Fasting and muscle mass

Superhero Training

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. Clear?

Does fasting burn muscle?

Macro-oxidationSo 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. 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”. 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, because protein turnover is reduced, but not no, it is not zero.

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 30 years ago that protein is not ‘burnt’ for glucose.

UreaIt 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.

Fasting switches the metabolism

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.

MuscleBreakdownFrom 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.

Triglyceride-300×181Where 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.


Despite all the physiology, the proof is in the pudding. The best way to get an answer is to put people on a fast and measure their lean body mass. Some people have claimed that every fast over 24 hours burns 1/4 to 3/4 of a pound of muscle.

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?Catenacci1

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.). According to the fear mongerers, there should have been approximately 15 pounds of lean tissue lost. In reality, there was zero. 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?

Recently, a randomized study published in 2016 by Catenacci et al also fasted people who fasted every other day for 32 weeks, which is more than half a year. These people fasted approximately 36 hours and compared to those with caloric restriction alone. At 32 weeks, there was 1.6 kg lean tissue lost with calorie restriction, but only 1.2 kg with fasting. As a percentage, 0.5% increase in lean tissue percentage in calorie restriction (because of fat mass loss) and 2.2% increase in fasting. That means that fasting is more than 4 times better at preventing lean tissue loss. Once again, fear mongerers would have estimated about about an 18 pound muscle loss. Sorry, welcome to the real world, where our body does not store food energy as fat and then burn muscle.Catenacci2

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.

For good measure, from that same study, what happens to RMR (Resting Metabolic Rate). During caloric restriction, the number of calories burned by the body at rest goes down by 76 calories per day. During fasting, it only goes down 29 calories per day (not statistically significant from the start of the study). In other words, fasting does not slow your metabolism. But guess what? Calorie restriction will slow your metabolism as sure as night follows day.

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. BUT after fasting, the high GH will encourage the body to rebuild that lost lean tissue. If you simply estimate muscle losses by looking at breakdown, you entirely miss the fact that the body is rebuilding it afterward.

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.

No, fasting does not mean you burn protein for glucose. Your body will run on fat. 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.

Jason Fung


Intermittent fasting for beginners

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  1. Eddie
    Very interesting, really enjoyed reading that post. It certainly has always been something I was concerned about. I am currently reading Dr. Fung and Jimmy Moore's book, the complete Guide to Fasting. Unfortunately though there is not a lot in the book on fasting for people with Type 1 diabetes. As a T1 diabetic, I would love to know what the consensus is on fasting for T1 diabetics or is it something to be avoided if you are T1 diabetic?
  2. Patrick
    So, to lose weight on hflc I assume that I burn the fat I eat, before my stored fat to lose weight, so when burning body fat I am still restricting calories? (less calories in than used). Does this not slow my metabolisim too as this appears to be calorie restriction. Am I missunderstanding? should I be worried?
    Replies: #3, #4
  3. Martin
    The fat you eat in a meal(say 30g) will go to mostly to fat, muscle and liver tissue to storage for later use. Then you will burn a mix of the recently acquired fat and your previous stores. Phinney in lectures ( and books show "the four phases of a well formulated ketogenic diet" ( So this person is burning 50% of his energy from stores and 25% from diet fat in the first phase. No apparent slowed metabolism at this stage. This is because your fat storage in the absence of excess insulin is free to leave the fat cells and then is plenty of energy in the blood stream in the form of free fatty acids, ketone bodies and glucose.
  4. Gentiann
    If you restrict the fat on a low carb diet, your body will see it as a signal to "shortage of food" and, in response, may slow you down , saving your fat stores....It's not about calories. Do not skip the fat in the diet.
  5. Robert
    It would be good to see an explicit debate between Jason Fung and Stephen Phinney, on extended fasting especially. There seems to be little controversy in the low-carb scientific community, that daily intermittent fasting is perfectly fine and even beneficial. A bit less so, as the fast extends into days or even weeks.
    Oversimplifying a bit of course, Phinney's emphasis is mostly on the scientific literature and fundamental biochemistry, but Fung has many years of clinical experience with fasting, successfully treating obese and diabetic patients.
    My sense is that their differences have more to do with the overall lifestyle question: "How does extended fasting beyond 24 hours on a regular basis, help or hurt a healthy individual, in the long run?"
    Reply: #10
  6. David
    This is dumb as shit. Diet doesnt affect muscle gain? PROTEIN doesnt affect muscle gain?!
  7. Nicklas
    So fasting is muscle preserving because of evolution, that makes perfect sense. But calorie restriction burns muscle how the FUCK does that make evolutionary sense?

