Fasting and Muscle Mass – Fasting Part 15

posted in: Fasting, Health and Nutrition | 102

It seems that there are always concerns about loss of muscle mass during fasting. I never get away from this question. No matter how many times I answer it, somebody always asks, “Doesn’t fasting burn your muscle?”

Let me say straight up, NO.

Here’s the most important thing to remember.  If you are concerned about losing weight and reversing T2D, then worry about insulin. Fasting and LCHF will help you. If you are worried about muscle mass, then exercise – especially resistance exercises. OK? Don’t confuse the two issues. We always confuse the two issues because the calorie enthusiast have intertwined them in our minds like hamburgers and french fries.

Weight loss and gain is mostly a function of DIET. You can’t exercise your way out of a dietary problem. Remember the story of Peter Attia? A highly intelligent doctor and elite level distance swimmer, he found himself on the heavy end of the scale, and it was not muscle. He was overweight despite exercising 3-4 hours a day. Why? Because muscle is about exercise, and fat is about diet. You can’t out-run a bad diet.

Muscle gain/ loss is mostly a function of EXERCISE. You can’t eat your way to more muscle. Supplement companies, of course, try to convince you otherwise. Eat creatine (or protein shakes, or eye of newt) and you will build muscle. That’s stupid. There’s one good way to build muscle – exercise. So if you are worried about muscle loss – exercise. It ain’t rocket science. Just don’t confuse the two issues of diet and exercise. Don’t worry about what your diet (or lack of diet – fasting) is doing to your muscle. Exercise builds muscle. OK? Clear?Macro oxidation

So the main question is this – if you fast for long enough, doesn’t your body start to burn muscle in excess of what it was doing previously in order to produce glucose for the body. Hell, no.

Let’s look carefully at this graph by Dr. Kevin Hall from the NIH in the book “Comparative Physiology of Fasting, Starvation, and Food Limitation”. Great title guys. Amazon probably couldn’t keep enough stock on the shelves.

But anywho, this is a graph of where the energy to power our bodies comes from, from the start of fasting. At time zero, you can see that there is a mix of energy coming from carbs, fat and protein. Within the first day or so of fasting, you can see that the body initially starts by burning carbs (sugar) for energy. However, the body has limited ability to store sugar. So, after the first day, fat burning starts.

What happens to protein? Well, the amount of protein consumed goes down. There is certainly a baseline low level of protein turnover, but my point is that we do not start ramping up protein consumption. We don’t start burning muscle, we start conserving muscle.

Reviews of fasting from the mid 1980s had already noted that “Conservation of energy and protein by the body  has been demonstrated by reduced … urinary nitrogen excretion and reduced leucine flux (proteolysis). During the first 3 d of fasting, no significant changes in urinary nitrogen excretion and metabolic rate have been demonstrated”.  Leucine is an amino acid and some studies had shown increased release during fasting and other had not. In other words, physiologic studies of fasting had already concluded that protein is not ‘burnt’ for glucose.Urea

It further notes that you can get increase leucine flux with no change in urinary nitrogen excretion. This happens when amino acids are reincorporated into proteins. Researchers studied the effect of whole body protein breakdown with 7 days of fasting. Their conclusion was that “decreased whole body protein breakdown contributes significantly to the decreased nitrogen excretion observed with fasting in obese subjects”. There is a normal breakdown of muscle which is balanced by new muscle formation. This breakdown rate slows roughly 25% during fasting.

The classic studies were done by George Cahill. In a 1983 article on “Starvation” he notes that glucose requirements fall drastically during fasting as the body feeds on fatty acids and the brain feeds on ketone bodies significantly reducing the need for gluconeogenesis. Normal protein breakdown is on the order of 75 grams/day which falls to about 15 – 20 grams/day during starvation. So, suppose we go crazy and fast for 7 days and lose about 100 grams of protein. We make up for this protein loss with ease and actually, far, far exceed our needs the next time we eat.MuscleBreakdown

From Cahill’s study, you can see that the urea nitrogen excretion, which corresponds to protein breakdown, goes way, way down during fasting/ starvation. This makes sense, since protein is functional tissue and there is no point to burning useful tissue while fasting when there is plenty of fat around. So, no, you do not ‘burn’ muscle during fasting.


Where does the glucose comes from? Well, fat is stored as triglycerides (TG). This consists of 3 fatty acid chains attached to 1 glycerol molecule. The fatty acids are released from the TG and most of the body can use these fatty acids directly for energy.

The glycerol, goes to the liver, where it undergoes the process of gluconeogenesis and is turned into sugar. So, the parts of the body that can only use sugar have it. This is how the body is able to keep a normal blood sugar even though you are not eating sugar. It has the ability to produce it from stored fat.

Sometimes you will hear a dietician say that the brain ‘needs’ 140 grams of glucose a day to function. Yes, that may be true, but that does NOT mean that you need to EAT 140 grams of glucose a day. Your body will take the glucose it needs from your fat stores. If you decide to EAT the 140 grams instead, your body will simply leave the fat on your ass, hips, and waist. This is because the body will burn the sugar instead of the fat.ADF MuscleMass1

But let’s look at some clinical studies in the real world. In 2010, researchers looked at a group of subjects who underwent 70 days of alternate daily fasting (ADF). That is, they ate one day and fasted the next. What happened to their muscle mass?

Their fat free mass started off at 52.0 kg and ended at 51.9 kg. In other words, there was no loss of lean weight (bone, muscle etc.). There was, however, a significant amount of fat lost. So, no, you are not ‘burning muscle’, you are ‘burning fat’. This, of course, is only logical. After all, why would your body store excess energy as fat, if it meant to burn protein as soon as the chips were down? Protein is functional tissue and has many purposes other than energy storage, whereas fat is specialized for energy storage. Would it not make sense that you would use fat for energy instead of protein? Why would we think Mother Nature is some kind of crazy?

That is kind of like storing firewood for heat. But as soon as you need heat, you chop up your sofa and throw it into the fire. That is completely idiotic and that is not the way our bodies are designed to work.

How, exactly does the body retain lean tissue? This is likely related to the presence of growth hormone. In an interesting paper, researchers fasted subjects and then suppressed Growth Hormone with a drug to see what happened to muscle breakdown. In this paper, they already acknowledge that “Whole body protein decreases”. In other words, we have known for 50 years at least, that muscle breakdown decreases substantially during fasting.

By suppressing GH during fasting, there is a 50% increase in muscle break down. This is highly suggestive that growth hormone plays a large role in maintenance of lean weight during fasting. The body already has mechanisms in place during fasting to preserve lean mass and to burn fat for fuel instead of protein.

So let me lay it out as simply as I can. Fat is, at its core essence, stored food for us to ‘eat’ when there is nothing to eat. We have evolved fat stores to be used in times when there is nothing to eat. It’s not there for looks, OK? So, when there is nothing to eat (fasting), we ‘eat’ our own fat. This is natural. This is normal. This is the way we were designed.

And its not just us, but all wild animals are designed the same way. We don’t waste away our muscle while keeping all our fat stores. That would be idiotic. During fasting, hormonal changes kick in to give us more energy (increased adrenalin), keep glucose and energy stores high (burning fatty acids and ketone bodies), and keep our lean muscles and bones (growth hormone). This is normal and natural and there is nothing here to be feared.

So, I will say it here, yet again.

  1. No, fasting does not mean you burn protein for glucose. Your body will run on fat.
  2. Yes, your brain needs a certain amount of glucose to function. But no, you do not have to EAT the glucose to get it there.

Continue to Fasting Part 16 – Fasting Lowers Cholesterol

102 Responses

  1. Great blog post Dr. Fung! Protein wasting with fasting is so ingrained in both the medical/nutrition as well as lay community! It makes it difficult to convince people otherwise so keep up the great work! Do you have any thoughts on the use of BCAA during fasting to promote muscle growth while following a two or three days a week strength program?

