HMB (hydroxy-methyl-butyrate) is the metabolite of the important branched-chain amino acid leucine that mediates its anabolic and anticatabolic effect. HMB has been shown clinically to increase muscle size, and strength and improve fat burning in response to exercise. It’s best to use HMB in conjunction with a strenuous training program or a strict diet regimen where catabolism and overtraining may easily occur.