Intermittent fasting: how many hours to fast to preserve maximal muscle

To fast for weigh loss is easy. You can fast for days on end and keep on losing weight.

The question is, how long should you fast in order to prevent muscle catabolism (breakdown).

Fasting increases proteolysis (the breakdown of protein) and the longer you fast, the more this process increases. Proteolysis breaks down muscle in order to provide amino acids to be converted into glucose for blood sugar.

As seen here even a 2 day fast increases proteolysis by 40% (R).

So what happens is, during a fast blood sugar becomes low and a hormone called glucagon and cortisol is released in order to increase blood sugar levels.

Glucagon then activates these three metabolic pathways – glycogenolysis, gluconeogenesis and lipolysis.

Glycogenolysis utilizes stored glycogen and converts it into glucose molecules and then releases them into the bloodstream. 49.7% of glycogenolysis is elevated after 16 hours of fasting and only 38.3% after 24 hours (R).

Gluconeogenesis generates glucose from lactate (4-5% contribution), glycerol, and amino acids. Gluconeogenesis is 50.3% activated after 16 hours of fasting, and 61.7% after 24 hours (R). 

  • Lactate is produced by the activation of muscles. Hence small amounts of lactate are always available as the muscles are constantly active, even at rest.
  • Glycerol is available when the body begins to use stored fat as a source of energy, as glycerol and fatty acids are then released into the bloodstream. Glycerol increases two fold after 12 hours of fasting (R) as marked increase in adipose triglycerides mobilization occurs between 12 to 18 hours. Glycerol competitively inhibits gluconeogenesis from amino acids.
  • Autophagy (the process of the body getting rid of dead/destroyed cells) is induced by fasting. Inhibiting autophagy will worsen muscle loss during fasting. Thus, autophagy flux is important to preserve muscle mass and to maintain myofiber integrity (R). Fasting and autophagy are very important for muscle health and to preserve muscle and the integrity thereof. The autophagy of muscles will result in a small increase in circulating amino acids, which will also supply gluconeogenesis with amino acids. This will not lead to a loss of muscle mass, however, prolonged autophagy will lead to muscle loss.


Fatty acids will become the predominant energy source after a period of fasting (12-18hrs), which will also lead to preservation of muscle mass. Uncoupling protein 3 (which increases fatty acid transport into mitochondria for ATP production) increases 5 fold after just 15 hours of fasting (R), which indicates a great increase in circulating fatty acids.


Assisting factors:

1) The elevated growth hormone during a short-term fast (shorter than 20hrs) helps to retain lean muscle mass (R).

2) Weight training helps to preserve muscle mass.

3) Increased protein intake reduces muscle loss during a caloric deficit.

4) Avoid being in a caloric deficit. It’s when you enter a caloric deficit that you might start to lose muscle. However, a caloric deficit isn’t even needed to burn fat when doing IF (intermittent fasting), as one can lose fat and build muscle while not even being in a caloric deficit.

5) Take anti-catabolic supplements such as BCAAs and/or HMB.


Let’s look at a few studies

In this here study, a 16 hour fast with an 8 hour eating window showed better fat loss and enhancement of lean muscle mass. No muscle wasting was noted.

As seen in this study, two groups ate enough calories to maintain their weight. One did 16/8 IF and the other didn’t. After 8 weeks the IF group lost 16% fat and gained 0.64kg lean mass, while the other group only lost 4% fat and only gained 0.48kg lean mass.

Even fasting 20hrs with a 4 hour feeding window showed that muscle mass was maintained just as well as the group that didn’t do IF (R). They only implemented IF on non-workout days, which suggests that IF does preserves muscle mass, but that IF might not be good enough on its own without resistance training for superior fat loss.


So there isn’t really any set time to IF, but here’s what you need to do:

  • Make sure to consume all your daily calories to avoid a caloric deficit
  • Eat enough protein – to ensure no muscle loss
  • Do weight lifting – for optimal fat loss


The thing is to do what works best for you. The body usually adapts to a new eating pattern after 3-6 weeks.

For me: I struggle to stuff all my daily calories into a small eating window. And the longer I fast, the less energy I have for my workout. So I personally don’t fast longer than 18 hours (usually only 16 hours).

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