HMB, a supplement used by athletes, may only help increase fat-free mass in those who eat less than 1.6 grams of protein per kilogram of body weight each day. For athletes consuming more protein, HMB does not appear to provide additional benefit.

Taking HMB supplements does not lead to a meaningful change in fat mass among athletes, based on data from multiple studies combining 128 participants.

Taking HMB supplements does not lead to a meaningful change in total body weight for athletes, based on data from multiple studies.

Taking 3 grams of HMB daily for four weeks or longer may be linked to a very small increase in lean body mass in athletes, but this effect is not statistically significant and may only occur if protein intake is less than 1.6 grams per kilogram of body weight per day.

Taking HMB supplements does not significantly increase muscle growth in people who already eat enough protein, but might slightly affect muscle growth when calorie intake is very low or training intensity is extremely high.

In healthy men aged 67 to 69, taking 3 grams of HMB-free acid daily while doing strength training led to a slightly greater increase in thigh muscle mass compared to a placebo, though the difference was not large enough to be considered statistically certain.