Studies: Intermittent Fasting Makes You Lose Muscle, Not Fat

I go over studies on how intermittent fasting (time-restricted eating, specifically) affects your health, weight, and body composition. In particular, I'm focusing on a recent study that is an RCT with way more participants than past intermittent fasting studies, and a meta-analysis of meta-analyses. Press "show more" to see the study link(s)!

[davidadcock]

I have been doing OMAD for 5 years. Eating mostly protein and my muscle mass has increased along with my much improved health. I have no plans to stop and go back to the way I was.

[joelm]

it doesn't matter what you believe, you can find dozens if not hundreds of videos that tell you that you are right - no matter what you believe. Go with what works for you and ignore the Youtube experts. Why do I say that? Because I started fasting a few years ago and lifting (light) weights every three days. I am now 75 and for the first time in my life I have arms that are well on their way to being very muscular. The trick is don't lie to yourself, do not start a routine and five days later say that didn't work for me. It took you a lifetime to get to where you are. Change will take months or even years.

[Joannacollie]

Hi Mich, you are my absolute favorite health YouTuber but I’m not convinced on this one.
I usually intermittent fast but having watched your other videos on early eating - I decided to try breakfast and lunch again . I ended up massively overeating and bingeing in the evening and gained body fat on my scales. This has been my experience on many many occasions. When I do IF, I eat a lot less and lose a lot more body fat with little difference to muscle when I work out.
I believe that people who are trying to maintain weight loss have a different reaction to food due to lower leptin levels than a ‘never been fat’ person. Eating can actually make you more hungry! This was the ONLY thing I changed - as an experiment so it wasn’t something else.

[ccuellar]

Intermittent fasting saved my life. I have never been healthier or in better shape in my life.

[thomasgrant]

I found that point about fasting increasing loss of muscle mass very interesting, being 70 and having been into strength training for a long time. There was a parameter in the study, however, that gave me pause. I and several other people I know use intermittent fasting periodically to lose weight. And it works very well. What confused me about the study is that I was unclear if the requirement for the fasters was that they eat what they normally ate in a day or that they eat what they want to eat while fasting during the study. The primary reason I find that intermittent fasting is successful is that it is an easy way to reduce calorie intake. When you have less time in a day to eat, you just naturally eat less, and when you start fasting, it only takes 3 or 4 day before you have no desire to eat outside of your eating window, whereas trying to reduce calories without fasting results in you craving food all the time. That's fasting's charm. Anyway, I just discovered your channel a week or so ago and have been getting lots of wonderful information from it, so thank you..

[greentree_]

Dr. Jason Fung said that you should NOT eat nonstop in your eating window, he says NOT to snack at all in between meals (breakfast, lunch, dinner). If your eating window is close to normal, you should not eat other than your breakfast, your lunch and your dinner. If your window is really really small you should eat only one meal. he says these studies on Fasting are done wrong because they tell people to eat freely, as much as they want, as often as they want in their eating window.

[larsd]

I have been practicing 16/8 for almost 5 years and feel great and strong. Now I have perfect levels of sugar, insulin, triglycerides, liver, kidneys, prostate, blood, vitamins, minerals while not taking supplements. I did not have that before. No illness at all during those years. I can go hiking or skiing a whole day without eating anything. 58 years old. Btw: LDL can be high when you're on keto when LDL is used to transport fat. Now worries.

[dcs]

I have been doing IF as OMAD for last 4 years. My Type 2 diabetic pills had to be stopped in first three months. My A1C test being conducted every six months is normal. I am on OMAD for 4-5 days a week, Sat I am on multiple meals and Sun I keep extended eating window of about 5 hrs to break the routine. I also indulge in long water only fast roughly once a month. I am exercising in gym 4-5 times in a week. I can still complete my gym routine. During the long fast of 3-4 days water only I don’t go to the gym.

[gregbannish]

I lost 70 lbs in 6 months doing pescatarian keto and alternate day fasting, and kept off for 3 years now with 20-4 eating window. It worked to lose the weight when nothing else worked, but definitely lost muscle mass and cholesterol shot way up. Now trying vegan keto, scared of carbs but may go there, and exercise, to lower cholesterol and build back some muscle. IF and keto proved to myself that i didnt have to be fat in middle age, i enjoy being thin, and so am happy i did it...

[BlakeDoesBusiness]

Theres an important question here: what was the level of physical activity among the study participants?

Muscles atrophy through disuse. If you are fasting this process would be accelerated.

I'm an extraordinarily active man (rodbuster + bodybuilding) and I do IF because from convenience: I eat after work so I dont have to bring a lunch.

I've lost roughly 15 to 20 lbs in the last 6 to 8 weeks through IF and caloric restriction - roughly 500 to 1000 calories fewer than my body burns per day, but with a high amount of protein (150 - 200 g per day).

My body composition is visibly improving but I'm curious: how could I determine whether or not I'm losing lean muscle mass? Cost effectively of course.

