Fasting Myths – Part 5

Fasting myths

Many myths are associated with fasting. These myths have been repeated so often that they are often perceived as infallible truths.  Some of the these myths include:myths

  • Fasting puts you in ‘starvation’ mode
  • Fasting will overwhelm you with hunger
  • Fasting causes overeating when you resume feeding
  • Fasting will make you lose muscle
  • Fasting deprives the body of nutrients
  • Fasting causes hypoglycemia
  • The brain needs glucose to function
  • It’s just ‘crazy’

Long ago disproven, nevertheless these myths still persist. If they were true, none of us would be alive today. Consider the consequences of burning muscle for energy. During long winters, there were many days where no food was available. After the first episode, you would be severely weakened.   After several repeated episodes, you would be so weak that you would be unable to get hunt or gather food. Humans would never have survived as a species. The better question would be why the human body would store energy as fat if it planned to burn protein instead. The answer, of course, is that is does not burn muscle as we discussed in the previous post. It was only a myth.

There is another persistent myth that brain cells require glucose for proper functioning. This is incorrect. Human brains, unique amongst animals, can use ketones as a major fuel source during prolonged starvation, allowing the conservation of protein such as skeletal muscle. Again, consider the consequences if glucose were absolutely necessary for survival. Humans would not survive as a species. After 24 hours, glucose becomes depleted and we become blubbering idiots as our brains shut down. Our intellect, our only advantage against wild animals, begins to disappear. Humans would have soon become extinct. Fat is simply the body’s way of storing food energy for the long term, and glucose/ glycogen is the short-term solution. When short-term stores are depleted, the body turns to its long-term stores without problems.

Consider an analogy. A freezer stores food in the long term, and a refrigerator used for short-term storage. Suppose that three times a day, everyday, we go to the market to buy food. Some goes into the refrigerator, but the excess goes into the freezer. Soon one freezer is not enough, so we buy another, then another. Over a period of decades, we have ten freezers, and nowhere else to put them. Food in the freezer does not get eaten because three times a day, we still buy more food. There is simply no reason to release the food from the freezer. What would happen if, one day, we decide not to buy food? Would everything shut down in ‘starvation mode’? Nothing could be further from the truth. We would first empty the refrigerator. Then the food, so carefully stored in the freezer would be released.starvation mode

So, in the body’s case, glucose is used for short-term energy and fat for long-term storage (the freezer). Fat is not burned when plenty of glucose is available. Over decades of abundant glucose, fat stores proliferate. What would happen if glucose were suddenly unavailable? Would everything shut down in ‘starvation mode’? Nothing could be further from the truth. Energy, so carefully stored as fat, would be released.

Starvation mode, as it is popularly known, is the mysterious boogieman always raised to scare us away from missing even a single meal. Over one year, approximately 1000 meals are consumed. Over a span of 60 years, this equals 60,000 meals. To think that skipping 3 meals of the 60,000 will somehow cause irreparable harm is simply absurd. Breakdown of muscle tissue happens at extremely low levels of body fat – approximately 4%. This is not something most people need to worry about. At this point, there is no further body fat to be mobilized for energy and lean tissue is consumed. The human body has evolved to survive episodic periods of starvation. Fat is stored energy and muscle is functional tissue. Fat is burned first. This is akin to storing a huge amount of firewood but deciding to burn your sofa instead. It’s stupid. Why would we assume the human body is so stupid? The body preserves muscle mass until the body fat becomes so low that it has no choice.

Studies of alternate daily fasting, for example show that the concern over muscle loss is largely misplaced. Alternate daily fasting over 70 days decreased body weight by 6%, but fat mass decreased by 11.4%. Lean mass (including muscle and bone) did not change at all. Significant improvements were seen in LDL cholesterol and triglyceride levels. Growth hormone increases to maintain muscle mass. Studies of eating a single meal per day found significantly more fat loss despite the same caloric intake. Importantly, no evidence of muscle loss was found.

The other persistent myth of ‘starvation mode’ is that basal metabolism decreases severely and our bodies ‘shut down’. This too is highly disadvantageous to survival of the human species. If, after a single day of fasting, metabolism decreased, then we would have less energy to hunt or gather food. With less energy, we are less likely to get food. So, another day passes, and we are even weaker, making us even less likely to get food. This is a vicious cycle that the human species would not have survived. It’s stupid. Why would we assume the human body is so stupid? There are, in fact, no species of animals, humans included that are evolved to require three meals a day, everyday. We have already seen in a previous post that resting energy expenditure (REE) goes UP, not down during fasting. Metabolism revs up; it does not shut down.

