Hydroxy beta-methylbutyric acid is a metabolite of the essential amino acid leucine and is synthesized in the human body. It is regularly used in the medical field to reduce muscle loss and by athletes of all types to increase strength, endurance, and lean muscle mass. 
Several studies indicate that HMB stimulates protein synthesis through multiple mechanisms including the mTOR pathway. It has been shown to double the results of weight training and lower LDL cholesterol, blood pressure, and the risk of cardiovascular disease.