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August 10th, 2018

Will I Lose Muscle Mass and Strength Under Intermittent Fasting?

Check out this answer from Consensus:

Intermittent fasting, when combined with exercise, particularly high-intensity interval training or resistance training, can help maintain or even increase muscle mass and strength while promoting fat loss. However, IF alone may not be sufficient to preserve muscle mass and could lead to suboptimal muscle protein turnover. Therefore, incorporating regular exercise is crucial for those practicing intermittent fasting to ensure muscle mass and strength are maintained.

Intermittent fasting (IF) is a dietary approach that alternates periods of eating with periods of fasting. It has gained popularity for its potential benefits in weight loss and metabolic health. However, concerns remain about its impact on muscle mass and strength, especially when not combined with exercise.

Key Insights

  • Muscle Mass Maintenance with IF and Exercise:
    • Combining IF with high-intensity interval training (HIIT) or resistance training can help maintain or even increase lean body mass while promoting fat loss .
    • IF alone, without exercise, may not be effective in preserving muscle mass .
  • Strength and Physical Performance:
    • IF combined with exercise improves physical fitness and strength, suggesting that exercise is crucial for maintaining muscle performance during IF .
    • IF alone does not significantly impact muscle strength or anaerobic capacity.
  • Body Composition Changes:
    • IF can lead to significant fat loss while maintaining lean body mass, especially when combined with resistance training .
    • IF protocols like time-restricted eating (TRE) can improve body composition by reducing fat mass and maintaining muscle mass .
  • Metabolic and Health Benefits:
    • IF combined with exercise improves metabolic markers such as glucose tolerance, insulin sensitivity, and lipid profiles, which can indirectly support muscle health .
    • IF alone can lead to overeating during feeding periods and may not be as effective in energy conversion efficiency, potentially impacting muscle maintenance.
  • Potential Drawbacks of IF Alone:
    • Prolonged fasting periods characteristic of IF may not optimally support muscle protein turnover and net muscle protein balance, potentially making it a suboptimal strategy for muscle maintenance without adequate protein intake and exercise.

Will I lose muscle mass and strength under intermittent fasting?

Robert White has answered Likely

An expert from University of Western Australia in Musculoskeletal Science, Neurodegenerative Disease, Stem Cells, Cell Biology

Excellent question!

I know I’ve selected “Likely”, but sadly, the answer is the horrendously ambiguous “it depends”. This is true for 2 principal reasons:

Firstly, there’s no single type of ‘intermittent fast’ and there is no ‘ideal subject’.

Intermittently fasting for 14 days per time will certainly reduce muscle mass. So let’s limit this to reasonable interpretations of ‘intermittent’. = >overnight but <2 days

Likewise, what is the nutritional state of the individual prior to the fast? Results will vary markedly between athletes, obese folk, and the well- versus mal-nourished.

Likewise again, the activity state of the individual will alter metabolic pathways involved in anabolism/catabolism. Muscle use (types of muscle use do differ) will instigate anabolism, muscle disuse will allow catabolism.

Secondly, what does it actually mean to lose ‘muscle mass’ and ‘strength’?

Skeletal muscle is comprised of 3 Things that have mass and contribute to strength; 1) contractile and calcium-handling apparatus (the part that is responds to stimuli and generates tension), 2) shock absorbers (the part that connects part #1 to your bones, and 3) fuel.

With reasonable intermittent fasting (before starvation), we can forget about Thing #2. Both Thing #1 and Thing #3 “can” be affected. The “can” depends on the magnitude of the fast.

Loss of Thing #1 is a massive problem for strength and takes time and training to fix. Any loss/gain of contractile apparatus or calcium-handling machinery is important and is difficult to adequately replace. So, it is protected and is one of the last things to go in starvation. We have exquisitely well-balanced mammalian target of rapamycin (MToR), insulin, insulin-like growth fact (IGF) and growth hormone (GH) signalling mechanisms to protect this (amongst others).

Loss of Thing #3 is a minor problem for strength (albeit rapid, short-term strength), and is rapidly fixed.  BUT, it is really heavy! Energy in high strength muscle fibres is predominantly stored as glycogen – glycogen is stored with 2x its mass of water (density =1), so when muscle mass is gained/lost rapidly, this is the reason; 1g of glycogen burnt = 3g of muscle lost. Glycogen is easy to use/replace, so it is one of the first things to go during starvation, intermittent or otherwise. The same MToR, insulin, IGF and GH signalling mechanisms are far less protective of this.

So, with reasonable intermittent fasting and assuming the individual is not subject to bed rest, your muscles will lose mass, but not lose too much strength. Muscle use will help to maintain this, but eventually, everything has its limits.