Fasting Physiology – Part II

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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.'

Physiology 

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

Insulin

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.

Electrolytes

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

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

2017-09-02T11:54:13+00:00 88 Comments

About the Author:

Dr. Fung is a Toronto based kidney specialist, having graduated from the University of Toronto and finishing his medical specialty at the University of California, Los Angeles in 2001. He is the author of the bestsellers ‘The Obesity Code’ and ‘The Complete Guide to Fasting’. He has pioneered the use of therapeutic fasting for weight loss and type 2 diabetes reversal in his IDM clinic.

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88 Comments on "Fasting Physiology – Part II"

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deirdra
Guest

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?

Cristi Vlad
Guest

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.

Tim H
Guest

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.

Susan Parker
Guest
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… Read more »
bob
Guest

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.

Heather
Guest
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. http://www.gestaltreality.com/2014/04/03/supercharge-your-health-by-sleeping-less/ Taking a tablespoon of apple cider… Read more »
Pam
Guest
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… Read more »
Doug
Guest
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… Read more »
Lori
Guest
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… Read more »
bob
Guest

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
Guest

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.

kfacwpup
Admin

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.

bob
Guest

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?

deborah
Guest

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

kfacwpup
Admin

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

Barbara
Guest

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?

Kristin
Guest

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!

Rebecca Latham
Guest
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,… Read more »
simsalabimbam
Guest

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.

http://www.ncbi.nlm.nih.gov/pubmed/17901540/
http://www.ncbi.nlm.nih.gov/pubmed/21816594/

Mary
Guest

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.

kfacwpup
Admin

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.

Barb
Guest

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?

Yasir
Guest

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
Guest

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.

Nate
Guest
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%?
kfacwpup
Admin

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).

Nate
Guest

Thanks, the body is so amazingly and wonderfully complex.

Irene
Guest

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.

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

Pat Leslie
Guest

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

kfacwpup
Admin

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
Guest
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?… Read more »
Monica
Guest
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… Read more »
kfacwpup
Admin

Sugars are good. Make sure sweating is not a sign of low blood sugars – if sugars are OK at night, then there is nothing to worry about.

Monica
Guest

Thank you so much for replying.

stenB
Guest
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… Read more »
stenB
Guest
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… Read more »
kfacwpup
Admin
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… Read more »
stenB
Guest

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 .

MarianeC
Guest
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… Read more »
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[…] Continue with Fasting part 2 […]

Scott Scarborough
Guest
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… Read more »
Shairaz a
Guest

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.

Rebecca Latham
Guest
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,… Read more »
Danielle
Guest

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.

Hunter
Guest

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

Pierre
Guest

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?

http://www.marksdailyapple.com/women-and-intermittent-fasting/#axzz3iAt3PLqn

renato
Guest

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

Bob
Guest
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… Read more »
Sheryl
Guest

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.

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[…] 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 […]

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[…] 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 […]

Tiffany
Guest

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!

Mari
Guest

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.

Jessica
Guest
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… Read more »
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[…] Fasting Physiology – Part II […]

Darrell
Guest
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… Read more »
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[…] 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 […]

Monica
Guest
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… Read more »
Monica
Guest

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

Emily
Guest
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… Read more »
Melanie
Guest

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
Guest

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

Rebekah Marquez
Guest

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
Guest

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
Guest
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… Read more »
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[…] 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 […]

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

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

John
Guest

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.

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[…] Fasting Physiology – Part II […]

FeedAllSpiritualThoughts
Guest
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… Read more »
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