HMB, short for beta-Hydroxy beta-Methylbutyric acid, is an active metabolite of the branched chain amino acid Leucine, the most heavily-studied of the BCAAs. HMB is found in minor amounts in several foods, including grapefruit, sprouts, and catfish; however, the amount contained in food is not sufficient to yield noticeable benefits in regards to muscle-building.
Concerning building muscle, Leucine is well-known as a potent stimulator of muscle protein synthesis and since HMB is the active metabolite of Leucine, it mimics many of the same effects as Leucine. However, the big difference between the two is that HMB appears to be significantly stronger on a gram per gram basis at stunting the rate of muscle protein breakdown but is less effective than leucine at promoting muscle protein synthesis. Because of this, HMB is typically marketed as an anti-catabolic agent rather than an anabolic agent.
HMB is most frequently found in two forms: one binds HMB to calcium salts (CA-HMB) and the other free acid HMB (HMB-FA), both provide better absorption than isolated HMB and are shown to more effective in the research.
General recommended dosing of HMB tends to be 1-3g daily for the purpose of attenuating muscle loss and improving recovery.