Technically "beta-hydroxy beta-methylbutyric acid," HMB is a chemical that occurs naturally in the body when the amino acid leucine breaks down.

Leucine is found in particularly high concentrations in muscles. During athletic training, damage to the muscles leads to the breakdown of leucine as well as increased HMB levels. Evidence suggests that taking HMB supplements might signal the body to slow down the destruction of muscle tissue.1 However, while promising, the research record at present is contradictory and marked by an absence of large studies.

HMB is not an essential nutrient, so there is no established requirement. HMB is found in small amounts in citrus fruit and catfish. To get a therapeutic dosage, however, you need to take a supplement in powder or pill form.