β-Hydroxy-β-methylbutyrate (HMB) is a naturally occurring metabolite of leucine, an essential amino acid. Typically, about 5% of leucine is oxidized to HMB, but due to the ability of HMB to increase protein synthesis and reduce protein degradation, it has been widely used as a nutritional supplement. HMB is important in muscle health. It is stored in the muscles and used to build, maintain and repair muscle cells.