HMB

Beta-hydroxy beta-methylbutyric acid, commonly abbreviated HMB, is a short organic acid produced in the human body as a downstream metabolite of the branched-chain amino acid leucine ^[1][3]. Only a small portion of the leucine obtained from food is converted into HMB; the pathway runs from leucine through the intermediate alpha-ketoisocaproate before HMB is formed, chiefly in the liver ^[2]. Because the body makes only a limited quantity each day, supplementation aims to raise circulating levels well above what diet alone provides ^[1].

Two forms are used in supplements. The older calcium salt, known as HMB-Ca, is absorbed comparatively slowly, whereas the newer free-acid form, HMB-FA, reaches the bloodstream faster and may be taken up by tissue more readily ^[1]. HMB is also incorporated into certain medical nutrition products intended to support muscle and wound healing ^[1]. It is typically taken around exercise, and reviews note that a period of consistent use appears to precede its measurable effects ^[1].

Research in exercising people has centered on HMB's ability to limit muscle damage and support gains in muscle size and strength ^[1][2]. Position statements from sports nutrition bodies conclude that HMB can reduce markers of exercise-induced muscle breakdown and aid recovery, and that it may enhance hypertrophy, strength, and power when paired with an appropriate training program ^[1]. Its benefits appear more pronounced in untrained individuals and in those undergoing periods of high physical stress, and are less clear in already well-trained athletes, possibly because their bodies have adapted to suppress the very proteolysis HMB acts upon ^[2].

A second major line of study addresses muscle loss in aging and disease. HMB has been examined for its capacity to preserve lean mass and function in older adults with sarcopenia and in wasting conditions such as cancer cachexia and prolonged bed rest, generally with the strongest results when combined with exercise or other nutrients ^[2]. Reviews caution that much of the clinical evidence involves combined interventions, making it hard to isolate HMB's independent contribution ^[2]. Some analyses also note that anabolic effects are limited in healthy people who are not exercising ^[2].

HMB is regulated as a dietary supplement rather than a drug, and it is not prohibited by major anti-doping organizations ^[1]. It is generally reported to be well tolerated across the amounts studied, with a favorable short-term and medium-term safety record in the trials conducted so far ^[1]. Supplements are sold as capsules, tablets, and powders in both the calcium and free-acid forms.

Very safe with a long track record and no meaningful toxicity at standard doses. Side effects are rare and mild. It's one of the more benign performance supplements out there.