September 1, 2019


Can you maintain muscle during fasting?

But in the short-term (at least up to a week for most people), I believe you can maintain muscle mass while fasting, particularly if you’re resistance training.

Read Time 4 minutes

In last week’s email, I discussed a study, “Effects of testosterone supplementation on body composition and lower-body muscle function during severe exercise- and diet-induced energy deficit: A proof-of-concept, single centre, randomised, double-blind, controlled trial,” in which a severe energy deficit resulted in losses of fat mass (FM) alongside gains & maintenance of lean body mass (LBM) in the testosterone and placebo groups, respectively.

If you recall from last week, these young men were prescribed daily exercise to the tune of > 1,500 kcal expended, and were instructed to slightly restrict the number of kcal/d compared to how much they were eating at a weight-stable baseline. On average, participants were facing an estimated daily energy deficit of over 2,000 kcal/d.

So these guys were in an “energy deficit” and yet many of them maintained (or even gained) LBM. One of the questions I posed in my previous email and want to address in this one: what happens to LBM during a water-only fast? In particular, what happens to skeletal muscle mass? Can you maintain muscle while fasting?

We know that if skeletal muscle mass is maintained, muscle protein synthesis (MPS) and muscle protein breakdown (MPB) are balanced overall. So, what are the ways in which you can boost MPS and/or limit MPB?

Eating a meal that contains protein with high quantities of all of the essential amino acids, leucine in particular, is a great way to stimulate MPS (and reduce MPB). If you’re fasting, this important contributor of MPS, and attenuator of MPB, appears to be out of play.

Another way to stimulate MPS is by exercising (resistance training in particular, but also aerobic exercise, perhaps to a lesser degree). However, this also stimulates MPB. If that’s the case, how can we build muscle if exercise both stimulates the synthesis and breakdown of protein? It turns out that exercise also sensitizes skeletal muscle to the anabolic effects of a protein-containing meal, which can result in an overall increase in MPS and LBM. But if we’re fasting, apparently there’s no protein-containing meal to be had.

Anecdotally, during my week-long water-only fasts where I engage in daily resistance exercise and 1-2 zone-2 rides, I don’t seem to lose any muscle mass. (Granted, I’m basing this off pictures and bioelectrical impedance scales for the most part, so there may be some small margin of error.) Given the apparent constraints above, how is this possible?

For one thing, fasting increases the production of ketones, beta-hydroxybutyrate (BOHB) in particular, and this may spare the body’s protein from oxidation. Infusing BOHB is associated with the maintenance and even increases in circulating branched-chain amino acids (i.e., leucine, isoleucine, and valine). Steve Phinney and Jeff Volek have pointed out that the primary driver of MPS is the availability of essential amino acids (especially leucine) and that in the keto-adapted state, blood levels of leucine increase. In other words, the keto-adapted state spares protein and preserves lean tissue. Couple this with the effects of mechanical stress (and myokine signaling, perhaps) from resistance exercise on MPS, and it could be a powerful 1-2 punch for muscle maintenance during a fast. But how can you synthesize muscle protein if you’re fasting?

During fasting, insulin and glucose plummet while counter-regulatory hormones increase (e.g., growth hormone, adrenaline), setting up a milieu that favors fat-oxidation and enhanced autophagy. While protein breakdown exceeds protein synthesis during a week-long fast, this does not necessarily mean that MPB exceeds MPS. All cells contain proteins, so when proteins are catabolized, this may be preferentially coming from skin and intestinal cells, for example, compared to muscle cells. It stands to reason that during times of famine, muscle preservation is favored over other cells that turnover more rapidly. (Jason Fung, a previous guest on The Drive, has a great post explaining muscle preservation during starvation.)

Globally, mTOR—described as the cell’s general contractor by my friend David Sabatini—is turned down during fasting. But, the mechanical stress from resistance training can turn up mTOR (mTORC1, to be more specific) locally (i.e., in muscle cells), independent of growth factors and amino acids (which are the two other well-known ways to turn up this protein complex). This may help partition some of the amino acids in the body that are broken down toward muscle, thus preserving muscle mass. I don’t think it’s out of the realm of possibility that you can “eat” a meal of protein that contains all of the essential amino acids during a fast. This may be one of the benefits of autophagy, which literally translates to “self-eating.” Your muscle cells may be dining off your skin cells, so to speak.