  2. Wenchypoo

    5 jumbo shrimp (my favorite oxymoron)

    What about MILITARY INTELLIGENCE, or POLITICAL WILL? 🙂 Hubby and I are fasting through this 3-day weekend–we started at 6 Friday night, and will end at 6 on Monday night.

  3. Wenchypoo

    By suppressing GH during fasting, there is a 50% increase in muscle break down. This is highly suggestive that growth hormone plays a large role in maintenance of lean weight during fasting.

    Does the mutant MTHFR gene play at all in this? If so, how?

  4. Zachary W.

    I’m a sizable fan of the blog (in enthusiasm, not girth). I’ve neither diabetic, nor particularly unhealthy (at this point), but your blog has been a wonderful source of confirmation (and compiling) of information, much of which, I’d come across over the years in my own progress with fasting and intermittent fasting. But as I am only a lowly Dr. of humanities, a medical perspective on the same research has proven an invaluable technical resource in reminding me why I eat the way I do, in spite of the incredulous looks I get from people who eat in the conventional (less healthy, IMHO) way and are always worried that I’m harming myself (or just) crazy for forgoing food for hours, days, even the occasional week.

    Anyway, on to the question/perspective: I am a casual weightlifter and former athlete, and I wonder hypothetically if much of the fear of muscle catabolism stems from experiences in the bodybuilding community, who attempt to maintain such unnaturally high levels of muscle mass (and low body fat) that the body’s metabolism *might* in those rare situations (i.e. extremely lean and muscular bodybuilders) look around at its muscle stores while fasting, and if not a terrible lot of heavy-lifting is going on while fasting, might say “hey we don’t need 40 extra lbs of muscle, let’s get rid of this metabolically hungry stuff now in case this famine runs long..). To this point both IF bodybuilding gurus (Pillon and Berkhan) are lean and muscled, but not nearly to the extent that is witnessed in professional competition, so fasting for them at ‘natural’ levels of muscle mass, allows the precise biomechanics the post above describes protect their muscle, where calorie restriction or fasting might not protect the “un-naturally” muscled.

    In other words, I wonder if quantity of muscle should be considered a variable in muscle preservation vis-a-vis fasting.

    • ZW:


      • Zachary W.


        These people might “who (might) give a s**t:” [WGAS]:

        Dr Fung: might like to hear that his blog is helpful to people beyond those just who are battling diabetes or extreme obesity. It is called a thank you. He might also be interested in addressing the question of fasting as regards to abnormal levels of lean muscle mass. He probably has considered this, but thus far has not addressed it, nor have I seen it really addressed well elsewhere.

        YOU, bill: clearly cared enough to respond with a snarky comment on a blog where people are civil and appreciative of this resource. In the future, consider do you care enough to put vitriol into a respectful discourse, and if you do, consider keeping it to yourself and not damaging the environment of Dr. Fung’s generous, helpful, and supportive community.


      • This is of interest to me.
        I have seen a paper where buffering with sodium bicarbonate and potassium chloride reduced nitrogen loss by a third in fasting subjects.
        Also resistance trained people generally have more myonuclei per muscle fiber than untrained.
        If the rate of muscle protein synthesis decreases due to lack of stimulus or lack of substrate the fiber will shrink but not disappear once you start eating it will refill quickly.

        Good review paper here on glycogen in fasting, looks like short intense training is best while fasting.
        “Post-Exercise Muscle Glycogen Repletion in the Extreme: Effect of Food Absence and Active Recovery”

    • This explains a great deal.

      Thank you for the insight.

    • Zachary: I have been casually weighlifting for decades 2-3 times per week, and have read many books which assert that fasting results in diminished muscle mass. However, in light of what Dr. Fung has written here with this post, I believe that the weightlifters’ alarm is misplaced or at least there is a different explanation for the “wasted muscle” appearance. I suspect that although there is the appearance of loss of muscle,* what is really going on is that because decreased levels of insulin causes the body to excrete large amounts of water from all cells (as we who fast can well attest to–the initial loss of numerous pounds is mostly water loss), that the muscle cells–comprised as they are of 70% water–have actually shed a lot of their volume. So it’s not so much that the muscle fibers themselves have somehow been catabolized by fasting, but just that the water that makes up most of the bulk of muscle has been excreted.

      *I myself have noticed that when I eat carbs all weekend, my muscles are far more full and pumped up looking; however, after a 3-4 twenty-three hour fasts each week, plus an additional single 45-hour fast per week, by Friday my muscles have a much slimmer, almost flat or deflated appearance–as if I stopped going to the gym a month ago–but which is likely the loss of water.

      • I’ll start by saying that I agree with Steve, on the water loss.
        I’ve been into strength training for a while now, and have just gotten into bodybuilding. A large part of bodybuilding is achieving the “pump” or a state of hypertrophy. Glycogen stores in the muscles are primary responsible for the storage of additional water, allowing for a “fuller look”. Higher intake of water with a combination of substances such as creatine allow for further water storage. While achieving a pump the muscle enters an engorged state in which excess blood and water are pulled to the area of work, allowing hypertrophy to take place. When the body is VERY LOW on glycogen like in a ketosis style diet, or while fasting, the excess water in the muscle mass is very quickly lost. Within the time span of 2 days of fasting I personally notice a shrinkage of muscle mass, similar to that “flat” look you are talking about. Upon returning to my normal maintenance diet of roughly 3400 calories, within a few hours I achieve my previous full look (with the added benefit of more definition) and am more than ready to train and achieve a pump the same day. As of yet, I haven’t noticed any decline in strength, if anything my power is still rising at a normal rate (although I am young, and that should definitely be taken into account).

        ***I’m just a student in college who is heavily interested in athletics and dieting, if i’m wrong about any of what I stated then feel free to correct me, it will help me much more in the end. I HAVE MUCH MORE TO LEARN!

  5. Such a great post, Jason! And you’re too funny – your exasperation is palpable! Thank you for all of the invaluable information you have on your site!

  6. Dr.S. Vijayaraghavan

    Dear Dr Jason
    On target yet again. I have gone through the frustration of not being able to lose weight despite working out almost an a day six days a week for three years.
    I stopped this nonsense, went on LCHF diet and HIIT ( 10 mgs twice a week) and lost 8 kilos in a month.
    Proof of the pudding!!!

  7. “Yes, if you fast for 7 days and then eat no other protein than 5 shrimps, you are fine.”

    Nope. Not at all.

    The 15-20 grams of protein per day you cite applies to prolonged starvation (about 20 days into the fast). In the same paper from Cahill, the protein loss per day after 3-4 days of fasting is 75 grams. That would be around 300 calories per day, which fits nicely with the graph you have near the top of your post.

    Let me repeat: according to your own data, we should expect around 75 grams of protein lost per day of fasting (at least for the first few days of a fast, in a normal man).

    According to the graph and the Cahill paper, during the first week of a fast, the body would burn at least 400 grams of protein. That represents 3 to 4 pounds of lean tissue (like muscle) once you account for the water bound with it. A normal shrimp contains between 1 and 2 grams of protein. I can’t find an official entry for “jumbo shrimp”, so let’s say 2 grams of protein per jumbo shrimp (it certainly won’t be 20 grams of protein per shrimp, that would require an 80-gram shrimp or so). That means you need to eat at least 200 jumbo shrimps after a week of fasting, not 5.

    The body does preserve protein as the fast is prolonged and ketosis increases (we’re talking about 20 days of fasting to reach the plateau). The problem is not really that the body furiously ramps up protein breakdown during a fast. The problem is that there is zero protein intake to balance that breakdown.

    • “Nope. Not at all.”