It’s unclear to me where this myth originated. Daily caloric restriction leads to decreased metabolism so people assumed that this would simply be magnified as food intake dropped to zero. This is wrong. If you rely on food for energy, then decreasing food will lead to decreased energy intake, which will be matched by decreased energy expenditure. However, as food intake goes to zero, the body switches energy inputs from food to stored food (fat). This significantly increases the availability of ‘food’ and this is matched by an increase in energy expenditure.starvation-mode-myth

So what happened in the Minnesota Starvation Experiment? These participants were not fasting. They were eating a reduced calorie diet. The hormonal adaptations to fasting were not allowed to happen. In response to a prolonged period of lowered food intake, the body makes the adjustment to lower TEE.

Everything changes when food intake goes to zero (fasting).  The body obviously cannot take TEE down to zero.  Instead, the body now switches to burning the fat stored on our bodies.  After all, that is precisely, exactly what it was put there for.  Our body fat is used for food when no food is available.  It’s not put there for looks.

Detailed physiologic measurements show that TEE is maintained or sometimes even increased over the duration of a fast. Alternate daily fasting over 22 days found no measurable decrease in TEE. There was no ‘starvation’ mode. There was no decreased metabolism. Fat oxidation increased 58% while carbohydrate oxidation decreased from 53%.   This means that the body has started to switch over from burning sugar to burning fat with no overall drop in energy. Four days of fasting actually increase TEE by 12%. Norepinephrine levels (adrenalin) absolutely skyrocketed 117% to maintain energy. Fatty acids increased over 370% as the body switched to burning fat. Insulin measurements decreased 17%. Blood glucose levels dropped slightly but remained in the normal range.

All the incredibly beneficial adaptations to fasting are not allowed to happen in a low calorie diet.

In fact, look how quickly the merest touch of glucose reverses the hormonal changes of fasting. Only 7.5 grams of glucose (2 teaspoons of sugar or barely a sip of a soft drink) is enough to reverse the ketosis. Almost immediately after consuming glucose, the ketones beta hydroxybutyrate and acetoacetate drop to almost nothing, as does fatty acids. Insulin rises, as does glucose.

What does this mean? The body stops burning fat. It has now returned to burning the sugar that you are eating.Glucose reverses fasting

Repeated concerns are raised that fasting may provoke overeating. Studies of caloric intake do show a slight increase at the next meal. After a one day fast, average caloric intake increases from 2436 to 2914. But over the entire 2-day period, there is still a net deficit of 1958 calories. The increased calories did not nearly make up for the lack of calories on the fasting day. Personal experience in our clinic shows that appetite tends to decrease with increased duration of fasting.

Does fasting deprive the body of nutrients? Most people have more than ample quantities of nutrients. That’s the whole point. To get rid of some of these nutrients – also known as fat.

If you are worried about micronutrients and minerals – you can always take a general multi-vitamin. A different regimen such as alternate daily fasting (ADF) can also alleviate concerns about nutrient deficiency.

The science is clear. The myths surrounding fasting were only falsehoods.

Start here with Calories I

Start here with Fasting I

Continue with Fasting part 6

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

94 Responses

  1. My experience has been:
    Fasting will overwhelm you with hunger
    Fasting causes overeating when you resume feeding

    • My experience was the same as Fred’s. In addition, my blood glucose meter confirmed that my blood sugar fell all day while I tried fasting. (Yes, I’d been fat adapted for months. And my blood sugar wasn’t high to begin with.)

      Fasting is great for some people–and if it works for them, that’s fine. Humans as a species (especially the young, healthy males used in studies) may do fine on fasting, but some individuals do not.

    • I think if you are on the government recommended low fat high carb diet, then fasting will be difficult. On a low carb high fat diet it’s easy. At Christmas I did a 3 day fast and I felt fine. The biggest issue was more psychological than physiological because I was accustomed to eating 3 meals a day and it felt odd to be not eating. However, I was did not feel overly hungry.

    • Blood sugar fell from what to what?

    • Barbara

      Personally, I don’t get all that hungry while fasting, and the second day is even easier. But I have been fat-adapted for quite a while, go into ketosis quickly. And I don’t overeat when I go back to eating. My stomach always feels smaller, and overeating would make me physically sick.

