How to Build (and Maintain) Muscle Mass While Fasting

Written and medically reviewed by Rich LaFountain, PhD

We know that fasting has many benefits, but the most popular one is, you guessed it, weight loss. However, if you’re truly interested in maximizing your healthspan, a more specific goal is fat loss, because if weight is all you’re focused on, you’re at risk for lean-mass loss, which has a negative effect on your body composition and long-term health.

All in all, muscle is your friend, and far too valuable to sacrifice in an effort to lose weight. Luckily, it’s easy to pair fasting with both muscle maintenance and muscle building. Here’s how to do it.

The Relationship Between Fasting and Muscle Mass

Fasting is often blamed for muscle or lean body-mass losses associated with weight loss. Although fasting often leads to spontaneous reductions in energy intake, calorie restriction (CR) diets do the exact same thing. The difference is that they’re definitely not spontaneous, and they’re notoriously unsustainable. In fact, the average adherence reported for time-restricted feeding (TRF) is between 80% and 87%, whereas adherence to diets that rely upon CR is between 53% and 70%.  

So, Does Fasting Cause Muscle Loss?

No, fasting does not cause muscle loss. In fact, there’s probably not one exclusive cause — rather, there are several factors that can contribute to muscle loss, which is clinically referred to as sarcopenia. According to the research, these include low protein synthesis, lack of regular physical activity, poor hormone balance, low neuromuscular activity, inflammation, and reduced cellular function. 

Increasingly, malnutrition is recognized as one of the main risk factors of skeletal muscle loss contributing to the onset of sarcopenia. In fact, even if you were to eat for 24 hours a day, but you were physically inactive and had low calorie and/or dietary protein intake, you would still be at risk of losing muscle over time. A recent randomized trial indicated that 12 months of CR resulted in almost double the lean mass loss than daily 16:8 TRF. Additionally, causal factors of muscle loss are not exclusively linked to fasting; many studies suggest that fasting promotes fat metabolism, increases the production of ketones, and preserves skeletal muscle mass. 

Regardless of your fasting habits, muscle loss is almost certain if you’re in a severe caloric deficit and malnourished, especially if you’re not getting enough protein. If you do choose to fast, make sure that your eating window is full of high-quality, minimally processed foods so that you’re not missing out on essential nutrients. And if that proves to be challenging, you can always expand your eating window to allow for more time to hit those protein goals.

3 Tips for Muscle Building and Maintenance While Fasting

Now that we know that we can build and maintain muscle on a fast, it’s time to get to the how! By following the tips below, you’ll be on the right path to healthy, fasting-fueled gains.

#1. Be Physically Active

Your body prioritizes efficiency, and like fasting, this ties back to simple survival. So, if your muscles are not regularly active, your body may determine that they’re not worth maintaining, let alone building — a use-it-or-lose-it mentality, if you will. The only way to combat this is to practice all forms of exercise that require skeletal muscle activity. 

After your adulthood, it’s important to focus on building muscle and strength reserves. Unfortunately, by the time you reach your 30s or 40s, your muscle mass is already on a gradual decline. You can slow or perhaps even halt this decline as long as you incorporate resistance training into your exercise routine.

Research suggests that resistance training is a powerful way to maintain healthy muscle mass while fasting. Trained males and females who adopted daily TRF for 8 weeks in addition to resistance training had comparable muscle and strength outcomes to individuals who did not restrict their eating window. Additionally, the participants in the TRF groups had greater fat mass loss than those who were not following TRF, so their body composition (a.k.a. body fat percentage) was improved. 

#2. Avoid Severe Caloric Deficits and Yo-Yo Dieting

We’ve already established that muscle is costly to maintain, and even more costly to build. So, when your body senses that energy supplies (i.e., high-quality calories) are low, muscle maintenance and building are affected, regardless of whether or not you’re fasting.

The research is clear: If you want to maintain muscle mass, it’s best to avoid severe energy deficits, which are stressful on the body and usually result in a higher proportion of weight loss being a reduction in lean mass. Additionally, yo-yo dieting, or the endless cycle of constantly losing and gaining weight, is linked with progressive declines in lean muscle fat loss. Yo-yo dieting can contribute to gradual increases in body fat percentage, and it can make maintaining weight loss harder because resting metabolic rate, or how many calories you burn throughout the day even when inactive, is linked to muscle mass.

So, how do you combat this? It’s a matter of retraining your thinking. Next time you fast, forget about counting calories altogether. Because fasting promotes a gentle and spontaneous reduction in calories, you’re better able to steer away from overconsumption and start being more intuitive about your eating habits and natural hunger cues — a practice known as mindful eating. 

#3. Provide Muscle-Building Materials

Michelangelo wouldn’t have been able to carve the statue of David without a slab of marble; similarly, you can’t achieve your muscle goals if you’re not supplying your body with the right materials. Even if you’re consistently meeting tips #1 and #2, muscle growth is a distant dream without the most important building block of skeletal muscle: protein.

A review of weight-loss strategies focused on the percentage of lean mass loss in response to various diets confirms that 100% fat mass loss (or 0% lean mass loss) is all but impossible. However, while most diets’ peak fat loss percentage is about 75% (or 25% lean mass loss), which is considered a great result, diets that are higher in protein (~1 g/lb body weight/day) are able to achieve 89% fat loss, which means that more lean mass is maintained even in a weight loss scenario.

So, if you want to build or maintain muscle, dietary protein is the most important macronutrient you need to consider. An easy target to shoot for is 1 gram per pound of body weight per day. That way, even in the context of an energy deficit or the inevitable aging process, you’ll still be covered. 


Responsible fasting is not a liability for muscle maintenance, and you may build muscle in conjunction with fasting so long as you consider the three essential conditions for supporting healthy muscle mass and growth. In addition to supporting muscle mass, fasting is advantageous for overall health and metabolic flexibility which also are crucial to living a longer, healthier life.

Rich LaFountain, PhD
Posted in Health & Science

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