      Perhaps you should take a little time off from your in depth fasting research and work on your manners. Maybe you have a point in what you’re trying to say. Maybe you don’t. It’s hard to tell. My brain shuts down as soon as I hear your rude and disrespectful tone. You ever hear the expression “what you are speaks so loudly, I cannot hear what you’re saying”? We know Dr. Fung’s credentials. What are yours? What makes you such an expert? What’s your background in this area? What in your background and accomplishments gives you the moral certainty to say so absolutely “no, you’re wrong” to a person that has so selflessly helped so many of us? Please enlighten us, because right now you come off as a bitter troll with an ax to grind.

      • “We know Dr. Fung’s credentials. What are yours?”

        See, that’s the scary part. I don’t have any credentials. I’m just a layperson who happens to have read some of the fasting literature (for my own weird health issues). Yet, I can spot Dr. Fung’s mistakes a mile away. Can’t you? Do gou not see the discrepancy between the graph (250-300 calories of protein burned perday) and the text (15-20 grams of protein burned per day)? Do you actually accept that 5 shrimps could contain 100 grams of protein?

        As for the tone, I’m sorry it causes you such distress that your brain shuts down. Since you are so sensitive, maybe you noticed that Dr. Fung himself is not exactly courteous towards people who don’t share his views. He paints them as a bunch of pitiful idiots (at least, that’s how I read it). Furthermore, I am having a hard time believing that the mistakes in this post and others are all unintentional. For instance, he shows Cahill’s table 3 (the 15-20 grams of protein during prolonged starvation) but does not mention table 2 in the same paper (that shows 75 grams of protein in early starvation, which applies to the example he gives). Another one: the ADF paper that supposedly shows fasting does not decrease muscle mass does not actually involve true fasting. If you follow the links, you’ll see that the so-called fasting days were actually dieting days (25% calories) where carbs were not so low (and carbs will spare proteins in that context). Seems disingenuous to me.

        • “I’m having a hard time believing that the mistakes in this post and others are all unintentional.”

          Statements like that are the problem. They’re rude and insulting. You’re accusing Dr. Fung of being some sort of evil, lying, conniving person going out of his way to deceive people. Yet there are many on this blog, myself included, whose life have been profoundly changed by Dr. Fung. I’ve gotten a whole new lease on life following his advice after spending years trying everyone else’s to no avail. His believe system is the only one that has actually WORKED. So when I read something he has written that I don’t agree with, I don’t automatically assume he’s lying or trying to trick us. I give him the benefit of the doubt, because so far he has proven he clearly deserves it.

          What I tried to tell you in my previous post, obviously unsuccessfully, is that your accusatory tone is not appropriate for this blog. This is Dr. Fung’s blog. Not yours. If you want to express your opinion’s you should start your own. While you’re here, if you find something that you think might be a mistake you should respectfully ask for clarification in a courteous manner. That is the way well-bred adults behave in polite society. It’s always considered bad form to be impolite to your host when you’re a guest in their house. This is Dr. Fung’s house. You’re a guest. Act like one. Then perhaps others will be more willing to listen to what you have to say. Including Dr. Fung. He might even thank you for finding an inadvertent error he may have made. No one is perfect. We’re all human.

          • I’m curious as to why Dr Fung would be making intentional discrepancies..this sounds like a tin foil hat conspiracy theory. Mwahaha Dr Fung is plotting to make us … less. I’m sick and tired of the nutrition wars online with so many trying to prove others wrong. What is this an ego thing? I don’t agree with 30 bananas a day man, nor vegans or low fat promoters and so simply I don’t bother to comment on their blogs, what’s the point? If you don’t like it press the esc key.

          • You are seriously triggered and sound like a fanboy for this doctor.

        • I agree with Valerie. I just finished a 7 day water-only fast. And I can tell you that you indeed lose muscle. I did a DEXA scan (gold standard for body composition) prior to, and immediately after my fast. I lost 5 lbs. of muscle and 1/2 pound of fat. I was shocked.

          This article makes complete sense to me. I understand it. But the body is much more complex than we realize and each person is very unique. I think the fear of losing muscle is genuine, and it does have some basis from others who have done so in the past. Don’t easily dismiss them because you can explain it with science or math. Our bodies are not that simple.

          I have no idea why this happened to me, I am puzzled. I am a relatively lean active female (5’3, 115 lbs), playing tennis 4-5 times a week, but no weight training. I tracked both blood glucose and blood ketone during this fast. I was eating LCHF prior to fasting and fasted intermittently before, so I went into ketosis on day 2 (higher than 1 mmol/l). My blood ketone continued to rise and was steady at 5-6 mmol/l day 3-7. My blood sugar steadily declined to 55-65 mg/dl after day 2 and stayed there. This is exactly what I expected from everything I’ve studied. By all measure I should have lost much more fat. Why else would ketones get so high if its not from fat metabolism?

          By day 3 I began to feel aches and pains in my thighs and hips. It became so painful that I had to take Advil. By day 7 they were quite intense. It was a sort of a burning ache that is unlike anything I’ve felt. Sort of similar to the body ache one gets while having the flu but more intense and it was limited to my thighs and hips with no other accompanying symptoms. After ending the fast and eating regularly again, the aches suddenly disappeared completely.

          What happened? I don’t know. I suspect it had to do with other hormones in my body. Maybe my growth hormone was suppressed as I was on 500mg of Metformin 2x a day? I began to take Metformin a month prior, as I was showing some signs of pre-diabetes with fasting glucose of 100 in the mornings. I also read a great deal about the benefits of Metformin and longevity, by suppressing mTOR and insulin and insulin-like growth factors. That is my best guess. But even so, I still expected to lose fat BEFORE muscle, or at least more so in relation, so this was a complete shock to me.

          Dr. Fung (or others who are experienced), can you explain what may have happened in my case? I am not menopausal, very healthy in every respect (other than the elevated glucose) and take no other medication other than Metformin which I stopped during the fast. All of my blood lab tests show normal ranges and I used the same DEXA machine both times. I am highly educated, and well read on nutrition, diabetes, and ketosis. I am careful to not “explain” or conclude the wrong things by a singular personal experience that may have been irrelevant; I try to examine everything carefully and scientifically. But for ME, (n=1) fasting DOES indeed lose muscle….dramatically, and with pain.

          This experience has definitely made me realize how about our bodies are so complex and everything cannot be so easily explained by science (and the limitations of our science) at all times.

          • Traci

            You didn’t mention what your body fat vs. lean mass percentages were before the fast but at 5’3 and 115 lbs. you’re thin to begin with. I’m only 5’0 and the last time I hit 117 lbs., I looked very thin. I speculate that in your case, your body fat was low enough that you’re body decided to preserve it so that you could feed a child if you were to become pregnant. Also, lean mass isn’t just muscle is it? It’s organs, tissue, bones, blood vessels, etc. Maybe a lb. or 2 of that muscle was actually extra stuff floating around in your body that you didn’t need? Maybe you had some benign tumors floating around in there that your body ate up? Where were you in your cycle when you took the scan? Again, I’m just guessing. Why were you fasting anyway with so a low body weight to begin with? Because of the elevated glucose? I find it interesting that you would do the DEXA scan prior to the fast if the blood sugar was your only concern. Are you trying to reach a certain body fat %? Curious.

      • Tony, perhaps you should set aside your butt hurt emotions and maybe focus on the question Valerie legitimately poses. Your loyalty to Dr. Fung is noted. Now how about moving on to helping this blog be informative, (not snap your fingers saying oh no she didn’t).
        With all due respect Dr Fung, the chart seems to indicate a higher daily oxidation of protein than 25. We could be and likely are looking at it wrong but please acknowledge this at least. Thank you for all your dedication. Also thank you Valerie and Krisztian for speaking up.

        Dr. Jason Fung: You may quibble with the numbers all you like. Every study will be different. My point, as Valerie notes, is that the body does not ramps up ‘muscle burning’. There is a daily turnover of protein, but urinary losses fall dramatically, as do fecal losses. Modern diets will overwhelm these losses in a single meal, let alone a week of meals. Further, protein losses do not equal muscle losses. There are many other proteins in the body that are not muscle and some that are defective and need removal (autophagy). So what’s your point? Or are you just arguing for its own sake?