    • Same experience. One meal a day for 2-3 years, no problem… except when that one meal ( withing a 2 hour eating window) rolled around. Everyday I was overwhelmed with hunger and over-ate, continued snacking after the meal, which is something I can avoid when eating 3 meals a day.

      Overall, my conclusion is that one meal a day might work for really obese people but not for the general population.

  2. Simon Thompson

    I put it a little crudely: “Don’t BlueBall your metabolism”. Do not subscribe to the sexy CICO theory- there is never a “Happy ending”.

  3. Joe Smith

    Jason, I appreciate your research and comments on your fasting series, and agree with it logically. However, your research and thoughts seem in contradiction to your series on starvation in 2013. Namely, the idea that one’s metabolism does not increase versus how one’s metabolism does greatly decrease while fasting. Can you explain why your opinion has changed?

    • Fasting causes metabolism to be stable or increase. Caloric reduction (reviewed in 2013) is not the same as fasting and causes metabolism to decrease.

      • I would like to try fasting to assist with diabetes control please confirm that this will help and what is the best fasting regime you would recommend for this? I also am on metformin two times a day so how should I take my medication if I am only eating from 5 – 10 pm.

        • Do 22hrs dry fasting then a window of 2hrs rehydration and eating,,,,bt less carb

      • Dr Fung, I am new to the fasting concept but am totally convinced about the health benefits. I am considering the intermittent fasting plan. I have always been an active person and generally feel great. I am 75 years old, not excessively overweight.. About 15 pounds. But I do want to get the waist fat down. I have coronary artery disease so feel that is really important for my health. Because of the CAD Is there any reason why I should not fast?

      • Dr. Fung, thanks for your comment about how fasting does not cause metabolic damage but caloric restriction does. However, I am very unclear about something: if I practice a daily 16:8 IF (skipping breakfast, no snacks, and eating a normal-portion LCHF lunch and dinner), isn’t that in essence the “caloric restriction” that causes metabolism to decrease?

        • That’s a great question Ashley. I’ve been wondering the same thing. Let me know if/when you find an answer to that!

  4. Ann Patterson

    Dear Dr. Fung, After years and years of “dieting” every which way and suffering with several metabolic issues including hyperthyroidism, high blood pressure and CKF, I found you because you were a speaker at the LCHF Conference. I noticed that your specialty was kidneys and was intrigued by the fact that a nephrologist was a part of the panel at the conference. I have been impressed by the clear, straight forward and heart felt message in your blog posts and videos and am grateful to have found you and been shown one more way to try to restore my health. Thank you so much! I have just finished my 4th fasting day and have periods of hunger, but they are not unbearable by any means. As I wrote to Megan, I miss the actual habit of eating, but am not “starving”. Again, thank you for your work and for sharing it so generously. Ann

  5. Dr. A.E. Kagal

    Most swamis in India eat only one meal per day …no second helpings. They also fast on new moon full moon and the 11th day of the lunar cycle…”Ekadasi”, with more prolonged fasts during the “Chatur mass”. Regular fasting throughout the year has been an essential part of the life of devout Hindus for centuries.

  6. Dr. A.E. Kagal

    Fasting, I think, is the most important but least appreciated part of spiritual growth. Since the brain and mind are sophisticated complex but nevertheless physiological functions of the body it is not surprising that they are vitally impacted by what we eat and how often we eat.

  7. deirdra

    REE is defined, but what does “TEE” stand for?

  8. Thank you for yet another great post about fasting. The subject is so fascinating, but how do you do it? How do people cope with the headaches, fogginess, feeling cold, bad sleep and so on that at least happened to me when I tried to fast.

    An article about the how-to’s would be greatly appreciated.

    • Margaretrc

      Start by getting yourself fat adapted first. The symptoms you describe are the symptoms of switching from a high carb diet to a low carb one. It’s not about the fasting, it’s about lack of the fuel the body is accustomed to getting. Switch to a low carb high fat diet, eat some coconut oil (for ketones) and wait it out. It takes a couple of weeks to adapt. Once adapted, you’ll be able to fast without those symptoms. I fast at least 16 hours every day, no problem.

  9. During fasting times what liquids besides water do you suggest? Is black coffee ok, or does caffeine raise blood sugar?

    • Supposedly caffeine can raise blood sugar, but it doesn’t seem to for me. Your best bet is to test yourself if you want to have black coffee, or stick to water.

