Beta-hydroxy-beta-methylbutyrate, or HMB, is a downstream metabolite of the branched-chain amino acid leucine. Many studies have shown that leucine is by far the most potent single amino acid in inducing muscle protein synthesis. Several years ago scientists at the University of Iowa studied the metabolic pathways of leucine and deduced that the role attributed to it was in fact due to its metabolite, HMB. Several studies involving animals showed that HMB seemed to have potent effects on protein production.
Early human studies, using mainly untrained subjects, seemed to show that HMB also worked well for humans engaged in resistance exercise. HMB hit the commercial market in the late ’90s, but the results were less than stellar. Since then, most of the bodybuilding community has pegged HMB as an expensive but useless fad supplement. Oddly enough, however, many nutrition scientists continue to extol its virtues.
The suggested mechanisms for HMB point to its being an effective anabolic supplement. One thought is that it blocks the cellular activity that’s involved in protein breakdown. Another theory says that HMB is converted into cholesterol, which is noted chiefly for its involvement in cardiovascular disease but which is required for the stability of cell membranes. Recent studies also suggest that cholesterol may have anabolic effects in muscle, which isn’t surprising when you consider that it’s the precursor of all steroid hormones, including testosterone.
An early theory suggested that HMB stabilized muscle cell membranes to the extent that it blocked excessive muscle protein breakdown. Still another theory is that it may even aid bodyfat loss by increasing muscle cell fat oxidation, although precisely how that’s possible isn’t known.
In the latest HMB study, a randomized, double-blind, controlled experiment, 22 men, average age 24, got either three grams a day of HMB or a placebo for nine weeks. The men had an average of more than three years of training experience and trained no fewer than three times a week. Strength tests involved one-rep-maximum lifts in the leg extension, bench press and preacher curl. Other tests included body composition.
After nine weeks those using HMB had an average overall strength increase of 1.6 percent. When the results were isolated to lower- and upper-body gains, though, maximum leg strength increased by a substantial 9.1 percent. The gain in upper-body strength was deemed inconclusive. The HMB group also had bodyfat loss that the authors termed trivial. They suggest that HMB may be more useful for beginners, who are more susceptible to muscle damage than more experienced trainees.
— Jerry Brainum
Rowlands, D.S., et al. (2009). Effects of nine weeks of B-hydroxy-B-Methylbutyrate supplementation on strength and body composition in resistance-trained men. J Str Cond Res. 23:827-835.