        So the Dietary Reference Intake is 0.8 g/kg or 56 grams for a man and 46 grams for a woman. OH MY GOD! WE ARE ALL BURNING MUSCLE BECAUSE WE NEED 75 GRAMS/DAY. ITS A WORLD CONSPIRACY! WE ARE ALL GOING TO DIE! Jeez, relax about the number – we’re talking about GRAMS here, for god’s sake. Are you going to worry about being in deficit 50 grams of protein for 2 days while you’re diabetic and 50 pounds overweight for the last 25 years?

        Yes, the study using ADF defined it as 25% of calories. So what’s your point? There are many ways to do fasting, and this is how they did it. Are you just arguing minutiae to prove how smart you are?

        Tony – thank you for speaking up. I agree completely. I do not mind bringing up inconsistencies, as I will make mistakes, but there is no call for rudeness.

        • Quibbling with the numbers? That’s a significant difference, which was exactly the point I thought we were proving here? Basically the chart refutes everything the article says about protein loss during a fast other than it slows. Maybe I’m confused, but now this article seemed to say x doesn’t happen… But then someone says the chart shows x… Oh, well then x doesn’t matter. Completely avoiding the issue it seems.

    • Elaine in Big D

      Valerie, thank you very much for your comments and for sparking the little debate. I appreciate your being willing to comment on this blog since it’s where much of the fasting info is being offered. I used to fast 10 days at a time back in the early 80s. In later years (after that fasting craze kinda blew over) info came out that organs could be damaged if fasting longer than three consecutive days. I felt it did damage to my heart muscle. Doctors don’t know, you can’t ask and get any kind of sensible answer. In any case, your thoughtful reflections have inspired me to stick with my own limit of three consecutive days. I’m definitely thinking of beginning alternate day fasting. I’m a big IF fan, LCHFMP, with two full fast days a week, having only one small meal a day of real, organic free range, and etc. I do my own beef bone broth (perpetual in a slow cooker makes it soooo easy) and consume it on my full fast days.

      Dr. Fung is correct (not that he needs my affirmation), getting a handle on blood sugar and obesity is top priority.

      You’d think fans of this blog would know that the truth can stand up to any question without fear. Rude is so rude and speaks poorly of the rude.

      Thanks again, Valerie! Godspeed your journey!

  8. Bernard P.

    According to my calculations, this post should be titled PART 15 🙂

    Part 14 is here:

    Dr. Jason Fung: Thanks. I’ve corrected it.

  9. Thanks again Dr. Fung for another very informative post! Your ongoing work is greatly appreciated. I just listened to your podcast with Jimmy Moore and this latest article fills in some of the gaps for me. I hope you offer your thoughts on the use of BCAA’s as mentioned above. Since we are not eating protein on an extended fast (say 2-week fast) does the protein needed for muscle repair, and growth if we are weight training, come from the autophagy process? Thanks again!

  10. [email protected]

    Another really good post, Dr Fung. Thanks for laying out so clearly. Do I notice a “tad” of exasperation creeping in? ?

    Dr. Jason Fung: HaHa. I don’t mind lay people asking me. However, I get doctors who have never fasted or supervised a fast telling me that I’m just ‘wrong’ and fasting breaks down muscle.

  11. As you say, the glycerol, goes to the liver, where it undergoes the process of gluconeogenesis and is turned into sugar, which means that the liver has the ability to produce sugar from stored fat, even if you are eating zero carbs, even if you are fasting.
    My question is : if you have high insulin resistance, could that liver produced sugar raise your blood sugar over the limit ?
    Please clarify that point.
    Thank You

  12. Magnus Løining

    Great post Dr. Fung. This subject is always an issue when it comes to fasting, and many are arfraid because of it. You really succeed with the way you present it here, and make it understandable for people. This shows that our biology is way more complex than just “you must eat all the time to have enough energy”. We are complex machines that are able to store energy for later use, and produce our own energy from our storage, without “breaking down” the body, which make perfekt sense. How would we survive our evolution if we didnt have this capability?
    Keep it coming Dr Fung.. You really help changing the world 🙂

  13. Hi Jason,
    I don’t think our brain needs so much glucose (” the brain ‘needs’ 140 grams of glucose”).

    Have a look at Figures 1 and 3 on this article (pages #7 and #9). Read, please, the text under Fig 3.

    Thanks for your work.

    Dr Jason Fung: I agree that the brain likely could get by with less. However, this 140 grams number gets bandied around a lot by dieticians, which is the reason I used it.

    • MachineGhost

      Not sure where 140 came from, but the IOM set the RDA for carbohydrates at 130g.

  14. Krisztián Pintér

    the article says 15-20g protein per day, however the diagram says 250kcal/day from protein. these numbers are at odds with each other. 250kcal comes from over 80g. how do you reconcile?

  15. Dr Fung, thank you for helping us better understand our metabolism. Today, I am not sure to understand your point properly.

    At first sight, it seems that you are suggesting that, all the weight loss, of a person loosing let’s say 50 pounds on a 100 days fast will “only” come from body fat and water? Wich would be about 454 (g) / 2 = 227 (g) per day.

    On the other hand refering to approximate data from the graph at the beginning, on day one an fifteen, I think that you are saying that, more or less 250kcal or 63 (g) of that weight loss while fasting, will come from lean body mass or protein. The remaining 164g coming from water and body mass? This gives us a ratio of about 2,6 (g) body fat for each (g) of lean body mass or protein.

    From this perspective, what I understand is that from you point of view, considering damage caused by insulin resistance, from carb intolerance, loosing 50 or 63 (g) a day lean muscle in reversing that process is not something we should worry about, because of the wonderfull benefits we will get from it. In other words, IF needed, it will always be POSSIBLE and EASIER to put muscle back once we have been able to get over insulin resistance.

    Am I correct in my interpretation and/or understanding of your point of view doctor?

    Sylvain M
    Val-d’Or, Québec, Canada

    Dr. Jason Fung: The main point is that the body does not increase the breakdown of protein. There is a certain amount of baseline breakdown. Much of the broken down protein will be reabsorbed as urinary losses and fecal losses drop. So, there is indeed a protein deficit, but it is small and can easily be made up in the meals that follow.

    After all, many civilizations eat 10% protein (compared to 20-30% North American) and do fine. So we eat double or triple the protein requirements for decades and then to worry about 7 days (for example) of no protein is ridiculous. 60 years of double protein intake is 20,000 days of protein excess, and people worry about those 7 days?

    The numbers vary from study to study, and remain small in all studies.

  16. Dear Dr. Fung,
    I am still working on understanding.
    What if someone has been very inactive for quite a while?
    If so, and one is insulin resistant, Is it necessary to engage in some form of strength training to retain our lean mass while fasting? In order to To rebuild lean mass as a matter of course during the daily protein turnover process, instead of losing it? Or, if not “strength training,” then at least moving around more?

    If lean mass is protected by growth hormone, what about older folk, such as those over 60 years old? Don’t they have less growth hormone to fall back on? Naturally, they would be even more worried about losing muscle and bone mass as a result of fasting! And I mean !!!worried!!!

    What about “protein-sparing” fasts? Are those more effective at preserving lean mass?

    I hope my questions are clear enough to elicit some clarification. After all, if it is hard for doctors to wrap their minds around these concepts, it is understandable that the rest of us would find it a challenge.

  17. […] faire bonne mesure, aussi celui ci qui indique clairement qu’il n’y a pas de fonte musculaire en cas de jeûne de quelques […]

  18. From Cahill’s study, you can see that the urea nitrogen excretion, which corresponds to protein breakdown, goes way, way down during fasting/ starvation. This makes sense, since protein is functional tissue and there is no point to burning useful tissue while fasting when there is plenty of fat around. So, no, you do not ‘burn’ muscle during fasting.