    So the body is only smart when there is no food, as soon as there is a little bit food, the body suddenly becomes RETARDED and stats burning more muscle?

    Replies: #21, #24
  8. John
    David and Nicklas,

    After reading your moronic messages, I felt the need to address your ignorance because it doesn't look like the owners of this website respond to people's comments.

    David, diet and protein don't CAUSE muscle gain. Suppose you just sat on the couch all day and did nothing, then someone suggested eating more protein to gain'd just gain more fat and not have a discernible increase in your muscle mass.

    That's because muscles don't grow just because you provide them with protein. They grow in response to use. Any protein not needed by the body gets converted into fat cells or eliminated.

    You can eat all the protein in the world, but if you don't exercise it won't do diddly.

    Nicklas, caloric restriction is different than fasting. Yes, they are both caloric restriction, but fasting is 100% caloric restriction, while caloric restriction generally refers to less than 100% decrease in calories.

    When calories are decreased but food is still present, the body can continue to work normally to some degree (depending upon the severity of caloric restriction). However, when fasting, the body has no choice but to completely revamp its systems to conserve protein while burning fat for energy.

    If you paid attention to the graphs and looked the studies linked to this article as I did, you would have discovered this for yourself.

    Reply: #19
  9. John
    Regarding fasting...

    I have fasted numerous times throughout my life for up to 22 days at a time. At no point have I lost any perceptible muscle mass. I felt just as strong and was able to lift as much or more weight when weightlifting.

    On the other hand, my fat level did go down significantly and visibly, and I felt and actually was healthier than ever after extended fasting.

    Prior to the 22-day water fast I had mildly elevated blood pressure. After fasting, it was normal and has remained that way. My cholesterol levels (which were never that bad), improved significantly with lower triglycerides and higher healthy HDLs.

    To top off the weight loss and improved health, I felt better than tasted better, my senses were sharper, and my mental processes were improved.

    I highly recommend fasting. Naysayers don't know what they are talking about.

  10. John

    You asked, "How does extended fasting beyond 24 hours on a regular basis, help or hurt a healthy individual, in the long run?"

    In my experience, extended fasting can be an awesome and healthy experience. I started fasting in my teens while I was a high school football player and weightlifter. I'm now almost 50 and have fasted for extended terms as long as 22 days many times. (My average fast was perhaps 3-5 days.)

    Despite what anti-fasting fearmongers would have people believe, I suffered no ill-effects from fasting. I didn't lose any discernible amount of muscle mass, but I did lose a considerable amount of fat each time. I usually exercised during these fasts--5-15 mile runs and weightlifting--and only experienced a minimal loss of energy DURING the fast, and none afterward.

    My health improved every time I fasted. As I have aged and let myself go more than I would like, I put on some weight and my blood pressure went up. I was on blood pressure medication, but after the 22 day fast, my blood pressure was normal and has stayed that way. No more meds for me! My doctor was thoroughly impressed with the positive changes in my health.

    The side effects after fasting were all positive: sharper senses, less illness, more energy and strength, and just a generally excellent and youthful feeling.

    It's certainly possible that fasting has negative effects on some people, but my own experience and the profoundly positive changes that fasting caused for me shows me that those who are opposed to fasting are either willfully ignorant or have another agenda.

  11. Adam
    David, I am 100% with you. There is no way anyone can say that your diet doesn't affect muscle gain. There is solid evidence that your body need the proper nutrition, calories and protein amount to gain muscle as well as other nutrients to help your body break down this protein to fuel your sore muscles after a workout to help them rebuild so please. DONT EVER BELIEVE THAT DIET HAS NOTHING TO DO WITH BUILDING MUSCLE!!! Sure you also need to workout and exercise but you will NEVER gain muscle if you workout and not eat protien. Please gain some solid evidence before you publish this work that is easily proved wrong by many others around the world.
  12. Jesse
    Dr fung Please reply. I have been on a fast for 5 days now and had creatine today. Did I break my fast thank you
  13. Jack
    I am on my 33rd day of non consecutive cyclic water fasting. I have lost about 50 lbs and dropped 3 inches off my belt in 43 days. None of my resistance training or cardio has suffered, all volume and weights has stayed same or gone up. I have about 30 lbs more to go, and this article has just psyched me up to see this through. This article was written by an extremely intelligent person. I am sick of all the "bro science" about how you lose lean mass when fasting and how it is so dangerous. Like this great man says, it is bs. Our bodies were made to fast. Thank you so much for writing this article and all your other works. You truly are a god among insects.
  14. Kenny
    I have a question in this regard.
    I understand that fasting won't cause me to lose muscle on it's own, but how will it impact muscle building ?

