Fasting guides
Success stories
Video
NewsFasting and muscle mass
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?”
NO, not for short-term fasts.
Here’s the most important thing to remember. If you are concerned about losing weight and reversing T2D, then worry about eating in a way that lowers your 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 frequently 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 and you will build muscle. That’s stupid. There’s one good way to build muscle – eat enough protein AND 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?
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. 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 a limited ability to store sugar. So, after the first day, fat burning starts.
What happens to body protein? Well, the amount of protein your body turns over goes down. There is certainly a baseline low level of protein turnover, but my point is that we do not start ramping up internal 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 others had not. In other words, physiologic studies of fasting had already concluded 30 years ago that protein is not ‘burnt’ for glucose.
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.
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.
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.
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?
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.
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
More
Intermittent fasting for beginners
Top videos with Dr. Fung
Earlier with Dr. Jason Fung
Obesity – Solving the Two-Compartment Problem
Why Fasting Is More Effective Than Calorie Counting
The Complete Guide to Fasting Is Finally Available!
How Does Fasting Affect Your Brain?
How to Renew Your Body: Fasting and Autophagy
Complications of Diabetes – A Disease Affecting All Organs
How Much Protein Should You Eat?
The Common Currency in Our Bodies Is Not Calories – Guess What It Is?
More with Dr. Fung
Dr. Fung has his own blog at intensivedietarymanagement.com. He is also active on Twitter.His book The Obesity Code is available on Amazon.
His new book, The Complete Guide to Fasting is also available on Amazon.
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?"
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?
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 muscle...you'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.
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 ever...food 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.
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.
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 ?
Thanks,
Kenny,
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).
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.
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.
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.
There are some problems with your references :
You wrote : "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."
In this study (https://onlinelibrary.wiley.com/doi/full/10.1038/oby.2010.54), it was a modified ADF protocol. The subjects consumed 25% of their baseline energy needs on the fast day, and ate ad libitum on the feed day.
They didn't lose lean body mass because they ate 25% of their energy needs (55 carbs, 25 fat, and 20% protein) on the fast day. That was not an ADF with a zero calories on fasting days.
About the second study, you wrote : "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."
In this study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5042570/) the subjects didn't fast on any other day for 32 weeks but only for 8 weeks. The following 24 weeks were a "unsupervised follow-up to assess risk for weight regain after completion of the intervention. » and during that period « Participants had no contact with study staff during the follow-up period and dietary adherence was not assessed ».
So it was a 8 weeks ADF and not a 32 weeks ADF.
What happened after the 8 weeks of the intervention phase ? (table 4) : subjects lost 3.2 kg of lean mass in 8 weeks and they regain 2.1 kg of lean body mass during the follow up, because they were not fasting anymore.
Through these studies, it seems that exercise and diet are both important to preserve lean body mass.
Thank you.
Your comment seems very well researched and makes sense. Can I ask in this case, what you would advise if someone wanted to lose bodyfat and gain muscle? Thanks
I can tell you from firsthand experience that it IS possible to gain muscle while losing bodyfat. I typically forgo all eating on Tuesday and Thursday and lift with a reverse pyramid scheme on Monday, Wednesday, and Friday. I have been at this for a few months, and I have continually gained or maintained on every lift while losing about 23 pounds. This stuff really works.
@Matt you sort of answered the question lingering around in my head. I watched a video of a dude who documented himself on Youtube, doing an ADF (Alternate Day Fasting) for 30 days. He already had some muscle on him and you could see his abs. At the end of the 30 days 15 days eating and 15 days fasting with no food and sometimes no water), he looked even more lean and shredded. There was no sign of muscle loss and he only went to the gym one time during that 30 day period. It does make sense that we would use our fat as fuel to make up for any day that we fasted and/or any day that we had restricted calories. I was thinking about fasting twice per week, and do three days of lifting and cardio and then the fourth day I would fast while doing light cardio such as swimming or walking that day and then repeat all over again.