    I just finished the book A Study of Prolonged Fasting (1918), and it pretty much says exactly what you said above. This book is very concise in the measurements they took–this was actually a textbook, and I didn’t know it. The only sad part is that they didn’t mention much on the urine glucose measurements, so I could get an idea of how many days of fasting it took to reach a zero level of glucose in the urine, and put it to work in Hubby’s case.

    Other historical fasting books I’ve read that list the basic chem values (including glucose) of the urine say that their subjects reached a zero urinary throwoff of the glucose in 4-7 days (depending on severity of diabetes). I know we have better instruments today for measuring BG, but I’m curious as to what BG level renders a zero level in urine glucose, and is there any way to measure urine glucose at home?

    I’m game to try historical methods for getting Hubby’s blood sugar under control without supplements or resorting to insulin. One way that was mentioned is the “yolk cure”, so I’m going to look for recipes that incorporate a lot of yolks to give that a try. If it worked in the days PRE-INSULIN, it ought to work now to some degree.

  19. Devialini De Souza

    Dear Dr. Fung
    As I have mentioned below, I am an ardent follower of your blogs and I find them very informative not to mention extremely useful.
    I do have a question though:

    When my husband was diagnosed with diabetes about 15 years ago, he had dropped from 94kg to 74kg in a matter of a month and a half. The doctor said that this was because he had diabetes (established by doing the Hb1Ac and his triglycerides were way over the top). He definitely lost a great deal of muscle as I remember his thighs being all that!!! His fat was located mostly around his belly.

    So if insulin resistance causes weight gain and then diabetes, how did he end up losing so much weight in such a short time with all of his metabolic markers going completely out of sync.

    He has never regained all of that weight as he plays tennis thrice a week and is relatively active. He went up to 81kgs and is now down to 75kg on the LCHF but his sugars are still high although better than before. The control on his sugar has been quite poor over the years despite following “recommended” guidelines. He is now, however, down to 20 units of insulin from 70 units.

    • I am not a physician or scientist.
      Perhaps he lost weight because his pancreas had stopped making insulin? A rapid weight loss is often one of the the signs that alerts people to the onset of Type 1 diabetes.

  20. Speaking of muscle mass, a lot of the historical accounts of fasting mention re-gaining the weight back during a re-feed when the fast was over. Wouldn’t this be the same as yo-yo dieting? Of course, people mainly used fasting as a detox and cleanout, rather than a weight loss method (although a few did use it for weight loss and diabetes control).

    Wouldn’t this whole scenario just be a lather-rinse-repeat for the continuation of bad (or wrong) eating habits? My, how far we’ve come medically.

    • [email protected]

      Wenchypoo, I typically will lose a pound a day during fasting and gain back .5 pound when I start eating again. So there is net loss of .5 pound per day when I do 3-7 days fasts. [others are successful with shorter fasts, not me] Dr Fung does warn us that we will gain back some of the weight when we start eating again so that is an expectation. The other part is that one is not supposed to continue bad eating habits. If a person does that, they should fully expect to see no improvement. We are coached to stay on a LC diet, which I have done.

      And lastly, I have a net loss of 15 pounds and have eliminated 70% of my diabetes meds. I can see a major improvement in my immunity, energy levels, blood sugar levels and mental clarity due to fasting.

    • I have a net loss of close to 50 pounds (depending on the day), but that includes 20-25 using low carb, and the rest using low carb and fasting. I’ve fasted up to five days, have done multiple 3 day fasts, and currently do 16:8 4 days per week, 20:4 two days per week, and one day where I eat 3-4 meals including breakfast. (16:8 = 16 hours with no eating and eating only in an 8 hour window.) I’d like to do longer fasts, but currently have to take aspirin, which must be taken with a meal. I may be able to stop aspirin, where I’ll go back to fasting one or more days. I personally find that I am not that hungry while fasting and after fasting. Today, I had scheduled a 16:8 fast and had to force myself to eat my “brunch” (not sure what to call it when it’s at 2pm).

    • For me its hard to be on LCHF all the time. I admit that I do go off the LCHF during weekends or if I’m unwell. One interesting thing I noticed after my first 3-day fast is that I can’t eat as much as I used to before. That said, I do plan to continue the 3 day fast once every month or fortnight at least till I reach my goal weight. Maybe even after.

  21. Winston Lee

    1 am 76 years of age, weighing 90 KG with a height of 170 cm. I have type II diabetes for 2 years and am taking blood sugar reducing medications prescribed by my family physician, but never any insulin. For fasting, what precisely are the foods I should take and at what quantities and timings? For me, low blood sugar seems to be more serious than high blood sugar. If I do not take my meals in time, my sugar reading often falls to around 4 or even below and I start to feel dizzy and have to swallow sugar cubes right away. Is this relatively mild DM at early stage reversible by fasting? How can I sustain the fasting? I live in Vancouver.

  22. thank you Dr jason i have tried your fasting way for curing diabetes 2 and have amazing results here in Zimbabwe . may God bless you, continue to do the good work. Stay blessed

  23. Dr Fung or anyone, is there any reason blood sugars would rise from Intermittent fasting? Not dangerous levels but higher baseline and PP’s than normal. Other markers are good ie sleep, mood, energy, good appetite control, not experiencing cold etc. Would this mean in a case like this the person is not good candidate for fasting?

    • I had a “fasting” blood sugar test done after fasting for four days, and it was 62. The “normal fasting” (meaning about 12 hours of fasting) test I had done about three months before that was in the low 90s.

  24. Dr Fung, thx a lot for your great articles. But as i’ve read your good article about Fasting and also this one muscle mass fasting, i have to say the clear meaning of “FASTING” is unknown. research done on 7 day or one month fasting bases does not express all this good result comes out from which kind of fasting??? taking less than 50 calories a day , or 16:8 window diet, or carb diet….. Pls give an advice or fully article about “what is fasting metabolicly, what make your fasting broken ”

    thanks so much

  25. Well, I spent much of Sunday reading all of these fasting articles, after a huge meal of eggs and bacon around 2pm. It’s Weds night and I have not eaten anything since that meal on Sunday. I have never fasted longer than maybe 18 hours in my life, and I’m 53. I lost a lot of weight about four years ago on the so-called paleo diet, but I’ve slowly regained about half of it back. After reading Fung’s fasting articles and the entire hormonal series tonight I’m pretty sure that despite very good blood lipid and glucose numbers that I’m still insulin-resistant, which makes sense once you realize that it’s INSULIN that causes fat gain. So I’m hoping that this big fast and future ones will break that cycle, and I will eliminate all dairy in the future — I was eating huge amounts of cheese — except for pastured butter to see if that will reduce excess insulin production. The other fact that supports insulin resistance is that I was a fat kid, back when that was very unusual despite neither of my parents being fat, and was a fat adult until the paleo thing. I don’t think I was ever clinically obese until just before I started paleo. I dropped 50lbs within six months, but instead of being slim and lean I was skinny-fat, and I’m not that tall (5’8″) so dropping from 200lbs to 150 in six months was probably too fast and destroyed a lot of muscle.

    Oh and I feel great almost four days in; my feet seem cold most of the time, but otherwise I have had more energy and greater mental acuity this week than in a very long time. I think I can make it until Friday night and I have only had 2-3 serious hunger episodes. I’ve weighed daily but it’s all over the place so I think I’ll stop doing that.

    Given that even tasting sweets can cause insulin release, I have to wonder if the flavorings in e-cigarette liquid could also provoke it. I quit smoking in April and switched to an e-cig with no relapsing.

    Anyway, glad I found this site.

  26. I freakin’ love how comprehensive your posts on fasting are.

    I’ve been fasting for 2 years now and I’ve noticed that the best way I can preserve my muscle while fasting is to eat a very high protein meal to break my fast every day. I backload my carbs for night time. This has helped me IMMENSELY with losing fat while still keeping my muscle.

