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HMB is a naturally occurring compound produced in the body by a liver enzyme. The enzyme (a-ketoisocaproate oxygenase) reacts with leucine, an amino acid, ultimately to produce HMB. While the precise mechanism of HMB metabolism is not completely understood, it is believed by some that HMB inhibits the breakdown of muscle proteins during exercise and other physically strenuous activity. For this reason, HMB has become popular as a dietary supplement for humans, particularly bodybuilders and other athletes who are trying to increase muscle mass. New work has evaluated the effects of HMB on protein synthesis and some of the associated molecular signaling pathways. In these experiments it was clearly shown that HMB given alone increased muscle protein synthesis. When protein synthesis was inhibited by drugs, HMB prevented the decline. It was shown HMB affects the regulation of several of the initiation factors. Similar to leucine, HMB also activates mTOR - the critical switch in muscle that turns on protein synthesis. HMB has been studied since the 1950s, and new incites are continuing to evolve like these recent findings showing positive effects on muscle protein synthesis.

Eley HL, Russell ST, Baxter JH, Mukherji P, Tisdale MJ. SIGNALING PATHWAYS INITIATED BY {beta}-HYDROXY-{beta}-METHYLBUTYRATE TO ATTENUATE THE DEPRESSION OF PROTEIN SYNTHESIS IN SKELETAL MUSCLE IN RESPONSE TO CACHECTIC STIMULI. Am J Physiol Endocrinol Metab. 2007 Jul 3; [Epub ahead of print]