    • Sandy, I Dr. Fung has endorsed drinking coffee, tea, and low-calorie broths (as well as lemon water and spices in any of these drinks); here’s an article he wrote about that: Beverages – How to Lose Weight VI

      Also, in this PDF, Dr. Fung goes into more details about some fasting protocols including what to drink: Fasting: Guidelines

      • Christoph, thanks for the link to the beverages! I do have a question, if you know the answer… As a diabetic, I follow a ketogenic lifestyle and one of the suggestions was orange slices in water, wouldn’t that give you too much fructose?

      • Thank you for the fasting guidelines – very helpful.

      • Hello. Hoping someone can answer this for me. I understand the benefits of bone broth in terms of gaining minerals and electrolytes, however if we are drinking bone broth periodically throughout the day, does this not impact the “zero” calorie fast and become more of a reduced calorie diet? Or do we still attain the same hormonal benefits from a water fast, thus leaving our metabolism unaffected?

        • My understanding is that the impact on insulin is minimal with bone broth with the added advantage of minerals and electrolytes.

    • Guido Vogel

      Broth, for the electrolytes.

  10. Kathy from Maine

    Dr. Fung, where have you been all my life? I first heard of you in an interview you did with Andreas Eenfeldt, and you have skyrocketed to one of the people I most trust with my health (including, but not limited to, Dr. Eades, Gary Taubes, Andreas Eenfeldt, Jeff Volek, and Stephen Phinney). Like them, you explain things so that I can truly understand them.

    About fasting, I have a quick question. I get how fasting is different from calorie restriction. That makes total sense to me. What I got out of your writing is that when you fast for 24 hours, from dinner to dinner, you eat a regular dinner again rather than trying to match the calories you “require” based on your basal metabolic rate. True? It would sure be hard to get 2000 calories in at one meal!

    Most days I have my first meal around 11:00 am, and finish eating by around 6:00 pm, so I fast for 17 hours and eat during the remaining 7 hours. I always felt that I had to still reach my BMR “need” for, say, 2000 calories. I usually come in around 1200 – 1400 calories when I do this, sometimes more, depending on my hunger level. (I should note that I typically keep carbs down around 20 – 30 g daily, have adequate protein and fat, and eat no processed foods).

    So if I do this virtually every day, isn’t this calorie restriction disguised as fasting?

    Might I be better off to do one or two 24-hour fasts (dinner to dinner) a week, and eat (whole foods) according to hunger the other days?

    But what if I’m not hungry enough on the “regular” days to eat 2000 calories? Do I try to force myself so as not to succumb to possibly a 600 calorie deficit daily (long-term calorie reduction)?

    As a postscript, I find it incredulous that I have to even ask these questions. Eating should be a natural event, not something for which you have to analyze your every bite.


    • “It would sure be hard to get 2000 calories in at one meal!”

      SONIC disagrees.

      • Kathy from Maine

        You got me there! That Ruth’s Chris steak and martini I could go for, but not any of the other dishes. I “treat” myself to a Wendy’s Baconator with fries and a diet (!) Coke once a year, and that’s more than enough for me!

  11. lasslassiter

    I’ve fasting for 5 years and have nothing but great success. I walk around all year in the 6.5% body fat and am fully capable of at 45 of Deadlifting, twice my body weight. All these myths are just that; if your having difficulties you may need to look at your choices of food you consume when you break fast.

  12. R. peiris

    you are doing professional excellent teaching for the world, thanks to you, now We know what is the diabetes,Our doctors treading for sugar and Insulin,but doesn’t work ,We are really admire your good Qualities,
    thank you
    R peiris

  13. Dr. Fung, I would be interested in your opinion of Dr. Goldhamer’s video on fasting. The difference is his regimen is water only, complete rest, medically supervised fasting at his clinic in Santa Rosa, California. And after breaking the fast, he recommends eating a whole food, plant based (vegan) diet. But he has seen wonderful results. Thanks!

  14. Katherine P

    I have very much enjoyed your teachings!

    I had always thought that the significant reduction of food after gastric bypass (which I would never consider) was the primary factor in the elimination of diabetes post surgery. I just finished the following article and I wondered what your thoughts would be regarding this and also if fasting triggers a similar hormonal response. At any rate I thought that you may like to read it and glean whatever information that you could from it!

  15. I was listening to this interview of yours with John Kiefer and Rocky Patel, MD.

    I learned a lot. Thanks.