[ronih]

Hi, I tend to disagree with you on this one. First of all, the controlled group in this study is flawed, since they all lost weight. I understand your point that this is to be expected of all control groups in researches of diets, but it is still flawed IMO. Second point is that IMO the main purpose of intermittent fasting is to stop the habit of overeating.
Something that over-eaters tell themselves is that they cannot survive if they don't eat constantly (this is also done through hormonal mechanisms). Intermittent fasting gives people this huge message which is - JUST STOP EATING. It won't hurt you, you can live without food for awhile, it is OK to be hungry. This is an important message which other diets miss.

[emeafterlist]

Eating 2 meals a day or sometimes 1 meal a day definitely doesn’t cost me more money than eating 3 meals plus snacks per day.
But it does cost the food industry to make less money.
Are you funded by the food industry?
You should debate this with Dr. Jason Fung.

[thatonegamergirl]

Were these normal weight people or overweight and obese? That is a huge factor. If your body already has the amount of fat that it seems necessary for survival of course it will break down muscle for energy. If there is a large amount of excess fat why would the body break down muscle instead of fat? Also were these people asked to do any physical activity? Also a big factor in if you lose muscle while fasting. Was this true muscle loss or glycogen loss from muscle tissue? I don't think this proves anything either way.

[sebacatana]

Love your vids and logic. Dietitian from the EU here. :) Would be interesting to see a study with a bit more protein (1.2-1.5 g/kg) and a bit of resistance training. I'm willing to bet there would be no muscle loss. Just sitting at a desk all day and eating protein will probably do nothing.
Keep up the good work!

[Choco-un]

I did the 20 hours fasting for 4 days, got a body scan before and after and I lost 2.7kg of body fats and gained 0.4kg muscles mass. So this video is incorrect. Not just that though, I can focus more at work and my moods changes completely! My anxiety is gone as well. And I stop bloating.

[MichePhD]

Fledge Fitness response: He didn't talk about the meta-analysis OF meta-analyses that formed the basis for my whole video (It's in JAMA, a top-tier journal). The individual studies I talked about were examples to be able to discuss individual protocols and illustrations. Unfortunately the smaller meta-analysis he cites is in a journal that is known among scientists to be unreliable and is even becoming predatory (Frontiers; it's a pay-to-publish model). Also I have 0 clue why he thinks I'd be biased against IF--I really don't care either way. My channel isn't about IF at all. I'm just biased against misinformation. Meanwhile he has sponsorships and is selling things to people based on his entirely pro-IF channel.

Original:
I just focus on sharing the best science I can here, no matter how it comes out--the current best evidence we have is that IF is not very helpful for most people. If you have found it personally helpful, it is definitely possible you're an outlier and it works great for you. The point here is that it's not ideal for people on average. If you enjoy IF, as I say in the video, you should keep doing it!
If the tide of the scientific evidence ever changes in favor of IF, I will share that too.
Longer clarification:

I just want to clarify that the point of the video was not to say that no one should do IF, but rather, that people should not feel like they need to do IF. I was trying to push back against the tide of “gurus” saying that IF is the magic weight loss bullet for everyone, given that the science says the opposite. I was not trying to say that IF is universally bad for everyone, but I apologize if I came across that way.

I have no doubt that there is a subset of people who gets great effects from it because it fits well into their lifestyle, circadian rhythm, etc., and in those cases any muscle loss might be outweighed by the benefits of the fat loss someone experiences in the long-term. My only point is that the studies—including the meta-analysis that I cited—suggest IF (specifically time-restricted eating) is not going to be useful on average for weight loss, diabetes, blood pressure, and/or blood lipids (e.g., cholesterol).

Also, I should have been more clear that the only health parameters I was talking about whenever I referred to "health" were the specific ones I discussed: diabetes markers, blood pressure, and blood lipids. IF could theoretically have beneficial effects for overall longevity, etc. even if it does not help with the aforementioned parameters, but that was beyond the scope of the video (and I haven’t done a thorough lit review on that yet so I can’t comment either way).

[bernkbestgirl]

It is hard for me to argue with studies, but these results seem very counter-intuitive with how the body works. Why does the body go through the effort of storing fat in the first place, if a brief 8 hour pause in eating is enough for it to start burning muscle instead of fat, even though there are still fat stores remaining? It seems wildly inefficient and doesn't make sense. Would love to hear a reasonable explanation for this.

[kevjackson]

Love your channel, Miche...The study did not note Macros or what type of exercise these participants may or may not have done. I have done IF and OMAD for periods on and off...I lift weights at high intensity and regularly. I never lift fasted but when I do IF I don't think I've lost muscle b/c my total cals & macros and protein were recorded....these participants were likely not monitored 24/7 understandably....

[peterreeves]

Hi Miche! I'm curious about long term fasting like 36, 48, 72hr. Any good data points on this? Thank you in advance -Peter

[truthpop]

If you eat enough protein and calories, along with some resistance exercise, I.F. wont cause muscle loss, but it wont necessarily be a healthier lifestyle. Fasting starts benefitting after you burn up your last meal. With I.F. you don't have time to get there and then rebuild muscle.