HMB stands for β-hydroxy-β-methylbutyrate, a by-product (metabolite) from the breakdown of the amino acid leucine HMB is produced naturally by the body and can also be obtained through diet, but the amount needed to improve the body exceeds the amount which a reasonable person would get through diet alone. Because of this, HMB supplementation is popular among bodybuilders and those trying to gain muscle mass and reduce body fat. HMB has been shown to be effective in increasing muscle mass while reducing muscle breakdown. Other benefits include reduced body fat and cholesterol as well as improved muscle recovery. After training.