β-Hydroxy-β-Methylbutyrate (HMB) is a metabolite of l-leucine (an essential amino acid). As we get older, muscle size, strength, and performance decline. This can occur even if we continue to exercise. HMB has been used as an ergogenic substance (i.e., something that supports improved sports performance) for more than 20 years, with most of the research focused on supporting muscle performance and helping muscles age better.