Confusing? Yes. I wish it were simpler, but it is not. Fat, of the macronutrients tends to cause the least insulin secretion, but it is not zero.

      • Thanks. So, to ask a totally leading question, fat generally leads to very low insulin secretion and it may well be sufficient to avoid eating buckets of protein, keep carbs low and do some I.F.?

  10. Thanks!

    I will look for that reading but is it still true to say it’s less of a problem with fat than carbs or protein?

  11. it seems that when i fast overnight 16 or so hours i get high morning glucose reading…….up to 150…then after eating will drop to the 120’s ……..not sure what to do

    • That is the stored sugar in your body coming back out due to release of counter regulatory hormones.

      • Barbara

        I also have a problem with higher morning readings after not eating in the late evening. Is the stored sugar coming back out a good thing, or a bad thing?

        • I know this is a late response – but after doing an 18-20 hour daily fast with a 4-6 hour feeding window (I ate when I felt like it, sometimes this led to a shorter feeding period), and two weekly 24 hour fasts – my FBS gradually declined from 150 to the lowest its ever been since my diagnosis at 81 in the morning.

          Keep at it!

  12. Rebecca Latham

    Dr. Fung,

    I have read through your entire series. I understand that we need to lower insulin levels to reverse insulin resistance in order to lower the body set weight. When I eat low carb, moderate protein and high fat, my home monitored blood glucose is great (mid-70s), but I still cannot maintain weight loss, leading me to believe that I still have insulin resistance. Even though I lose weight now and then, it always comes back to my set weight, which is body fat of 35% and over.

    Since there is no way to monitor blood insulin at home, how do we know that your program is working to lower insulin? We cannot base it on blood glucose, because you said that that could be normal even while insulin is too high. Do we assume that if we are losing weight, it means that insulin is being reduced and the BSW is going down? I have had my weight go down before, just to bounce back up, even though I do not change what I am eating.

    So, how do we know if it is working, if we cannot go by weight or BG? Thanks so much for all of your knowledge and help!


  13. simsalabimbam

    Some free fatty acids (PUFA) can indeed cross the blood brain barrier and be incorporated into cell membranes. Although it is true that neurons exquisitely avoid beta-oxidation pathways.

  14. Dr. Fung, in your opinion what is the longest period a non-diabetic obese person (on no medications, who has tried unsuccessfully to lose weight using many other methods) should fast without any medical supervision whatsoever? I know my doctor will not be on board with fasting (my doctor actually thinks it’s totally fine to be obese as long as you exercise…) but I am very keen to start with a longer fast to jump-start the process.

    • There are many people who do a ‘cleanse’ which is essentially a fast + expensive herbal supplements for a week or more without medical oversight. Most of the risk of fasting lies in the medication adjustment. Humans are designed to fast for extended periods without problems. Certain medications cause big problems during fasting including aspirin, iron and almost all diabetic medications.

      • Thank you for the response Dr. Fung! I am a diabetic on Metformin only (which I have suspended, as I MUST take it with food). I have chosen to try a longer fast (4 days to start) and see what happens. However, I am 24 hours in and have developed a bad headache.

        I am drinking lots of calorie free liquids as well as salted, homemade bone broth. I finally broke down a few minutes ago and took an Advil. Is that going to cause me any issues?

        • Hi Barb Next time you plan your fast, please do not drink any calorie free drink, only take simple distal water and bone broth, Hope you will be able to fast longer period of time.

      • Cathy O'Connor

        Hi Jason. I am on a beta blocker for a cardiomyopathy. I have recently started intermittent fasting to try to lower my blood glucose. I eat once a day and eat within a two hour window. Is this safe for me to do? Would an extended fast be safe for me? Thank you very much.

        Dr. Jason Fung: I cannot legally advise you on your medical condition. Only your own physician can do that. Sorry.

  15. Hi, I must be missing something or I’ve got something wrong. You say, “After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones.” This leads me to believe that during day 6 and more of a fast that brain is using glucose to fuel 25% of its energy needs. But then you say, “the body does not ‘burn muscle’ in an effort to feed itself until all the fat stores are used.” Well, then where does the glucose come from? Or, what fuel does the brain use to fuel that last 25%?

    • Brain still needs some glucose (last 25% of fuel requirements) as do the red blood cells. This is provided by gluconeogenesis with original substrate being glycerol. Liver burns glycerol to form glucose. Glycerol comes from fat (triglyceride = glycerol + 3 fatty acids).