    I'm trying to lose weight, but also gain muscle mass, so I work out at the gym 2 times a week.

    So when should I plan my workouts ? Should I do these while in fasted state or not, and how will that impact fat loss and muscle building ?


  15. karthik
    In many places, I read that the body starts reacting (burning fat, ketones production, increasing GH, etc) start only after 24-36 hours. If I eat before 24 hours (as in 16-8 fasting or 24-hour fasting), won't this process prevent burning the fat? After 24 hours, if the fat burning would start in hour 25, but then if I already eat food at hour 24, then will the fat burning never happen? I'm just trying to find the logical answer.
  16. bill
    Your fat burning stops in the presence of insulin.
    Eating fat doesn't elicit an appreciable insulin
    response, therefore you can eat fat before 24
    hours and still burn fat.

    This is the reason this website has been confusing
    people because of its focus on fasting. I wish it would
    get back to LCHF (Low Carb High Fat).

  17. Forrest
    I have read both of the books and loved them! But I am not sure I buy this idea. I am on day 7 of an extended water fast. Intending to go 20 days for detoxifying etc. and if any weight stays off that is good, especially if it is fat. However, on the day before the fast, my body fat % was indicated as 29%. Now it is at 32.2%. Granted, without advanced testing, it's not fully accurate, but each day it increases, and the net result is it shows I am not losing any fat, but am losing muscle etc. At the end of the 20 days, if I don't see any change, I will throw away the books.
    Reply: #20
  18. Dan
    So we dont need a 'calorie surplus' to build muscle? if so, that is interesting. If anyone has experience/answer to this i would be grateful. many thanks
  19. Dan
    So we dont need a 'calorie surplus' to build muscle?
  20. Irina
    I have the same experience. 1% muscular loss per day.
  21. Karen
    Please don't use the "R" word.
  22. ThatGuy
    It would be idiotic to try to muscle bulk while trying to lose weight. Choose one or the other people.
  23. 1 comment removed
  24. Christina
    I know this is old and John did a good job responding, but left out something critical in his response. And I'd like to add for anyone else looking up fasting.

    In caloric restriction, on a standard diet, insulin is being produced. When insulin is present, sugar is burned and fat is not. When insulin leaves the bloodstream, the body is supposed to switch to fat.

    For someone with insulin resistance (a great many people given our obesity epidemic), the body produces too much insulin for the amount of sugars consumed and, once the sugars are used as energy, forces the body to burn protein instead - because the insulin locks out the fat burning.

    The reason why fasting and lchf don't have that effect is because they do an end run around insulin and keep it out of the picture. Simple caloric restriction does not do that, and thanks to the catabolization of protein, lowers metabolism.

    So caloric restriction results in loss of muscle mass because it doesn't control insulin and fasting does not result in loss of muscle mass because it controls insulin.

  25. Sonia
    I have a question; i just started the intermittent fasting last week and is great! but... i want to start working out: boxing.
    i’m doing 16/8 but if i start working out, my training would be in the middle of the 16 fasting period... can i still have my protein shake after working out? or that would break the fasting? how does this affect the protein intake i should have?
    ( i am doing 16/8 and keto as well)
    Reply: #26
  26. Kristin Parker Team Diet Doctor

    I have a question; i just started the intermittent fasting last week and is great! but... i want to start working out: boxing.
    i’m doing 16/8 but if i start working out, my training would be in the middle of the 16 fasting period... can i still have my protein shake after working out? or that would break the fasting? how does this affect the protein intake i should have?
    ( i am doing 16/8 and keto as well)

    A protein shake would definitely break a fast. Most people find they don't need to supplement anything to incorporate exercise. Also watch out with the shake as many have artificial sweeteners and other ingredients we recommend avoiding. Keto is moderate protein and incorporating the protein shakes may make your protein intake too high.

  27. Pierre Lichtlé
    So basically I can build muscle while on a caloric deficit? I don't need to bulk like I've always been told to? Yes I've gained a bit of fat from bulking, but muscle as well! Would I just not have gained fat if I'd eaten normally?
  28. Robert
    Hi Pierre, Dr. Fung sums it up as follows:
    Weight loss and gain is mostly a function of DIET.
    Muscle gain/ loss is mostly a function of EXERCISE.

    So you can lose weight (fat) and add muscle mass with the correct combination of exercise and macro-nutrients. We do it all of the time.
    Adding muscle mass during a prolonged fast is probably impossible, but should be easy using an intermittent fasting approach.

  29. Vlad
    Do these results hold in the case in which one were to do weight training?

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