Many people think that their muscle will waste away if they skip a meal or two, but this isn’t the case. Ultimately, your risk of muscle loss depends on how long you go without eating and the style of intermittent fasting (IF) you choose to follow.
There are many versions of intermittent fasting (IF), with the length of the fast varying between each. For example, some people may choose to eat over an 8-hour window and go 16 hours without eating (16/8 fast), whereas others may go as long as 48 - 72 hours without eating.
Research has shown that there is an increase in the rate of protein breakdown in the body between 15 and 30 hours into a fast, but studies done using an alternate day fasting protocol (24 hours on and 24 hours off) showed no significant loss in lean body mass. Thus, even though lean body mass might start breaking down as early as 15 hours into a fast, it’s likely that this loss would only become substantial after a fasting period exceeding around 36 - 48 hours.
Currently, more research needs to be done in the area to make any claims with certainty, but if preserving muscle is important to you it might be wise to stick to a 14 – 18 hour fasting window. This shorter fast is more manageable, carries a lower risk of muscle loss, and is more conducive to an active lifestyle.
For more on the benefits and drawbacks of IF, read our article.