    It was especially interesting to hear your point about one-meal-per-day IF plans. That sure, they increase fasting blood glucose, but that doesn’t tell you anything because of course they would if you just ate one big meal at night. It doesn’t tell you what their average BG is.

    Also, you made an important distinction between high protein and high fat, low carb diets, with the former being insulinogenic due to protein’s raising insulin, but the latter not.

    You talked about the beneficial effects of fiber in plant based diets. Here I’m going to agree, but also add something: I think there’s a lot of evidence for extremely micronutrient-dense plants in particular being healthy, including for diabetes. For example, look at the effect of a mere teaspoon a day of Amla powder:

    Amla vs. Diabetes

    Amla has lots of soluble fibre, sure, but it has to be more than just that. Amla is the premier herb in traditional Indian medicine for a reason.

    Joel Fuhrman, who you talked about in that interview, and Michael Greger both make a big deal about nutrient density. Fuhrman especially talks about how micronutrient dense foods prevent fat gain somehow. It seems to me that they may have a beneficial effect on insulin, and that a lot of the benefit ascribed to fiber may really be due to polyphenols, etc., and not merely the action of fibre.

    So I agree (and I’m sure Fuhrman and Greger do as well) with your criticism of McDougall’s tolerance of simple carbohydrates such as white bread and white sugar. It may be true in some trivial sense that, absent much fat, these can be tolerable, but they can not ever be ideal. Nutrients in plenty are ideal.

    Anyway, I just wanted to draw your attention to the possibility that micronutrient-dense food per se may be a factor in insulin sensitivity, which you probably have considered, I’m sure; and also I had a further thought.

    Michael Greger, Neil Barnard, and others believe that it is the saturated fat within muscles and organs that lead to various forms of insulin resistance and diabetes. You doubt that saturated fat in the diet is the culprit. What I’m thinking is that it’s the micro-phyto-nutrient-dense food’s action on reducing insulin that reduces fat within the muscles and organs and that while they might be misunderstanding the mechanism for how a plant-based diet reduces this lipid, that they’re right that the lipid in the muscles, liver, pancreas, etc., is very important … and that it’s reduced when insulin is reduced. Since insulin’s job is to put energy into muscles, organs, etc.

    So in other words, that these diets all work, as you say, and that the insulin sensitivity is increased largely because of decreased fat in the muscles, liver, kidneys, and pancreas, as they say.

    Your work’s very intriguing. It will be interesting to see how it develops over the coming years as more science is down and different disciplines can interact with each other!

    • Cassandra

      I have Fuhrman’s books, and have tried to eat according to his regimen and failed miserably due to constant, gnawing hunger. Huge quantities of plant matter made me feel sickeningly distended but never allowed me to feel satiated after meals. The effect on my blood sugar was not great, and it was mentally very stressful.

      YMMV, as always. I find it easier to fast for a day or two or three and eat meals based on fatty animal proteins. My cravings go away 100%, I feel satisfied and calm enough to embark on a new period of fasting. Side note: I wonder if cravings for carbohydrate foods (though this is a dopamine issue, I understand) can be correlated in some way with the degree of insulin resistance?

      Let me add my immense heartfelt thanks to Dr. Fung here for all that he does for our health and wellbeing. Itching to pre-order the book!

  16. Hi Dr. Fung.

    This is a question for you but anybody else that knows the answer is welcome to chime in. You say that during fasting muscle is not wasted like it is during caloric restriction which I believe to be true. But how does the body get the protein it needs to retain muscle while fasting? I could understand on a shorter fast (say 36 hrs) how to body may preserve muscle but what about longer? The body has to have protein in order to preserve it’s muscle mass so where does it get it? Is there residual protein in stored body fat? Is there some mechanism to use body fat to generate protein?

    • i believe he mentioned how growth hormone protects against Muscle Wasting during fasting.

    • The body does not need protein to “retain” muscle. It needs protein to “build” muscle. The old scare story about fasting was that your body will break down muscle when fasting for the purpose of creating glucose from the protein. Dr. Fung has stated that the body creates the glucose from the glycerol backbone of the triglycerides. Your body stores the fats as triglycerides. IF Dr. Fung is correct then the triglycerides are used to create the ketones for those cells that will survive on them and also for the glucose for those cells that need glucose.