  27. I am on day 9 of a water fast and doing great. My question is, can the beta cell burnout be reversed with a c-peptide result of .57 or is the pancreas too far gone by then?

  28. Hi Dr. Fung,

    I have couple of blog entry ideas.
    Is there any chance you could do a blog post on much longer fasts? I mean fasts lasting multiple days and weeks (even months…). I ask for two reasons. First, your very comprehensive content is amazing, but tends to apply mostly to fasts lasting less than 42 hours. Second, I have noticed in the comments that many people are fasting for many days and sometimes weeks at a time (like Jimmy Moore). It would be great to have specific advice for this.

    Also, I think a blog post that helps people to find a “maintenance” plan of eating would be great. I know you spend a good deal of time exhaustively outlining best practices for weight loss, but where many of us struggle is in finding a “steady state” style if eating. I know it’s complex- you point about this is not lost on me. Yet I know that many would love to hear about a good method for maintaining their weight.



  29. Lo Carb Faster

    Awesome post you are a fantastic person and in time I hope many people especially doctors and nutritionists take a reality check and change their ways.

  30. Dr. Fung,

    Do you have any thoughts about the Protein-Sparing Modified Fast developed by Drs. Bistrian and Blackburn?

    This is from the beginning of the abstract.

    “Total fasting reduces hunger and induces rapid weight loss,2 but half the weight loss in the first month and one fourth to one third thereafter is from muscle tissue,3 and hepatic, renal, and endocrine function is affected. For these reasons fasting regimens are generally recommended only for inpatients.

    Long-term follow-up of persons who previously fasted has demonstrated a high rate of recidivism5 due to obligate regain of lean tissue or the lack of a weight-maintenance program or both.”

    November 17, 1978
    Clinical Use of a Protein-Sparing Modified Fast
    Bruce R. Bistrian, MD, PhD
    JAMA. 1978;240(21):2299-2302. doi:10.1001/jama.1978.03290210081040.

    Dr. Jason Fung: Many of the older studies on fasting started with a 14 day fast, which was never again repeated. They then followed a low carbohydrate diet, which has repeatedly failed.

    What they did NOT do, is repeat the fasting throughout the study period, such as is done with alternate daily fasting studies. Also, historically, fasts are repeat events. Whether it is done yearly or weekly these fasting periods were not one-off events. Do I think a single fast followed by a year or two of low calorie dieting works? No. Absolutely not.

  31. […] Continue to Fasting Part 15 – Muscle Mass […]

  32. Sascha Heid

    Would a single protein-shake a day reduce the hormonal benefits of a fast?

    My reason for asking is not that i’m terribly concerned about muscle-loss.
    I did a 5-day water-fast about 6 weeks ago and have not visibly lost muscle.
    However, i am currently very focused on Lifting and workout several hours a day. Very happy to have holidays now and be able to do that with getting plenty of sleep and reading-time. But if you work out really hard it is mentally really hard to not get any “gains” from that or even just lose a little bit of muslce. I am also doing cardio, so… would it be ok to have a protein shake in this scenario?

    Thank you!

  33. Ramana Babu

    Hello doctor, I am a type 2 diabetic since 1 year and I am on Metformin 500 mg plus Voglibose 0.3 mg, my question is how to go about the fasting which you are recommending and do I need to stop my medications on the days of fasting. I am overweight also (89 kg for a height of 5 feet 5 inches), pls clarify

  34. Dr Fung,

    I understand your logic and the science behind the minimal loss of lean mass during long term fasting. I am on day 8 and have noted a significant loss of lean mass in the 8 days. I track my weight daily and use an Omron body fat scanner. My weight started at 200.5 lbs with 18.3% BF. I am now at 190 lbs and 17.4% BF. This means I have lost 3.63 lbs of fat which is great but then 6.87 lbs of lean mass. Not good. I have reduced but not eliminated my weight training and have reduced my cardio even more.

    Any ideas as to why the reversal of what your post suggests should happen?

    Thanks much


    • Rhonda Kaye

      Hi Mike,
      I’m not an expert, but I think there is another possibility you might not be considering. Body fat scanners work by sending a small electrical pulse through your body and reading how quickly it travels. Fat slows down the pulse, but muscle and water both conduct the electricity very quickly, and the meter reads them as being the same. Therefore an electrical fat meter or scanner can only give readings about fat levels. It cannot tell you if the non-fat weight you have lost is indeed lean muscle mass or if it’s actually water weight loss.

      The other thing that comes to mind is that since you have reduced your levels of exercize it’s quite possible that you have lost some muscle as a result. If that’s the case, the muscle loss would be a result of changing your activity levels and would have happened regardless of how you are (or are not, in your case) eating.

      I hope this helps.

  35. I am also wondering why my husband was visibly wasted after a medical fast for seven days for pancreatitis. No nourishment- not even IV – for seven days. He lost 20+ pounds and looked like death. He has been much less muscular ever since. That will be three years ago, in March.

  36. Hi Dr. Fung,

    Excellent article as always! I did have one clarification question I was hoping you could shed some light on.

    Bodybuilders and other athletes often talk about losing strength and lean muscle tissue when cutting, traditionally done through calorie restricted diets. From your article, my understanding is that the preservation of muscle tissue during fasting is due to increased growth hormone (GH) as a result of the fasting. Is it then the case that muscle loss can occur on traditional calorie restricted diets as a result of suppressed GH, since the subjects are not employing fasting to keep GH high?

    And if that is the case, could you please explain why it is that calorie restricted diets lead to loss of muscle mass/strength in the first place? It seems to make intuitive sense that regardless of caloric intake, if the subject were consuming an adequate amount of protein, that alone should spare lean tissue mass. I wouldn’t think that GH would have to be elevated to preserve muscle tissue if protein is provided through diet. Thank you, and thanks again for all your hard work you do writing these articles.

  37. […] seems to be the complete opposite of what most people would predict, but when you align it with the current trend for intermittent fasting (eat most of your food in a short window each day and fast the majority of the day), it actually […]

    • I have not seen or heard Dr. Fung say before that the human body operates in three different states when it consumes energy to function: (1) the fed state, (2) the restricted-calorie fed state, and (3) the unfed or fasted state. The body when in the unfed state has gone without calorie intake for 6 hours or more. This is when human growth hormone kicks in to save the muscle from being consumed to make sugar for the blood. And I have read in several places that muscle will not be consumed much if any during a fast up to 72 hours.

      The problem with most diets and with cutting performed by bodybuilders is they are trying to lose weight while in restricted-calorie fed state. They don’t consume enough calories to meet their bodies’ energy needs, but they consume food as meals and snacks which keep the body in a fed state. This means that insulin continuously is secreted into the bloodstream which prevents the body from tapping on stored-body fat for energy. So what is the body to do – it is basically starving! Of course it is going to tap on muscle for energy. When in either the fed state or the calorie-restricted fed state the body doesn’t naturally produce all that much human growth hormone. This is especially true as we get older which is when we are more than likely to develop insulin resistance.

      Common sense tells us that if we lose muscle mass, then we will lose strength. The more muscle we have, then the stronger we will be. And the less muscle mass we have, then the weaker we will be.

      Keep in mind that the extended fasts (30 hours to 72 hours) should not be done all that regular after you have gotten to your target weight. These longer fasts are not conducive to a healthy life style since you cannot count on getting the proper amount of nutrients into your body when doing heavy resistance training, HIIT workouts, high intensity cardio, and moderate intensity cardio. These workouts all require considerable amounts of carbohydrates to be performed properly. And they are the kind of exercise a healthy person should strive to perform. But low intensity cardio (walking, biking, swimming, and the like) is excellent to be done during a 3-month weight loss program involving limited carbs in the diet, regular fasting each day, along with some 30-72 hours fast mixed in. Low intensity cardio is ideal for burning fat while in the unfed state. The body can get enough oxygen to process the fat for energy while fasting.