Right now I'm about 6'1 to 6'2, and 175 to 180 lbs. I'd like to get leaner but I started a bulk to gain more muscle after a year off of lifting from a rotator cuff injury (if anyone wants to know how to naturally heal that, send me a message, I can give you tips). I've been vacillating between doing a bulk or just maintaining muscle while dropping fat (I've been doing Intermittent Fasting for many years now so it's easy for me to maintain weight whether I'm exercising or not). But when I heard about Alternate Day Fasting, it seems to be the 'sweet spot' for fat loss while still being somewhat capable of gaining muscle if you're getting the right amount of protein on the days that you're eating.
I think I will start next week and save this article. I'll come back and let you guys know how it goes around Christmas or New Years (about 2 and half months from now). I'd like to see any of your own before and after result pictures if you have them and your daily regimen as well.
I'm definitely interested in your natural rotator cuff repair. I love to lift weights but seem to have really aggravated my right shoulder and want to use my upcoming fasts as time to heal. Any tips would be appreciated. Thanks!
I workout 5 days a week i lift heavy weight broken up different body parts over monday to friday (can not workout on weekends) my heart rate is always around 120 to 156bpm during lifting. I can not perform high impact cardio due to knee injury so low impact cardio is all i can do.
I was doing the 8/16 fast for 4 weeks i decided to get all test done. My resting metabolic rate is an atrocious 994 calories even though i have never eaten below 1200 cals on non workout days and appro 1355 based on my current weight and height. So i didnt do this to myself by starving myself my genetics are just bad.
Height 161cm
Weight 62.5kg
Bodyfat tested via dexa scan 39.1% even though i do weights and exercise regularly.
Heres the kicker get ready for this. My RER of which i had fasted from 7.41pm the day before meaning and metabolic test was performed at 11.15am the next day. And i had only consumed 55grams of carbs over the course of the day before was a resounding 1.00 so please explain to me why after a 16 hour fast my body refused to use fat as its oxidation for energy. I was told by the tested that my body JUST USED CARBS my body refuses to burn fat hence why my bmi is healthy but my fat percentage is obese and my muscle is low. I eat the accurate amount of protein for my lean muscle mass and body fat composition. Doctors are useless they say well your bmi is healthy WTF????
I have a normal low blood pressure
High hdl and ldl due to low carb dieting
Bradycardia due to the amount i workout not heart related problems. Resting heart rate anywhere from 49 to 57bpm just watching tv.
So why cant i burn fat huh???
If I'm following you correctly, it sounds as if you may be eating too many carbs. If you are wanting to reach ketosis and become fat adapted, you may want to try eating at keto levels. Here are a couple of links that should be helpful for you.
https://www.dietdoctor.com/low-carb/how-low-carb-is-low-carb
https://www.dietdoctor.com/low-carb/keto
I would recommend seeing your doctor regarding any issues with your heart rate.
I would get checked for diabetes - if your body fat % is really that high 39.1% it's a risk factor, and your symptoms fit. IF your not diabetic, then instead of fasting you could try low carb for a while and see if that help fast loss ( some people genetics via gene expression suit different modes of fat loss, its not 'one size fits all'.). If that doesn't work, you may have a complex metabolic/gene issues that diet alone cannot address and you could try from exercise. You will need to raise AMPK to express different pathways, which your current heavy lifting does not as well. This is what cardio does (raise AMPK), but your knee issues prevent that. I would try metabolic circuits - lift a weight you can lifting for 12-15 rep range - move quickly to the next machine/or free weight with minimal rest between exercises, no more than 60 seconds max rest - and do as many as you can for 30-60minutes. This will give a strong 'cardio' type effect, raising AMPK and firing different gene's than your current 'heavy lifting', and may help. You could also try swimming if you have access, and I would try a high intensity/low intensity workout I used - so you do short sprints of 30-60 seconds, following by 2-3 minutes of slow swimming, for 30 minutes or so. Note. The main aim of the exercise is not direct 'fat loss' but to try to improve your hormonal and metabolic responses, so your diet is more effective.