  17. I am curious how did bodybuilders drop body fat to competition level with 6 or more meals a day (mostly high carbohydrate low fat), without fasting… And natural bodybuilders do mostly the same.

    • Fasting is not the ONLY method that works – it is one of many. Nobody NEEDS to fast, but it is one more weapon in the arsenal against obesity. My comments regarding weight loss mostly apply to people trying to lose weight rather than elite level athletes. If you are working out 6 hours a day, then eating several whole pizzas won’t matter – like Michael Phelps. However, most of us are not elite athletes, so different rules apply.

      • “However, most of us are not elite athletes, so different rules apply.” And I would add elite athletes with healthy metabolisms. A couple of the LA Lakers discovered they had pre-diabets or diabetes. Going on a LCHF diet help their health and their game. I don’t know that they knew about IF.

      • I have been relatively low carb for more than 10 years when I discovered that I was diabetic. I then started Dr. Steve Parker’s Ketogenic Mediterranean Diet which helped me lose 17 lbs and helped to keep my BG low. I am still considered obese and could not seem to move my weight any lower until I discovered Dr Fung’s 24 hour fast. I have since lost 20 Lb’s lowered my BG even further not to mention all of my lipid numbers. I do feel hungry sometimes but I find it is very doable. I will be keeping it up in the hopes of coming off my Metformin and losing more weight. I might even shake it up a bit by trying the 36 hour fast occasionally. Thanks Dr Fung!

      • I have never had much luck with anything other than fasting.

  18. Hasan Zaman

    Hi Dr Fung,
    I am very much interested about the fasting method. I have diabetics for 7 years. I am on Medication but when the blood sugar will normal rang? I had fasting for 4 days and my morning sugar rage 8/9. When and how long it takes to be normal rage of my blood sugar. Sorry I am not good in English. Thanks,

    • Hassan, I guess it’s individual and it depends, in great extent, of how much abdominal fat you still need to lose. I have lost 15 pounds and by sugar did not budge, but I still have a long way to go. Keep it up!

  19. DR. Fung, In the study you referenced regarding reduced meal frequency, a significant rise in blood pressure was shown. I’m fasting 1-2 times per week and experiencing a rise in blood pressure. Is this something to be concerned with, or will it drop again with weight loss. I’m down 34 lbs so far, LCHF with another 40lbs to go. Thank you

    • This may be the effect of the increased growth hormone and epinephrine. In our experience, it tends to be transient, but if it persists you may need to speak to your physician to adjust medications. More often, we reduce BP meds as weight decreases.

      • Dr Fung

        It’s the norepinephrine (NE) that rises, not the epinephrine according to the study you quoted previously. But in any case, NE does increase BP in its own right.

  20. nathanb

    Dr. Fung, Have you had any patients fail to lose weight or give up intermittent fasting for any reasons?

    • Yes, many, in fact. We can support people through their fasting, but cannot do it for them. Then they quit. It is much more rare to eat nothing at all for days and still fail to lose weight. It has happened, but I’m not sure about whether these people were truthful in saying they didn’t eat.

      • It has been my experience that weight loss during fasting does not occur in a smooth linear fashion. Rather, it will appear to stall for a day or two but then suddenly drop a couple of pounds on the third day.

  21. I have difficulty with fasting so I’m doing the best I can with a mini fast ie I must have my creamy coffees in the morning, I just can’t do without them, but then I mini – fast until dinner. After dinner I exercise to burn the glycogen and then fast until next morning. I seem to cope well with this and am getting better at it with much less hunger within the fasting period as time goes on.

    The second change was lowering my protein intake and increasing fat. LC to me always meant high protein but my BS were rising despite LC. Listening to Drs Fung & Rosedale I realised it was the protein. I’m happy to say with these 2 hacks my BS levels are absolutely perfect these days.

  22. Dr.Fung, I have noticed that you treat folks that have to decrease their weight and dependence on insulin…do these individuals change there fasting habits once their weight and insulin have normalized? So i guess what i am asking is, do they have longer periods of fasting more frequently until their metabolic markers change then change to a different fasting pattern?

    Thank you for all of your work and sound voice in a sea of nonsense.

    • Generally, as weight and insulin sensitivity get to a point that is acceptable, most people drop to a maintenance schedule – usually 1-2 24 hr fast/week

  23. Hasan Zaman

    Thanks Monica,
    I have been fasting for 5 Weeks now. I see my BG level morning is 6/7 and I am so happy to see that. Sometimes during the day I check my BG and it shows 5.7 and I did not see it in 7 years. Thank you Dr. Jason Fung for giving me the opportunity. I am fasting 4/5 (24h)days a week and I feel great and energetic.

