Preventing Muscle Loss While Fasting

by Nils Osmar. March 21, 2022

I’m a big fan of fasting and have experienced profound benefits from doing prolonged water fasting; time restricted eating; and fasting mimicking diets. I credit fasting with having helped me to recover from an autoimmune condition a few years ago.

But is fasting always benign and helpful? Or can we find ourselves losing muscle mass when fasting, while our body fat actually increases? According to Dr. Peter Attia, the answer to the latter question is a conditional “Yes,” However, there are ways of preventing or minimizing the problem.

To be clear, Attia is also an advocate of fasting, and has a history of doing one three day fast a month while also doing time restricted eating. (He’s not dismissing the benefits of fasting.) But in this interview with Tim Ferris, Attia discusses a patient who was doing a large amount of time restricted eating and ran into serious problems without being aware of them.

The patient had been fasting every day but doing almost no exercising. (I.e., he was not working out with weights. ) His body weight had barely changed over a few months’ time, but a DEXA scan revealed that his body fat had actually gone from 18 percent to 30 percent, and he had lost a corresponding amount of muscle. His visceral fat had also increased dangerously during the period between checkups.

His situation was probably aggravated by the fact that he wasn’t exercising. But Attia himself reports finding that his body fat went from 10 percent to around 16 percent during a period of time when he was doing a large amount of fasting. According to Attia: “I always exercise in the morning and don’t eat,… to exercise, then not provide yourself with any amino acids, every single day, to undergo muscle protein synthesis, is a bit risky. So I’ve been looking at other strategies.”

So What to Do?

There was a recent period in my own life when I was doing a great deal of fasting, while simultaneously building muscle and losing fat. Why did things work out this way for me, while Attia’s patient was going in the other direction?

During the eight months between August of 2020 and May of 2021, I was doing a great deal of alternate day fasting (eating no food at all) every Monday, Wednesday and Friday, then exercising (very intense workouts) on Tuesday, Thursday and Saturday. I was eating high protein, animal-based meals on my feasting days, sometimes right before working out, sometimes right after.

During this period, I went from 27 percent body fat to around 19 percent. I was visibly gaining muscle and losing fat.

I’m not claiming that I had some kind of deep wisdom, just that, for whatever reason, this approach worked for me, and that it’s definitely possible to prevent muscle loss while incorporating fasting into your life. And many others have had similar experiences. Why was I gaining muscle and losing fat, if excess fasting can sometimes lead in the opposite direction?

I think what made the muscle gains possible for me during this stretch was

The intensity of my workouts
The fact that I was usually working out fasted
The fact that I was eating high protein meals shortly after working out, and
The fact that I was not trying to restrict protein on my workout days. (I was eating omnivorous meals that included foods such as (grass fed/organically-raised) eggs, fish, poultry, red meat, organ meats from small family farms, and other sources of high quality protein within an hour or so after my workouts.)

I’m not saying everyone has to eat exactly like I did to offset the possible negative effects of fasting, just that eating these foods, while also doing a lot of fasting, did work for me.

What about mTOR?

Some people in the anti-aging community who’s read about my diet have responded that “Eating plants is better” and that “You’re probably over-activating mTOR.”

They could be right, but I don’t think so. I’m not disputing that eating plants and minimizing animal foods is one effective way of activating AMPK and switching off mTOR. But it’s clearly not the only way.

I’m not saying that I’m a carnivore; my diet actually includes more plant-sourced foods (by volume) than animal-sourced food. And I sometimes do vegan days and days when I’m eating zero food, both of which should be activating AMPK and suppressing mTOR activation.

But I have not seen any strong evidence that we need to, or should, be activating AMPK and suppressing mTOR full time. To me it’s about finding the right balance. My goal is to have AMPK activated at least 3/4ths of the time (by fasting and basing many of my meals around plants), but during the periods when I’m activating mTOR, to not be shy or tentative about it.

Whatever approach you use to prevent or minimize muscle loss while also incorporating fasting in your life, Attia’s experience, and what his patient went through, points out the importance of keeping track of changes in your body fat percentage. I didn’t have access to DEXA scans, but used a scale that calculates the percentage of body fat, water weight and bone density. Such scales aren’t necessarily accurate in a clinical sense, but they are useful in a relative sense, showing the changes clearly as the weeks and months go by.

Image by Sabine Mondestin from Pixabay