      Try to do anything more intense than low intensity cardio while fasting and your body will require carbs to function. And guess what it will try to do – yep, search out some muscle tissue and try to convert it to sugar as if the body is in a restricted-calorie fed state.

      Dr. Jason Fung: Jeff, I think you are completely incorrect.

      • Paul,Kelley

        Jeff, you need to read the book by Dr. Stephen Phinney and Jeff Volek, “The Art and Science of Low Carbohydrate Performance”

        • Hi Paul-Kelley,
          I’ve now read it. And I stand by my comments above. That book is primarily about living in a constant state of ketosis, and about training to excel at Ironman triathlon type events where one strives to maximize athletic performance at or near their aerobic threshold. Said another way, it is about endurance athletes competing while in the ketogenic state. There is nothing in that book that suggests burning fat is conducive to training for high intensity sporting events by doing resistance training and HIIT workouts. And besides, it takes 2 to 4 weeks to get to a ketogenic state for most people. So my comments above regarding intermittent fasts that last less than a week have nothing to do with that book. But thanks for your comment even though I think it is completely incorrect.

  38. so… “muscle” break down still occurs…just decreased. One, the tests were done on obese individuals, not body builders or those who just want to gain muscle mass who are not obese. Do you have a known research paper regarding my latter statement? If yes, It would be very nice and informative if you provide. Furthermore, what if a non-obese person fasts? Can you provide a research paper on that? it would help “us” understand your secondhand theories more. Second, The “Protein-retaining effects of growth hormone during fasting involve inhibition of muscle-protein breakdown” research, that you provided, states that “Muscle-protein breakdown was increased among participants who fasted without GH” Whether it had been suppressed or not, the abstract still proves my first statement is still correct – muscle breakdown still occurs, just decreased. I find your points misleading just to push your theories. But kudos to most of your information that helps. More power.

  39. I have a question about the “Days of Fasting” chart cited from Dr. Hall’s book.

    I noticed that x-axis begins with -5 days of fasting, and am wondering why is there a significant change in the oxidation rates of the macro-nutrients already happening before the fast begins at day 0.

    For example, why would the carb burn rate be going up sharply over the days before the fast even begins? Is there some other pre-fast dietary change at work? Or, is day 0 actually the start of the fast, or does it represent some other time, such as when the body has burned up the carbs available in the “fridge” (in the two compartment model)?

    Thanks to anyone still reading the comments who can shed light on this!

    • My guess… people knew they would be fasting for a while and ate more just prior.

  40. Dr fung thanks alot for your beautiful work. But could you please reply to JC objections ?

  41. Krystian

    Hello and thank you for this blog 🙂
    What would happen if a person performs strength exercises (normally done in order to increase muscle mass) during an extended fast, for example lasting 7days. Can body repair micro damages in muscle tissue during this fasting period? If yes, would this process be effective enough to provide muscle mass gain?

    • Fasting is not something that should be done with resistance training. The body has various energy systems it can pull from in order to function. Power lifting and resistance training use the two anaerobic energy systems which control exercises lasting between 1 second and 2 minutes. High intensity interval training (HIIT) workouts also predominantly use the anaerobic energy systems. And these systems burn glycogen (carbs) stored in the body’s liver cells and muscle cells. Fat cannot be used to fuel strength exercises. And if you are on a 7-day fast you are only going to have fat to burn as an energy source.

      You ask what will have if a person tries to do strength exercises (power lifting and resistance training) while doing an extended fast. Well the body will search for carbs to burn and come up empty-handed. Such exercise will damage the muscle, and instead of repairing the muscle it will use that same muscle as a source of carbs. Basically the body will operate as if it is in the calorie-restricted unfed state. See my reply herein above regarding the three states: fed, calorie-restricted fed, and unfed.

      You don’t need a bunch of lab-rat tests on this subject. Just look at Tour de France cycling athletes and Ironman athletes competing in Hawaii. On more than just several occasions you see athletes who have not taken on sufficient nutrition and faltered before the end of the race. Sometimes this is called a “bonk.” The body caves in on itself. It tries to regroup and consolidate its existing resources to live. If you do resistance training while on a 7-day fast, then you will bonk. It’s just that simple. And a bonk is not a healthy thing to experience.

      Dr. Jason Fung: Jeff, I think you are completely wrong.

      • MachineGhost

        You’re conflating resistance training with endurance training. The latter is horribly catabolic which might as well be the same thing as low calorie starvation.

        • Hi MachineGhost,
          Isn’t resistance training catabolic if you don’t consume protein, the building block for muscle, soon after completing your workout? And isn’t that similar to low calorie starvation? And what if you do eat soon after the workout, then why was it so important to do the workout during a fast? I was not “conflating” aerobic workouts with anaerobic workouts like you suggest. Did you think I was because I used the term “bonk?” Bonk simply refers to doing athletic endeavors and running out of glycogen stores and therefore the brain doesn’t get its normal fix of glucose. This can happen during endurance events which is when it is usually referenced. But it can also happen if you try to do high intensity workouts with no glycogen stores. I’m definitely not “completely” wrong.

      • Hi Jeff,
        If a person is overweight, then that person has excess protein in their system besides just muscle tissue. Skin and all the extra connective tissue to support that skin consists of proteins correct? So maybe, just maybe it would not only be ok for overweight people to weight train and/or perform HIIT while fasting, but encouraged to increase autophagy of all that loose skin? Also in the examples you listed, these people that are already super lean. THAT I think is the difference.

        • Hi Traci,
          Weight training and/or HIIT workouts are fine if the person does them while they are in the fed state and have some glycogen stores. Consume some whey protein after the workout, and then commence on your fast. Everything will be fine. But when you are in the middle of a fast and your glycogen stores are depleted you simply do not have the appropriate fuel to use to do quality high intensity workouts. You might be able to do a high intensity effort (or at least think you are) while having only fat to burn for fuel. But you simply canNOT train at a high intensity without glycogen and carbs.
          I appreciate Dr Fung writing his book Obesity Code. And most of what he says in it I agree with. However, to me it seemed to be a book about old sick people who were certainly not athletes. And the book is about losing weight, not about being physically fit. The book says somewhere that exercise has little to do with losing weight. And I agree 100%. But I think Dr Fung’s experience and research has been limited to insulin and insulin resistance in a sick person – not insulin and insulin resistance in a middle aged male who is very active physically, overweight but not obese, who wanted to lose weight. This middle aged male has applied what Dr Fung talks about in his book and does his resistance training and HIIT (high intensity) workouts when in the fed state. And he does his low intensity endurance workouts when in the fasted state. He is unable to have quality (meaningful) high intensity workouts while in the fasted state. This person is 5’4″ and weights between 155 and 160 lbs, but would like to get down to 135 lbs after competing in a national cycling championship in two weeks. Losing weight and building muscle don’t really go hand in hand.

  42. i think it’s important to keep your eyes on what’s important. as dr. fung says, so what if you’re protein deficient for a few days? people fasten their attention onto little “defects” in the strategy to avoid facing the big idea of fasting as a way to get healthier.

    similarly, there are a lot of questions about exercise and muscle maintenance and mini-adjustments in diet and fasting regime. i doubt that these questions have answers. if there were answers they would be statistical and not necessarily applicable to any particular individual. there is enormous variability among individuals. i suspect that the only answers to these questions will be individual answers that will have to be discovered by the individuals themselves.

  43. Mark Twain

    26 jumbo shrimp not 5. Jumbo shimp is 17% protein, not 100%. You would need to consume 26 jumbo shrimps to compensate for the lost protein assuming it really is “only 100 gram”. But I’ve read some other critcisms on this page that make me doubt a lot you say.

    Usually it gets attacked with “But Fung is a Doctor, he must be right!”

    Which doesn’t say much, do you have any idea how many incompetent doctors I know? Fung is far from incompetent, but definitely not infallible and it isn’t the first time I feel his positive representations of fasting are skewed too much towards the positve part, while skewing or ignoring a lot of negative facts.