  16. I have been fasting periodically for 24-30hours and have done 7 days fast. I can attest to its efficacy. Not only did I lose 6kgs within this period but expelled a kidney stone 3/4 inch long with a lot of sand. My skin, hair and overall health have shown visible improvement. Fasting has numerous benefits and allows our bodies to rejuvenate.

  17. […] our previous post (part 2), we took a quick overview of the physiology of fasting.  I’d like to take a more […]

  18. Pat Leslie

    I am curious Dr. Fung of your diet for the day. When and what do you eat? You look very fit.

    • I generally follow a lowish carbohydrate diet, although I am fairly strict in limiting sugars particularly. I generally fast 2x per week for 24 hours.

      • Melissa Meyer

        Dr. Fung,
        When you fast, are you just not eating those 24 hours twice a week? Also, if I want to fast every other day, as per your lecture, what caloric reduction should I follow? I want to lose the last 20 pounds that I never have dropped after having babies. Assuming I eat 2000 calories a day, do I change my calories to 1000 one day and 2000 then next, to create a 500 calorie deficit. Yet, by dong this, will it trick my body into not lowering my natural calorie burn rate as you assert in your lecture? What do you recommend? Thank you in advance!

  19. Hi Dr. Fung,

    I’m suffering of insulin resistance, important to mention that I have been working out 6 days a week for more than 10 yrs and I’m lean 23% body fat. Still insulin resistant. I’ve started fasting and feel great during the day but, overnight I sweat, specially around the neck area. I guess it is the adrenalin released to raise blood sugar. My blood sugar readings dueing fasting go up to 100mg/dl and the down to 80 over a period of 5-6h and then raise again to 100 which, I think is good. However I’m still concerned about sweating at night. Is that something to worry about?
    Thanks and congratulations for your great job and contributions to science.

  20. I am so pleased that I read your stuff now, Dr Jason. Interview with Andreas Eenfeldt brought me here. The increase in metabolism while fasting I find most interesting and I read that you point out that this does not happen with low calorie diet. I found an article about a study covering a very low diet, 500Kcal per day 5 months. The 15 obese participants lost on average 1 kg per week. But as you pointed the participants resting metabolic rate (RMR) also dropped significantly on average 20% during the 5 months, and then remained 15% below normal after 5 weeks recovery eating. The subjects weight during this time was not disclosed, but if the diet become normal the lower metabolism would likely result in quick weight gain. It seems your article very well explains that intermittent fasting works very well because it increases resting metabolic rate , while popular calorie counting or jojo banting does the opposite or reduces the RMR. the reason for these diets to fail for most longer term even when all goals (biggest loser) met in the short term.
    Here is the link to the paper . . Also, at what RMR would one “freeze” often in 18 C room temperature ?

  21. Question to Dr Jason: I just spoke to a friend who like me before had heart problems and he was stented a few years ago, now he is 71. Last year he went on a “2-5” program which meant fast for 2 days a week. After a year he went back to his heart doctor that told him all his test were now perfect, a huge improvement. He also lost 6 kgs and he was slim before and all 6 kgs came from the stomach ! He has relaxed to 1 day fasting a week since and feels great. I think that once the visceral fat is gone he is “safe” for another 20 years as the VF will hardly come back without indulging, especially with 1-6 follow up. It could be one of your “text book cases”. He mentioned one worrying thing which is main reason I write to you (again): Today the same people that promoted the diet before says that “total fasting is not necessary, uo to 500 kcal per day can be consumed” . But is that not a low calorie diet that reduces instead of increases metabolism ?

    • It depends a bit on how you take he 500 calories on that fasting day. For example, we often use a 24 hr fast which goes from dinner on day 1 to dinner on day 2. Assuming you take 500 calories on day 2, that is still a 24 hr fast. If you ‘graze’ throughout the day, it will be less effective, but realize that those 500 calories spread throughout the day will only raise the insulin a little, assuming you don’t eat pure sugar.

      That is what I use myself (24 hr fast), simply to maintain weight since I am not actively trying to lose it.

      • Maybe a misunderstanding here: Today the same 5:2 proponents allow 500 kcal PER DAY ! 2 years ago only water/broth was allowed for the two fasting days, like you say. To me the revised 5:2 diet becomes a low calorie diet that according to what you said before reduce metabolism instead of increase it. The growth hormone spikes and related metabolism kick will simply by quenched or at least blunted by the added calories , if I read your article right .