  24. I am suffering from confirmed severe reactive hypoglycemia and regularly have BG of 50 although my endocronologist is unconcerend because my Hac1 is 5.3. I have tried fasting and I get into a terrible state. I am totally unable to eat fat as a major component of meals because I also have chronic pancreatitis. Is there a way for me to fast safely? I feel I am destined for diabetes if not. Thank you.

  25. […] In the early 1900s, Drs. Folin and Denis described fasting as ‘a safe, harmless and effective method for reducing weight of those suffering from obesity’. Great. That’s exactly what we need. Something safe, harmless, and effective. The fact that fasting has been performed (mostly for religious purposes) for several thousand years only reinforced the long history of safety. It’s hard to argue that fasting is dangerous if people have been doing for the 5000 years. May as well argue that using soap is dangerous. Yet, myths about the dangers of fasting are everywhere. […]

    • Aside from the link to the study that said eating one meal per day significantly increases blood pressure, does anyone have additional data or links on meal frequency’s effect on blood pressure?

      I take daily blood pressure medication and have been eating one meal per day for 3 years. My weight has stayed steady but my blood pressure has increased. I don’t want to increase my medicine dosage and am thinking of going back to 3 meals per day.

  26. […] this compensation isn’t complete. One study showed that people who fasted for a whole day only ended up eating about 500 extra […]

  27. […] this compensation isn’t complete. One study showed that people who fasted for a whole day only ended up eating about 500 extra […]

  28. […] this compensation isn’t complete. One study showed that people who fasted for a whole day only ended up eating about 500 extra […]

  29. Fasting helped to reduce A1c but it’s made me loose so much weight and became lean, after several months A1c is again spiked. Now I can’t reduce any further weight as it makes me underweight. How does the weight loss (by fasting) will help for long-term?

  30. […] this compensation isn’t complete. One study showed that people who fasted for a whole day only ended up eating about 500 extra […]

  31. […] this compensation isn’t complete. One study showed that people who fasted for a whole day only ended up eating about 500 extra […]

  32. “Norepinephrine levels (adrenalin) absolutely skyrocketed 117% to maintain energy”- is this desirable? The context seems to imply it is positive. Or is one taxing their hormone levels?

  33. […] “lost” during the fast by eating more during the next few meals. However, this compensation isn’t complete. One study showed that people who fasted for a whole day only ended up eating about 500 extra […]

  34. […] a couple of meals isn’t going to kill me. And no, I’m not starving myself to lose weight. Going into “starvation mode” when fasting is a myth.  Fasting is perfectly safe and practiced by millions around the world for thousands of […]

  35. […] Fasting myths – Jason Fung MD […]

  36. You said, “only 7.5 grams of glucose is enough to reverse the ketosis”. Was 7.5 grams just an example or an exact threshold that causes your body to switch over to sugar? I ask because I eat multivitamin and calcium gummies which contain sugar. Together, they may add up to 3 grams of sugar. If I take these gummies in my fasted state, will it shut down the fat burn and growth hormone release? By the way, I’m doing intermittent fasting. I’m eating from 12:00pm to 6:00pm and I take the gummies at 8:00am. I’m wondering if I should move the gummies to inside my eating window.

  37. Dr Fung

    Thank you for the great series about fasting, but you keep confusing norepinephrine and adrenaline. I quote from above “Norepinephrine levels (adrenalin) absolutely skyrocketed 117% to maintain energy”. Norepinephrine is also known as noradrenaline and not adrenaline, which is epinephrine. In the study you quoted it’s true the NE levels skyrocketed but the adrenaline levels remained relatively stable (marginal decrease).

  38. Another inaccurate statement I noticed above “Only 7.5 grams of glucose (2 teaspoons of sugar or barely a sip of a soft drink) is enough to reverse the ketosis”. In the study you are referring to (The effect of carbohydrates on ammonium and ketoacid excretion during starvation. J.Clin. Invest. 51:2093–102) it was noted that the ingestion of 7.5 grams of glucose decreased the urinary excretion of ketoacids (ketonuria) but the blood levels of butyrate and acetoacetate remained constant, i.e the state of ketosis was still maintained. Even after ingestion of 15 g of glucose the urinary levels dropped further but the blood levels remained stable. The authors explained this fact by an increase in renal reabsorption of the excreted ketoacids. A beneficial effect of glucose ingestion was also noted that urinary excretion of ammonium was reduced indication of nitrogen-sparing effect. So all in all a small amount of carbs during fasting could have a beneficial effect rather than abolish the benefits as you mention.

