Intermittent fasting (IF) is an eating pattern that involves fasting for typically 16 hours a day and eating for 8; it’s not a ‘diet’ as it doesn’t tell you what to eat, only when to eat it. If you’re trying to put on muscle, it’s likely that you’re a little put off at the prospect of going without food for 16 hours of the day, you barely skip your protein shake post workout fearful of going into a catabolic state.
But what does the science say?
A study published in 2015 found that when participants intermittent fasted they lost body fat, reduced cholesterol, triglycerides and improved blood lipids which are all great benefits. Another outcome was the loss of up to 9% of total body weight (TBW) i.e. including muscle mass.
A study by Varady et al (2013) found that in 32 subjects, those who engaged in alternate day fasting lost body fat (up to 1% of TBW), but retained the same lean body mass.
These studies, like most research on IF, were focused on its outcomes for fat loss. They did not ask participants to engage in resistance training or monitor their food intake during the eating window; making it difficult to give an insight into the effects of muscle gain.
The two studies show different results for lean mass retention, but it may be because less total body weight was lost by the participants in the Varady study (only 1% vs. 9% in the 2015 study). The more body fat is lost, the more the total body weight is reduced (including muscle mass).
Another study by Varady (2011) found that among intervention groups doing a daily calorie restriction and those doing intermittent calorie restrictions, the same fat mass was lost (4-8% vs. 5-8% respectively). But in the intermittent calorie restriction group, less fat free mass was lost (i.e. muscle mass). This study showed that IF can be beneficial for retention of lean muscle mass, but no insight on if it helps you gain muscle.
There is very limited research on IF as a tool to gain muscle, because muscle is gained optimally in a calorie surplus and sufficient exercise stimulus to promote muscle growth. Thus, the pattern of eating becomes less relevant.
Nonetheless, a recent 2017 randomized controlled trial had participants resistance training and either fasting or eating normally. The study found that the fasting group maintained lean body mass and increased strength, but the normal group gained 5 lbs of lean body mass alongside strength gains. This would suggest that IF is not the best method for muscle gain.
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