Beta-Hydroxy beta-methylbutyric acid
β-Hydroxy β-methylbutyric acid (HMB), or β-hydroxy β-methylbutyrate, is a metabolite of the essential amino acid leucine and is synthesized in the human body. Its part in protein synthesis was discovered by Steven L. Nissen at Iowa State University. It has been used in scientific studies to purportedly increase muscle mass and decrease muscle breakdown. Nissen held the original patent on the metabolite as a nutritional supplement. It was discovered in pigs and small quantities can also be found in grapefruit, alfalfa, and catfish. As a supplement it is usually sold as the calcium salt calcium beta-hydroxy-beta-methylbutyrate.
Research published in the Journal of Applied Physiology has shown that HMB may have an effect on increasing muscle weight and strength. A review in Nutrition & Metabolism provides an in depth and objective analysis of HMB research.  The same study lists as HMB's proposed mechanisms of action the following:
- Increased sarcolemmal integrity via conversion to HMG-CoA
- Enhanced protein synthesis via the mTOR pathway
- Depression of protein degradation through inhibition of the ubiquitin pathway
The human body produces about 0.2-0.4 grams per day. Standard doses in research studies have been 1.5 to 3.0 grams per day, usually divided into two doses.