    • Aside from the link to the study that said eating one meal per day significantly increases blood pressure, does anyone have additional data or links on meal frequency’s effect on blood pressure?

      I take daily blood pressure medication and have been eating one meal per day for 3 years. My weight has stayed steady but my blood pressure has increased. I don’t want to increase my medicine dosage and am thinking of going back to 3 meals per day.

    • Lucille

      Han, thank you, I asked below about that very issue before I read your post.

  39. Aside from the link to the study that said eating one meal per day significantly increases blood pressure, does anyone have additional data or links on meal frequency’s effect on blood pressure?

    I take daily blood pressure medication and have been eating one meal per day for 3 years. My weight has stayed steady but my blood pressure has increased. I don’t want to increase my medicine dosage and am thinking of going back to 3 meals per day.

  40. Curious: Is there any information on REE or TEE after the patient reaches a normal weight and stops IF? Say I reach my goal weight and continue eating a very low carb diet but I discontinue intermittent fasting and return to three meals a day. Is there any indication that my TEE will be less then it was before I began to lose weight and fast? Will I then have to eat less calories to maintain my new weight?
    IF and low carb is certainly helping correct my blood sugar problems and lose weight, my only concern is being able to maintain that loss long term. Thanks for any guidance you may give.

    • Why would you continue with a low carb diet? If you believe Dr. Fung’s idea that weight gain is caused by high insulin levels and insulin resistance, eating low carb will not necessarily keep this from happening as ALL foods cause an insulin response, some zero carb foods foods such as steak cause a very large insulin response. The only solution is to go longer periods between meals and eat only 1 to 3 times a day. It is the frequency of the insulin response to foods more than the response itself that is the issue. Some supposedly high carb foods such as “pasta” actually have a much smaller insulin response than steak. The bodies response to pasta is affected by how it is cooked. If it is cooked correctly-al dente- it will cause a much slower blood sugar rise. If one eats a salad with a vinaigrette dressing before the pasta, this will also lessen the GI and the Insulin response.

  41. shanda wallis

    Hi Dr.Fung,

    just a quick question about hypoglycemia – I’m LCHF/ keto and do 24 hr fasts once a while as well 20/4 most days. HOwever over the last week BGL drop to about 2.5 by late afternoon, feeling hungry and sleepy. In this last week I’ve finely seen the stubborn plateau break and am down a couple KGs. HbA1c is 30. Any ideas?

  42. “If you are worried about micronutrients and minerals – you can always take a general multi-vitamin.” I was onboard until you said this. Paleolithic people wouldn’t be popping multivitamins… So, what then?

  43. Please help with this question. I’m T2D. Even during the day after taking Toujeo my BS will rise 10-30 points when I don’t eat. Why does this happen? When I googled it, every article stated not to skip meals because BS will rise. I need some sort of understanding about this.

  44. I am doing alternate day fasting, and it does seem to be working as I’ve lost weight without changing my low carb diet. Sometimes eating in the morning and waiting until the next day in the morning, almost 24 hrs, or doing 16/8. I am confused about what happens on the days you eat more regularly like the 8 hour window of eating or even adding a regular day of eating 3 meals into your regimen. ON those days you would no longer be in ketosis so you are switching back and forth?? It’s my understanding that you can’t switch back and forth so easily from burning glucose for fuel to burning fat for fuel and that some people require a long period of low carb high fat eating to even begin to get into ketosis and can easily fall back out of ketosis. I know that I never seemed to have much luck with staying in ketosis. I seemed to be in ketosis (urine test very dark purple) but I wasn’t losing weight, didn’t seem to be in a fat burning state, and would become discouraged after attempting this for months. I’ve been low carb, mod protein, higher fat (lowered it recently) for many years.

  45. Lucille

    I have given up artificially sweetened sodas and soft drinks and drink water and unsweetened tea. I want to put a slice of lemon in the water but I noticed that Dr. Fung says that the ‘merest touch’ of glucose reverses the hormonal effects of fasting and lemons have some sugars. Is a slice of lemon going to undo my fasting efforts?

  46. very nice website and informative

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