Hydroxy-methyl-butyrate. HMB is thought to be the active form of leucine, an amino acid that plays a role in regulating protein metabolism. In theory, if you supply HMB as a supplement, you may be able to increase muscle mass while reducing muscle breakdown during intense exercise.
There is some evidence that HMB reduces muscle catabolism and protect against muscle damage. For example, creatine kinase, an indicator of muscle damage, is reduced following exercise in subjects consuming HMB. This may indicate a reduced level of muscle damage and could lead to improved muscle function.