Obviously, the longer you fast, the more you drain your internal sources of food, and the more likely MPB will exceed MPS over time. (This may also suggest that the fatter you are to begin with, the longer you can eat yourself, and presumably maintain muscle mass for longer.) If you want to ADD muscle mass to your frame, fasting is obviously not the best strategy. Getting leucine, and all of the essential amino acids for that matter, from outside sources is a must for long-term growth and maintenance of muscle mass. But in the short-term (at least up to a week for most people), I believe you can maintain muscle mass while fasting, particularly if you’re resistance training.

– Peter

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  1. “Anecdotally, during my week-long water-only fasts where I engage in daily resistance exercise and 1-2 zone-2 rides, I don’t seem to lose any muscle mass.”

    I really want to push back on this. I’m guessing you DID in fact lose muscle mass. Let’s be honest, you could have easily lost a few pounds of muscle and not noticed as you were losing fat otherwise. To assume you must have lost 0 LBM simply because you didn’t detect it with casual observation seems a bit presumptuous. We don’t have reason to believe you lost DRAMATIC amounts of muscle- but “not dramatic” doesn’t equal “I’m guessing zero”.

    Something that stands out to me is the fact that the subjects in your study “during severe exercise- and diet-induced energy deficit” were fairly chunky lads. Both groups were over 21% bodyfat to start. There’s relatively good data very obese people are far better at metabolizing fat, to the extent LBM loss is nearly nonexistent under high protein conditions, or gains can be seen with the addition of resistance exercise. It’s not unreasonable then overweight people would still show some enhanced LBM protection, to the extent a placebo group showed only nonsignificant reductions in LBM.

    The point that I’m driving at is this- I think that preservation of LBM in already lean people under caloric deficits shifts to being an increasingly harder uphill battle that hasn’t really been examined and could have serious implications in trying to extrapolate results to “normal” or athletic lean individuals.

    One case study on the extreme end would be a case study of a competitive bodybuilder, who lost about 75% lean mass during a deficit despite lifting and taking a huge cocktail of anabolic substances The fact that he started at 4.5% bodyfat really might suggest that it becomes increasingly difficult to further reduce fat mass as one becomes leaner and leaner. Sure, it’s some junky case study with skinfold calipers, but let’s be honest- if it were possible to hit low bodyfat percentages with just lifting and fasting, bodybuilders (with the help of tons of anabolics) would lose almost no lean mass getting into contest shape.

    I think keeping in mind the STARTING body fat percentages in all studies can help to contextualize these results. Whether people are giga-obese, obese, overweight, “normal”, athletic, lean, or ultra-lean may dramatically impact their LBM-preservation prospects and the precautions that needs to be taken to further reduce body fat.

    I hope we see more experiments on sub 15%BF individuals, but I don’t think the research funding is really there.

    • Hey, little late to the party here, but I’ve done long fast few times. There is some muscle loss. However, most of what people perceive as muscle loss is really a muscle glycogen loss. And autophagy plays a big role, especially in older people.

  2. If one of your purposes of fasting is to inhibit mTOR to increase cellular autophagy and if resistance training turns up mTOR, wouldn’t that suggest that you shouldn’t do resistance training while fasting?

    • I am sure author means autophagy of other-than-muscles cells. There is a good amount of benign growth, especially as we age. It’s another benefit of fasting.

  3. I Am a female and recently competed in two bodybuilding shows and experimented with fasting 1 day a week 20 weeks out, then increased it to 2 days a week 10 weeks out and 3 times a week 8 weeks out. I still trained pretty intensely with not much cardio and always said I would stop if it affected my strength levels. I ended up dropping 5% in bodyfat and increased LMB by 6lbs as measured by dexascan between Jan and mid May. I don’t think that approach is appropriate for everyone but I did it as an experiment on myself and to prove to others that you do not lose muscle during fasting!

