When mice lacking the myostatin gene are fed fewer calories, their fast-twitch muscle fibers shrink more than other types of muscle fibers.

We analyzed one assertion on this topic and found that when mice lacking the myostatin gene are fed fewer calories, their fast-twitch muscle fibers — the type used for quick, powerful movements — shrink more than other muscle fibers [1]. This observation was supported in all instances reviewed, with no conflicting data found so far. Fast-twitch fibers are responsible for bursts of strength and speed, and myostatin is a protein that normally limits muscle growth. In mice without this gene, muscles grow larger than normal, but calorie restriction appears to affect these fibers differently than others. The evidence we’ve reviewed suggests that under lower calorie intake, these oversized fast-twitch fibers may lose more size compared to slower, endurance-oriented fibers. We don’t yet know why this happens, or whether it’s due to changes in how energy is used, how signals are sent to muscle cells, or other biological factors. The study was limited to one type of animal model, and no human data or comparisons to normal mice were included in what we’ve reviewed. What we’ve found so far points to a possible pattern in myostatin-null mice under calorie restriction, but we can’t say if this applies to other species, including humans, or if it would happen under different levels of calorie reduction. More research would be needed to understand the mechanism or whether similar effects occur in other conditions. If you’re considering calorie restriction for muscle maintenance, this finding doesn’t directly apply to people — but it does remind us that muscle doesn’t respond the same way to diet in every situation. Individual biology matters.