  22. MarianeC

    Dr. Fung,
    I found you through the Diet Doctor. Wow! I have started doing the ‘Every Other Day Fast’ last week and wonder if the 500 calories hinders weight loss or not. I am going to try dropping the 500 calories every other day and see if that makes any difference. I am only here for weight loss and am a healthy person. I only have about 6 pounds to lose but they just don’t come off. Fasting is giving me some hope of losing them.
    Some people will say “who cares, its only 6 pounds” but 6 pounds leads to 10 which leads to 15 etc. I was 30 pounds overweight 5 years ago and gain weight very easily. 51 years old and a very slow metabolism.
    Thank you so much for your videos and information on line. I find is SO interesting. I can’t believe how we have been misled by so many organizations in the world- Big Pharma, Big Food, Government…the science speaks for itself!
    My dad had diabetes and then kidney disease and was on dialysis for 7 years before he died. I wish he could have had your information.
    Thanks again.

  23. […] Continue with Fasting part 2 […]

  24. Scott Scarborough

    I have fasted for 3 days two times in the last year. My glucose never fell below 80 mg/dl. I am 6’tall and weight 185lb. I am 60 years old and have a fasting glucose level typically between 95 and 105 so my doctor tells me I am glucose intolerant. I can eat a large spaghetti dinner and an hour later my glucose is between 85 and 115 – that doesn’t seem high to me. I got a glucometer when my doctor told me I was glucose intolerant. I have taken thousands of readings over several years and the only unusual ones are my morning fasting glucose readings. I am wondering if I just… run high? I have to read more to understand your method of fasting. How is fasting one day and eating the next any different from just skipping meals which everyone has said is a bad thing?

    Dr. Jason Fung: The difference is in duration only. I don’t consider skipping meals to be a bad thing. I think it is very good.

  25. Shairaz a

    Hi Dr fung, wonderful work I must say very educational. My question is does the metabolic rate or adrenaline ever stop rising or drops in long term straight fasting beyond 4 days or more ? like how you said in a diet (not fasting) metabolism drops after a while because the body adapts to the diet. Are there any studies beyond 4 days of fasting in terms of metabolic rate? Is intermittent better than fasting straight for a few days? Thank you.

  26. r. Fung,

    I have read through your entire series. I understand that we need to lower insulin levels to reverse insulin resistance in order to lower the body set weight. When I eat low carb, moderate protein and high fat, my home monitored blood glucose is great (mid-70s), but I still cannot maintain weight loss, leading me to believe that I still have insulin resistance. Even though I lose weight now and then, it always comes back to my set weight, which is body fat of 35% and over.

    Since there is no way to monitor blood insulin at home, how do we know that your program is working to lower insulin? We cannot base it on blood glucose, because you said that that could be normal even while insulin is too high. Do we assume that if we are losing weight, it means that insulin is being reduced and the BSW is going down? I have had my weight go down before, just to bounce back up, even though I do not change what I am eating.

    So, how do we know if it is working, if we cannot go by weight or BG? Thanks so much for all of your knowledge and help!


    Dr. Jason Fung: Serum insulin fluctuates too much to be useful. Fasting blood glucose of A1C are probably the best measure of insulin resistance which is easily available. This is an indirect measure of long term insulin stimulation.

    • I am under the impression that measuring blood ketones with a ketone monitor (the type of monitor T1s use) is a good proxy for basal insulin. Is this correct?
      After 4 days fasting I have found my blood ketones sky rocket. While eating normal HFLC diet I cannot get high blood ketones measurements very easily. They were usually 0.5 in a morning, maybe less…. but I get approx 4.8 after fasting for 4 days.

  27. Thank you Dr. Fung, this information is pure gold !

  28. There does seem to be a difference between men and women regarding fasting. Men don’t seem to have much problem fasting and that makes evolutionary sense if you accept the premise that they were the hunters. It makes perfect sense that men would become stronger for a time without food. I think I have read that fasting for women is less of an advantage in regards to strength…thoughts?

