Journal Title (Medline/Pubmed accepted abbreviation): J. Int. Soc. Sports Nutr.
First Page: 6
doi (if applicable):10.1186/1550-2783-10-6
Summary of the report:
Background: Beta-hydroxy-beta-methylbutyrate (HMB) is formed naturally in the body as a minor metabolite of leucine. Leucine is a potent activator of protein synthesis and, concurrently, a potent inhibitor of protein breakdown. It is thought that HMB also contributes to enhancing net protein synthesis.
HMB is a commonly used supplement in the athletic population acutely for increasing strength and power and quickening the rate of recovery. It is also taken habitually for increasing lean body mass. Many controlled research studies, though not all, show benefits of HMB. The purpose of this report is to review the scientific literature on HMB including its mechanism(s) of action and to recommend usage of HMB to athletes as appropriate.
Summary of research findings:
- Approximately 5% of leucine is converted to HMB, thereby making it impractical to consume enough high quality protein to inherently produce the recommended effective dose of HMB per day (3 g).
- HMB supplements are currently available as a calcium salt (Ca-HMB) and, more recently, as a free acid (HMB-FA). The free acid form is taken up into the body from the gut at a faster rate and may be retained in the body better. At this point, however, (February 2013), many more studies have been performed using Ca-HMB and there are not solid grounds for comparison between the two forms in regard to efficacy in athletic performance.
- HMB has been widely studied for safety. All human studies have concluded that HMB is safe in regard to adverse effects (very few and/or minor) and clinical parameters as measured in the blood (no adverse changes). Only one rodent study showed that HMB supplementation of 320 mg per kg body weight per day (27.2 g/day for a 187 lb. person) for one month decreased insulin sensitivity. However, these results have not been corroborated in humans.
- Many recruiters for research studies limit their recruitment to trained or untrained participants. In general, when assessing the ability of HMB to reduce muscle damage and/or enhance recovery from intense exercise, HMB is very effective in untrained populations and is only effective in trained populations when the exercise is intense enough to cause muscle damage (often different that their normal routine).
- HMB is most effective at inhibiting protein breakdown when taken daily. It takes about 2-3 weeks for HMB to be effective at attenuating muscle damage and reducing soreness.
- Ingestion of 3 g per day is the typical dose that is tested in studies, and is more effective than 1.5 g per day. Increasing the dose to 6 g per day (performed in only one study) did not have greater benefits that 3 g per day.
- In regard to when to take HMB, it is most effective to consume HMB in close proximity to the workout. However, absorption is most effective in smaller doses (ex. 1 g dose) spread out throughout the day.
- Of the studies that have not shown benefits of HMB, workloads varied greatly between participants and/or compliance to the training routine was low. The large variability in gains from the program likely led to differences between groups that were not discernible with statistics.
- During short-term calorie restriction, HMB can help preserve lean body mass and augment fat loss.
- HMB has shown benefits in a large range of ages- adolescents, young adults, adults, and “master” (elderly) athletes. In fact, HMB may be able to preserve muscle mass in the elderly, attenuating normal age-related muscle loss.
- In addition to resistance training, HMB aids in endurance training, too. It can help increase maximum oxygen capacity (VO2max) by increasing mitochondrial mass and stimulating the provision of energy from fat.
- HMB is able to biochemically stimulate protein synthesis through stimulation of the mTOR pathway. It is thought that HMB decreases muscle protein breakdown by down-regulating the ubiquitin-proteosome pathway, which is responsible for degrading excess/damaged proteins. There is some evidence that HMB activates muscle satellite cells, thereby increasing generation of new muscle cells.
Key practice applications: HMB is a safe and effective supplement for increasing muscle size and strength and quickening recovery from intense exercise. Daily consumption of 3 g of HMB daily in 1 g doses starting about one hour before exercise is the optimal dosing regimen at this time.