HMB (beta hydroxy butyrate) is a product of leucine metabolism. The body produces it naturally during the metabolism of this amino acid; about 5% of leucine is transformed to HMB in the body. Its use as a supplement stems from the theory that the body is unable to produce enough HMB to support physical activity. It doesn’t really qualify as an anti-aging supplement, but it could have positive effects on muscle mass.

Nissen was the first scientist to publish research on HMB: He found that three grams daily taken in three separate doses reduced muscle catabolism by 20-60% more than in a placebo group and increased muscle mass by 2.3 kg. (vs. just .8 kg in the placebo group). Unfortunately, we have about an equal number of studies denying its effects as confirming them. When taken with Omega-3s, HMB may have enhanced anti-catabolic effects (as reported by a 2004 study), but it’s pretty tough to call it essential with such variable findings in the literature. Individuals should evaluate its use with the help of a nutritionist for an objective assessment of its personal effectiveness.

Amino acids such as leucine and its metabolite α-ketoisocaproate (KIC), are returning to be the focus of studies, mainly because of their anti-catabolic properties, through inhibition of muscle proteolysis and enhancement of protein synthesis. It is clear that these effects may counteract catabolic conditions, as well as enhance skeletal muscle mass and strength in athletes. Moreover, beta-hydroxy-beta-methylbutyrate (HMB) has been shown to produce an important effect in reducing muscle damage induced by mechanical stimuli of skeletal muscle. This review aims to describe the general scientific evidence of KIC and HMB supplementation clinical relevance, as well as their effects (e.g., increases in skeletal muscle mass and/or strength), associated with resistance training or other sports. Moreover, the possible mechanisms of cell signaling regulation leading to increases and/or sparing (during catabolic conditions) of skeletal muscle mass are discussed in detail based on the recent literature.

HMB is a well-established sports supplement, with a lot of research showing that it increases both strength and muscle in athletes doing strength training. Over more recent years however, we have seen mounting research on the impact HMB can have on endurance performance.