(Not to be confused with β-Hydroxybutyric acid.) β-Hydroxy β-methylbutyric acid (HMB), otherwise known as its conjugate base, β-hydroxy β-methylbutyrate (hydroxymethylbutyrate, HMB), is a naturally occurring metabolite in humans that is used as an ingredient in medical foods and sold as a dietary supplement. It is used in certain medical foods to help provide nutritional support for people with muscle wasting due to cancer or AIDS and to promote wound healing. In clinical research, supplemental HMB has been shown to inhibit the loss of lean body mass in muscle wasting conditions, particularly in individuals with sarcopenia and during bed rest, and is often used in addition to resistance exercise. Supplemental HMB is also used by athletes to increase exercise-induced gains in muscle size, m