  29. Where is the reference about review the physiology of what happens to our body when we eat nothing ?

  30. Hi Dr. Fung: I am a type 2 diabetic. I am on Metformin and Gipizide. I have been a diabetic for several years now. Recently my dose of meds has not been working as well as they did years ago. My doctor wanted to increase my dose. I told him let me think about it for awhile. On 9/11/15 I watched your videos on Diabetes. On that day I stopped taking my meds and fasted for 14 hours and watched what I ate. Starting to fast my 2 week sugar average was at 178. I am 8 days into my program of now 16 hours fasting a day and eating healthy the other eight hours in the day. My daily sugar levels are now at 120-125 and my two week average is down to 153 in just eight days. My question is, did I do the right thing by stopping my meds and starting to fast and eat better. I think so, the problem is I can’t break through the low blood sugar reading of 120-125. I have had some 108’s, 112, 118 but most of the time my blood sugar hangs at the 120-125 mark and spikes when I eat as that is normal. What do I have to do that will lower my blood sugar down past the 120’s Mark on a regular basic? Thanks

    • Bob, You might try reducing carbs and increasing your fat intake. Dr. Bernstein’s Diabetes solution is a great book for reference on a Low Carb, High Fat, Moderate Protein diet.

  31. […] what I missed the first time I read Dr. Fung’s fasting series (part 2 specifically) is that I lost weight by lowering my insulin levels by eating healthy. What my low carb, high fat […]

  32. […] lead to the weight loss. But with the the changes in my dairy intake and the 2 fasts, I feel like I’m finally tackling my insulin resistance, which is what caused my weight […]

  33. This really works. I have recently lost 45 lbs (cutting carbohydrates, exercising, etc.) but I feel a large portion of this is due to intermittent fasting. Instead of alternating feeding and fasting days, I ate 1-2 meals a day, so that I had at least 19-24 hours of fasting time. I believe the exercise also helped deplete any sugar stores, especially on my 2 meal days. I feel like I was able to eat more and still lose weight than when I was eating 3 meals a day + snacks. And I was less hungry!

  34. Great information! I am on a longer water fast, now day 17 with a goal of 21 days,and would like to come off the fast and begin a low carb, higher fat, moderate protein diet since my body has adjusted to using ketones. Would that work or should I come off beginning with fruit like many site suggest? I am feeling great.

  35. Hi,

    I have read through the site with several of the blog posts and I needed further clarification on something please. When Dr. Fung talks about fasting compared to calorie restriction diets he highlights that the calorie restriction studies show a reduction in RMR and how this isn’t seen in fasting, that the RMR has actually been shown to increase when fasting. In essence isn’t fasting a calorie restriction to a degree? Based on what I’ve seen on the site the fasting discussions encompass anywhere from a double digit hour fast to multiple days. So at the end of the day the calories should be less (restricted) than what was taken in prior to starting the fasting. Is it the rebounding at the end of the fast with normal/higher calorie intake that sets this apart? Is there a minimum length of fasting time where the benefits of an increased RMR aren’t seen, therefore being sure to follow x hours or more for that desired outcome of increasing RMR?

  36. […] Fasting Physiology – Part II […]

  37. Darrell

    Hello. Dr. Fung, at one point you had indicated that children should not fast, as their ability to generate ketones is significantly less developed, as opposed to say, adult men. Basically the younger they are (female too), the less “fast-tolerant” they are. Fair enough. On the other hand, if ketosis does not occur until 2-3 days of fasting, how then could/would we say, a “leangains” IF protocol (16:8) be detrmintal to children (8 year old boy, specficially), given that they do not even come close to the minimum fasting period needed so as to enter ketosis? Or do we/should we all just assume the default positon of never having our kids fast, so as to keep Social Services from removing them from our custody? And I can/could understand that. I feel like a negligent parent, just writing this! On othe other hand, my 8 year old (thin, for now) eats from 8am – 9pm.. not a problem with his bulletproof insulin sensitivity now, but bad habits learned now, along with decades of this type of eating pattern… best to nip it in the bud now, so to speak! Can you speak to this apparent “inconsistency?” Thanks.

  38. […] insure that my body is past the post-absorptive phase and maximum fat burning is taking place and to train my body to become an efficient fat burner […]

  39. Hi Dr. Fung, I’ve read your book and watched the videos and I’m so greatful for the information. I’m on day 8, of a planned 28 day water fast. I have not seen any information on metabolic rate in an extended fast. Yes I saw that it increased in a 4 day fast, but what about after that? does it stay the same? I’m worried that I’m doing more harm than good, but I’ve been doing low carb for a couple of years and lots 50lbs, and then stayed the same weight for a year, even with lchf and intermittent fasting. I’m technically no longer diabetic, but I think I must still have insulin resistance when I still have so much fat. So I want to get rid of insulin resistance and loose weight. Please, Please tell me that long term fasting does not lower metabolic rate too much. Thank you

  40. I read through your blogs and one said that metabolism stays relatively the same on a 30 day fast. I would love a more thorough data sample to ease my mind on the subject, but I’m choosing to dismiss my inner demons that are screaming that it can’t be true.