  44. […] במהלך צום, האם מדובר בפירוק שריר ושריפת חלבון? גיליתי מאמר שהסביר שמדובר בתהליך גלוקונאוגנזה המבוסס על גליצרול, […]

  45. I trust Dr.Fung and switched to water fasting now for to toal of 60kg weight loss

    atm I am losing about 700g-800g daily and still have 42kg to go

  46. Khalid Halloumi

    Great article
    I have a question though
    Some people fast by abstaining from food while still drinking water.
    I fast Ramadan for 30 days a year and it’s done with total abstinence from everything even water.
    Have there been any comparison between fasting with water or fasting without?
    Thanks in advance

  47. […] See also: Fasting and Muscle Mass. […]

  48. Steve Bergman

    Table ‘b’, at the top, indicates that the subjects did go into a ‘starvation mode’ while fasting, with BMR falling drastically. At the start, they were burning around 2500 kcal/d. By day 30, their daily caloric expenditure was down to around 1300 kcal/d.

  49. Dr. Fung: I found the article informative, but am still left with one question. I understand that muscle is conserved during fasting, and protein consumption drops. Any protein that is required for functional/repair work is obtained from tissue other than muscle, mostly. My question is – on alternate-day fasting, what would be approximately be the correct amount of protein i should be eating? Please help.

  50. You can’t build new muscle if you aren’t consuming protein. (You aren’t consuming protein on a water-only fast.) And you will lose muscle if you do anything that requires a high calorie burn – weightlifting, high intensity cardio, high intensity yard work, etc. Also, if you are already lean, you will tap into muscle for fuel a lot sooner than those that have large fat reserves. Don’t do any high-intensity work while fasting, especially if you are already lean. I’ve personally experienced very visible muscle loss on several occasions while trying to experiment with this. Take a casual stroll while fasting for fat loss. Eat sensibly if you want to build muscle or preserve muscle and need to get some work done. Seriously.

  51. About all that follows: I would be grateful even for a reference to any research papers that address the questions I have.

    After reading Dr. Fung’s article and all of the above posts, I am left with the question: During a long fast, say over three days, when your ketone levels rise above 3 (blood ketone test) can an obese person still create muscle with resistance training? To produce significant muscle tissue do you need to consume protein? Or is it sufficient to consume fat — either consume it from an external source or have the body metabolize it from it’s own internal intramuscular store of it? I know that all herbivores produce incredible amounts of muscle tissue without eating one scintilla of protein other than the odd few insects and single celled animals caught up in the pond water and the grass they consume. They have the required enzymes for the breakdown or hydrolysis of the cellulose they consume and I speculate by this the cellulose is converted to a carbohydrate of one form or another. I know carbohydrates are long chain sugars and are converted to simpler short chain molecules by the body. This then must be converted to the protein they are made of. But can human beings convert intramuscular fat while they are in ketosis to protein. Can fat be converted to glucose? Or must this all come from other stores of protein. And if from protein, from which form of protein in the body? I am 70 and overweight now so I water fasted for 8 days and felt great, in fact better than I have felt in a long time. After 8 days I got hungry and just started to eat again with a diet designed to keep me in ketosis. I plan on fasting to this extent again soon and lifting weights and cycling, weather permitting, during that same fasting period. I have in the past and am now cycling extensively — 20 to 40 miles on a day ride with very steep hills sprinkled about to keep it interesting. What will happen to the muscle tissue I have already built and what will happen beyond the normal “loose a few gain a few” ounces of muscle tissue?

  52. […] periods of fasting (over 24 hours) results in some loss of lean mass. However, the loss is way smaller than most people believe, and under most circumstances it’s simple to regain that lean mass between fasting periods. […]

  53. Paul Kelley

    Dr. Jason Fung: I’ve been a member of the IDM LDP since May 2015. I was pre- diabetic and overweight. I tried for years to lose weight following many diets. None worked long term. Thank you for your great work, wisdom and courage to stand up to conventional medical treatment.

  54. Paul Kelley

    Furthermore, Megan Ramos is a great help to all of us on the LDP IDM program. She is to be highly commended.

  55. “Don’t worry about what your diet (or lack of diet – fasting) is doing to your muscle. Exercise builds muscle. OK? Clear?”
    This sounds extremely stupid, excercise will only provide the stimulus for muscle growth, diet is exactly what provides the muscle growth.

  56. Let’s see…….

    according to the graph, a person fasting uses about 250 cals/day of protein…. which is roughly 80 grams of protein. A pound of muscle is roughly 100 grams of protein (before anyone whines that a pound of muscle weighs 454 grams, remember that most of that is water). There’s only around 100 grams of protein per pound of muscle.

    0.8 pounds of muscle loss per day doesn’t sound all that great to me. That’s over 5 lbs per week.

    • I meant 60 grams of protein, or 4+ lbs of muscle loss per week

      • Matt, this matches my understanding too. I think Dr. Fung’s argument is that we should not worry about it as body will quickly rebuild the muscle after the fast. How quickly? I am not sure, I have not seen any studies on this. I would appreciate any feedback on muscle rebuilding rate after a prolonged (> 3 d) fast.

        • Continuing with this: It seems that muscle growth rate is about 0.5-1% of body weight per month. This is for a normal person doing some exercise. Thus, a 150 lb person could recover 0.74 – 1.5 lb of muscle per month. To recover 4 lb would take 3-5 months.

          This assumes that muscle growth rate after a fast is “normal” and not somehow faster.

  57. Why not do some heavy weight training (of low volume -reduced number of sets) during an extended fast (once you are in ketosis). Won’t the heavy weight training signal the body to consume fat instead of muscle? One reason for fasting is to reap the benefits of autophagy, preferentially consuming damaged cells. Can’t the body then recycle the amino acids to rebuild the muscle that was damaged during the weight training?

    • Tom,
      You cannot rebuild body muscle during fast: Muscle is broken down to generate glucose for brain. Body cannot convert fat into glucose so some muscle loss is inevitable.

  58. tim mccarthy

    i have wrestled with Jasons thoery that muscle can not be eaten by the body as it would be bad for the survival of the human..but testing shows rapid muscle loss as per Phinney and others. So I came up with this theory as Im sure others have.
    I now think that the body will immediately respond to less food by reducing muscle, because reduced caloric need will be beneficial to the human in the event of food shortage and also this allows the brain to remain on the highest point on the energy critical path. This is not to say fasting doesnt work at all but I know im supplementing my fasting with some protein regardless of the insulin pulse that happens. My fasting may not be a insulin holiday but its still helping me control the intake of calories.

  59. Dr. Fung,

    I think there are some additional considerations:

    1. The ADF study you cited was not a full fast. On fast day, the calories were restricted but subjects still ate. I do not think this result can be extended to prolonged water fast.

    2. Most starvation studies have been done on severely obese persons. The protein usage for energy generation is higher for normal or just overweight persons. A person with BMI=25 would get about 15% of the energy by burning muscles. Assume that person exercises during fast, caloric expenditure could be 2,500 kcal. 375 kcal would be from protein which is about 1 lb of muscle per day. Thus, the person could loose 7 lb+ of muscle in one week water only fast. Recovering this would take many months. Please see:

    Elia, M. R. J. S., R. J. Stubbs, and C. J. K. Henry. “Differences in fat, carbohydrate, and protein metabolism between lean and obese subjects undergoing total starvation.” Obesity 7, no. 6 (1999): 597-604.

    I certainly agree that fasting is beneficial but I would not advocate long water only fasts as a way to loose fat without loosing muscle.

  60. I intermittent fast. I do 16:8 6 to 7 days a week. I am just starting to do HIIT routine in the middle of IF. People are telling me that this is not good because I will lose muscle and my body will go through atrophy. I don’t know what to believe. I’m still going to do this to see for myself. I just wanted to know what people thought.

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