HMB, or beta-hydroxy beta-methylbutyric acid, is an active metabolite of the amino acid leucine, meaning a portion of leucine is metabolized into HMB once in the body. Leucine is the branched chain amino acid most responsible for protein synthesis, or muscle growth. HMB, in its supplemental from, is commonly used by bodybuilders or athletes who are interested in increasing the benefits of strength training.
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According to a Vanderbilt University health psychology report, researchers concur that, although the mechanism by which HMB effects muscle function is still unknown, HMB may inhibit protein degradation during periods of resistance training. According to a study published in the European Journal of Applied Physiology, HMB, though not as effective as leucine in building muscle, is significantly more effective in preventing muscle breakdown. Therefore, it is marketed by most supplement manufacturers as an anti-catabolic agent as opposed to simply a muscle building supplement.
HMB, though not as well known as other muscle building supplements, may turn out to be among the more effective in its realm. A study published in the Jul-Aug 2001 issue of the "Nutrition" journal found that HMB, when combined with creatine, significantly increased lean mass and strength gains. A review article, published in January 2008 in the "Nutrition & Metabolism" journal, indicates that multiple studies found that HMB had positive effects on strength, lean body composition and reduced soreness on trained athletes. The article also noted similar findings in untrained athletes. Supplementation and nutrition compendium Examine.com gives HMB an “A” rating for “power output” (based on an aggregate review of 12 studies) and “B” ratings for its effectiveness in lowering body fat and increasing lean mass (based on an aggregate review of eight studies).
A Iowa State University study published in the January 2003 issue of the "Journal of Applied Physiology" indicates that HMB resulted in a decrease in LDL, total cholesterol and systolic blood pressure. These positive effects on cholesterol and blood pressure imply that HMB could prove a useful tool in preventing heart disease and stroke, two diseases directly linked to high blood pressure and high cholesterol. However, more research is needed to determine HMB's role as a preventative health supplement.
Although limited research has been performed on humans, it appears that HMB is a safe supplement. However, the Vanderbilt report points out that HMB is a comparatively new supplement and consideration must be paid to possible side effects of upsetting a natural balance within the body. Examine.com notes that overall, normal doses of HMB seem to be tolerated well over extended periods of time.