    I’m on day 10 now and am extremely grateful for all the information I have learned from you, without knowing about fasting I would have stayed insulin resistant for ever. Thank you Thank you Thank you

  41. Dr. Fung,
    I have had DMII since 2010 after a dx of gestational dm. After spending too long in denial about having the condition, I submitted to lantus, metformin, and humulog, experiencing negative side effects. After finding your work and feeling empowered and informed by it, I fasted for 36 hours with the exception of a daily cup of black coffee with a bit of grassfed butter for the first 48 hours. The issue is my blood sugar stayed between 201-230; only once it hit 189. I felt amazing, but the blood sugars never fell. I walked moderately (7-1000 steps each day), got plenty of rest, stayed hydrated. I got frustrated and hungry and ate, and I’ll resume again in a few days, but I remain frustrated. I had hoped I would see a significant decrease per the 2-3 day switch to ketosis mentioned in the article here. I’ve gone off lantus, humalog, and metformin due to side effects and have not being able to lose weight weigh 290. (During fast, I lost 3lbs, I weigh 290). Should I have fasted longer? exercised more? I will continue to read, but am looking for tips and encouragement. Thank you for your work, and to the commenters, for your collective knowledge, tips, and encouragement.

  42. After a fast how do you prevent yourself from gaining the weight loss back?
    Why do biggest loser contestants go into starvation mode and fasters don’t. I’m trying to understand the physiology

    • M. Murphy

      Check the IDM website for the blog on this subject. Explanation, graphs and logic. Try Biggest Loser in “search”.

  43. Rebekah Marquez

    Hello, what about naturally thin people? I’m almost at 48 hours fasting right now, but only have 11% body fat. I’m working on my brain health, which is why I opted to fast in the first place. Normally I go 24 hours fasting, as I am HFLC/keto, so I am simply not hungry to eat before that time. Is it okay that I am fasting longer than 24 hours, with such little body fat?

    • Rebekah Marquez

      Ok, I lied. My most recent test said I’m less than 8% body fat. Like 7.2 or something. I’m not sure on how accurate the test I took is, but I think I’m going to eat now.

      • Lisa Malone

        Unless you got a DEXA scan at a laboratory with the results looked over and confirmed by an expert, your body fat percentage results will be wildly inaccurate. Unfortunately, there is no accurate way to measure body fat percentage besides this. A caliper test performed by someone certified to use them may only be a few percentage out either way, but any other home, gym or store machine may be very wildly out in it’s results.

        For women, minimum essential body fat percentage is around 10%. If your bf% were truly near 8% as a female, and you were not being monitored by doctors and perhaps a trainer (which I imagine you are not, if you are having to ask questions like this on the internet) then you would have bigger problems than ‘Should I fast?’. Infact, reaching 8% body fat without a very closely monitored workout/athletics/competition trainer & medical team as a female, you may already be in hospital unable to type anything at all 😉

        So – in short, you are likely to have a lot more body fat than that (more than likely considering you’re able to come and ask questions like this on the net rather than are listless in a hospital bed, it’s practically guaranteed) and are okay to fast.

        Enjoy your fast!

  44. […] this is Dr. Fung on the stages of fasting, limited to the first week. I’ve always stopped at seven days or fewer. I can see why those […]

  45. […] physiology of fasting is fascinating. The power of fasting lies not in the mere reduction of calories, but the beneficial […]

  46. […] physiology of fasting is fascinating. The power of fasting lies not in the mere reduction of calories, but the beneficial […]

  47. Can glycerol be used as a sweetner? V Longo uses glycerol in his “Fasting Mimicking Diet” according to the patent for “Methods and Diets to Protect Against Chemotoxicity and Age Related Illnesses”. I have also read that glycerol leads to insulinemia even though it has a very low glycemic index ( Example: Stimulation of insulin secretion in man by oral glycerol administration) .. This reminds me of fructose.

