HMB (Hydroxy Methylbutyrate)
HMB is a metabolite of the amino acid Leucine, effectively a more powerful version of Leucine for the purpose of preventing the breakdown of muscle (catabolic activity). Ultimately, a much smaller dose of HMB (roughly 1/3 the quantity) is required to have the same effect as a larger dose of Leucine.
While HMB has shown flashes of its ability to catalyze muscle growth during periods of anaerobic exercise, HMB's true benefits appear to be the supplement’s ability to prevent muscle breakdown. This has been shown in clinical trials where HMB has been shown to preserve muscle during extended periods of decreased activity (reference 4). While Leucine itself appears to be more effective for enhancing protein synthesis, HMB alone has a much more powerful effect on the prevention of catabolic activity (reference 3)
There is minimal concern for safety around supplementation with HMB, where various trials, including dosing of 6g daily for a month has been shown to be extremely well tolerated.
Clinical human trials have used 2g of HMB in conjunction with BCAAs, other amino acids and vitamin D. These studies have been shown to improve lean mass gains, muscular strength as well as enhanced protein synthesis (reference 1). Additionally, studies done pairing HMB with creatine (reference 5) and other amino acids have been shown to have additive benefits on lean muscle mass and strength.
Up to 3g of HMB may be used, but greater than 3g hasn't been linked to increased effectiveness (reference 2).
2. Gallagher PM, et al Beta-hydroxy-beta-methylbutyrate ingestion, Part I: effects on strength and fat free mass . Med Sci Sports Exerc. (2000)
3. Wilkinson DJ, et al Effects of Leucine and its metabolite, β-hydroxy-β-methylbutyrate (HMB) on human skeletal muscle protein metabolism . J Physiol. (2013)
4. Zanchi NE, Nicastro H, Lancha AH Jr Potential antiproteolytic effects of L-leucine: observations of in vitro and in vivo studies . Nutr Metab (Lond). (2008)
5. Jówko E, et al Creatine and beta-hydroxy-beta-methylbutyrate (HMB) additively increase lean body mass and muscle strength during a weight-training program . Nutrition. (2001)