    • Should I drink BCAA during my workouts or is it fine just to stick with plain water? I box, lift, and run. I typically workout twice a day. 30 minutes on my lunch hour and 1 hour in the evenings. I take 1, sometimes 2 days off from working out every week.

      • On a fasting day don’t use bcaa’s as they will kick you out of ketosis. On non fasting days before and during exercise

        • You don’t get into ketosis in a day. It usually takes 3 to 4 days for your body to go through liver glycogen. Depending on your metabolic state and how much your liver stored. Your liver will only start de-novo lipogenesis process in absence it’s glycogen and no insulin in blood stream.

    • Brooke is right on. I completed 2, 5 day bouts of FMD. One day per week between bouts of a 24 hr fast over 2 month period. Training and supplements did not change. Dexa results were +1lb muscle, -11lbs fat. Having a dexa at work is nice.

    • Yours is not the only comment I’ve read from women increasing LBM while fasting. Did you do alternate day fasting three times a week or did you fast three days in a row? I’m currently doing a four day upper/lower split, Mon., Tues., Thurs., Friday and want to fast but trying to find the best days to do so while also taking into consideration increases in growth hormone. Thanks!

  4. Hi I am Dr Ram K from India. I am doing IF to loose wt about 5 kg to drop fat from my abdomen. I am not obese and I am marathon runner. As I am taking less non-veg meal , and not possible to take non-veg daily , I take protein from eggs, almonds, fruits. As a runner, I do lot of cardio. I am 58 now.
    1)Is it necessary to take lots of protein to maintain my muscle mass ?
    2) Is it necessary to eat carb before going for long run like 10-15 kms?
    Thanks in advance.

  5. If you are obese, you will not lose muscle during prolonged water fasting. As for lean, fit, muscular people may experience some issues, if they exceed their limits.
    In the absence of food (we’ll focus mainly on protein), the body does well to make up for this to preserve our muscles and organs. After depleting carbs stored and glycogen stores, the body begins burning fat for energy. Certain hormones began to regulate and reduce. Around the 36 hour mark (if you’re active) and 72 hour mark (if sedentary) fasting triggers autophagy. During autophagy, the body began consuming damaged and dead cells. These cells are forms of proteins. Excess skin, thick tissue surrounding fat cells are all proteins used to preserve and prevent the break down/deterioration of muscle and organs. The more fit you are the less available proteins their are to be used. This is why prolonged fasting doesn’t benefit lean people as much.
    Feel free to research for more info on the subject. As well as autophagy.

  6. So I was looking into a new exercise protocol for this year that involves Resistance Training x 3/week, Cardio (HIIT) x 2/week and 24h water fasting X2/ week. I’m following Tim Ferris’ slow carb diet and I add in some supplements for mostly nootropic effects.
    My question is, given the content of the piece above, should I fast exclusively on days of resistance training? Or can I fast on Cardio days? And how long should I wait between fasts?

  7. I would like to experience the benefits of autophagy via quarterly 36 hour water fasts but have resisted as I struggle to maintain body weight and hard earned mass. I lost 10 hard gained pounds over a 2 week period of IF with a 6 hr feed window 3 days per week. I consumed over 2k calories while in the window. I’m a leanish-muscular 152 at 5’8”and 53yo. Any suggestions?

  8. This makes sense, and, fits my experience with fasting 7 days, or more. However, I think the best part comes after the fast, maybe for several weeks after. I always break my fasts with 14-15 ounces of rare beef liver — only. The next meal, might be all-I-can eat beef heart. From that point on, for about a week, every meal will be animal protein of some sort, until I get a craving for either salad or fruit. Then, I typically go vegan for up to three days at a time, with a series of animal protein meals here and there. The peak in muscle building seems to come weeks after the end of a ten-day water fast, not at the end of the fast, or even a week after the fast. That has been my experience.

  9. The above study has a big mistake and cannot be translated to ordinary people. It was done on young military men. These people have high levels of anabolic hormones that help maintain muscle mass. This is almost the same as injecting testosterone from the outside to the layman. I can confidently state this, since in my medical practice I raise the level of hormones to the layman with the help of a Carb cyclic diet.

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