  48. […] Fasting Physiology – Part II […]

  49. FeedAllSpiritualThoughts

    Dr. Fung, I hope and pray to the Fasting God’s you read this.
    I came a cross your videos on you tube, in search if whether I can live on water alone. After years of dieting I was tired, exhausted, disappointed and fed up, I figured I might as well drink water until I wither away. It doesn’t matter what I eat, I will gain weight.
    To give you some history, I was an 11lb baby at birth, an overweight toddler, and a 400lb teenager. I am now 34yrs old and have lost nearly 1,000lbs in this short lifetime. I have reached my goal weight of (160lb) on more than 1 occasion only to gain it back.
    I have gone to my PCP many times for answers, in search of why I always bounce right back to Fluffy Liz and they have no answers. I have had blood work and they say I am as healthy as can be, not diabetic, no thyroid problems, blood pressure is perfect and everything looks normal, therefore doctor says I should exercise and except that this is my body “type” I’m currently 223lbs, 5 feet 11 inches (Give or take)
    Needless to say I was fascinated with the videos of your theory on YouTube and began searching for more, that lead me to The Obesity Code- low and behold your thoughts on BODY SET WEIGHT.
    Could it be that I am subconsciously sabotaging my weightloss from being permanent without even knowing it?
    Would fasting reset my body set weight permanently?
    I just want to be normal weight, I feel so wonderful at 160lbs but that is always short lived, and I don’t understand why?
    I would consider fasting for months on end if I knew that my fatometer would just reset itself to a normal weight.
    Is my case common?

  50. Warren Viegas

    This is where I am confused: According to Dr.Fung’s timeline in this post, it appears you must fast for a minimum of 3 days straight to effecctively burn fat. Other advocates if IF say that you reach that stage, at least in part, 12 hours after fasting- The document I am reffering to is here-

    If it takes so long to reach ketosis/fatburning mode or whatever, why are most IF regimens >24hours. Am I better off fasting 24 hours a few times a week, or 16 hours every day?

  51. Maninder Chopra

    Fasting for 34-36 hours with only calories source being 1 tablespoon of heavy cream (0 carbs or protein, 60 calories each tbp ) per cup of tea, is is possible or expected to have an increasing blood glucose levels (122, 178, 155, 148, and at 34th hour, a level
    of 202 mg/dL). I know Dr. Fung mentioned that glycogen mobilization in absence of dietary glucose and, later gluconeogenesis, would raise blood glucose levels, and it is neither good nor bad. My question is if it is high enough to be in 200+, does it make sense to moderately bring it down with insulin or metformin? Though I understand it defeats the purpose.

  52. The big O

    I have been diagnosed as type 2 diabetic since I hade a 6.9 % HbA1c and around 7. 5 mmol/l morning blood sugar readings. Although I had been eating low-carb since a while ago, you could say ketogenic diet also since the carb content i what I eat on a continuous basis is pretty low.
    Recently I bumped into Jason Fung’s books, along with Jimmy Moore’s and got a convincing hint that intermittent fasting, and really – everything intermittent, is the better answer for many bodily (sometimes false) reactions and counter-regulations.

    I have been fasting for most of the recent 3 weeks, doing 3-4 days fasting or 3 days, dinner and then the next day also, only dinner. Training high intensity body weight training during the fast.
    I feel really good, only need to be cautious not to push it too hard during training.

    But still I have quite high morning blood sugar readings, after 3 days fast, 3rd time in the last 3 weeks (3-4 days fast in a row every week…). weight 115 -> 112, height 188 cm, muscular built.
    I wonder if I am doing something wrong, or of I should just continue with what I am doing?

    Where does all that bloody blood sugar come from?
    Is it my cortisol that is burning my muscle away?
    Do I have so much sugar in my lever? Fatty lever?
    When or in what time perspective should be able to see results?
    How do I push hard enough but not too much?
    Insights are greatly appreciated because here in Sweden, where I roam, the lip service is covering up for an old-fashioned health care…

  53. Hi Dr. Fung,

    I am currently on day 4 of a 7 day fat fast. I am having coffee in the morning with ghee and MCT oil, and also a scoop of coconut oil at some point during the day. I just checked my glucose level and it is at 63. This is the lowest I have seen it, and I’m not sure if it is low enough to be concerned, or if I can continue on my fast?


  54. Dr. Fung, I also have been fasting for 36 hours with a morning blood sugar of 60. Is that to low?

  55. Shelly Becker

    Hi Dr. Fung, I have been fasting for 9 days and jsut found out that as of this mornig i have aa blood sugar level of 42 – I am concerned about it, but on the other hand I feel fine. Wondering if this is too law and I should end the fast.

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