HMB (Beta-Hydroxy-Beta-Methylbutyrate)

HMB is a downstream metabolite of leucine — only about 5% of dietary leucine converts to HMB in the body, which is why direct HMB supplementation is studied separately rather than just telling people to eat more whey. The case for HMB is real but heavily segmented by who is taking it.

The strongest signal is muscle preservation in older adults under stress. Deutz 2013 in Clinical Nutrition randomized 24 healthy older adults to 3g/day Ca-HMB or placebo for 5 days before and through 10 days of strict bed rest. The placebo group lost lean body mass as expected; the HMB group preserved it, with a statistically significant between-group difference. The PROT-AGE expert group (Bauer 2013, J Am Med Dir Assoc) noted in their position paper that HMB "may attenuate muscle loss and increase muscle mass and strength in older people" while calling for more trials. A 2021 meta-analysis (Lin, European Geriatric Medicine, 9 trials, n=448) found HMB alone significantly increased fat-free mass in older adults, though the effect attenuated when combined with exercise (likely a ceiling effect — exercise alone already drives most of the available adaptation).

The original novice-trainee data is also strong. Nissen 1996 in J Appl Physiol — the paper that put HMB on the map — ran two studies with 41 and 28 untrained subjects on 1.5 or 3g/day for 3-7 weeks of resistance training. HMB attenuated exercise-induced muscle proteolysis (3-methylhistidine, CPK), increased fat-free mass, and improved bench press strength. The 2013 ISSN position stand on HMB (Wilson et al., J Int Soc Sports Nutr) endorsed 3g/day for muscle damage attenuation in trained and untrained populations and concluded HMB is safe and effective in novice trainees specifically.

The case in experienced lifters is the weakest part of the literature. Sanchez-Martinez 2018 meta-analyzed 6 RCTs in 193 trained and competitive athletes and found no significant effect on bench press, leg press, body mass, fat-free mass, or fat mass — effect sizes were trivial and confidence intervals straddled zero. Slater 2001 in 28 trained men on 3g/day for 6 weeks also found no effect on strength or body composition. This is the standard finding: in people who already have years of training adaptation, HMB does not add much.

Caloric deficit and cutting is an intermediate use case. The mechanism plausibly favors HMB here — HMB appears to act primarily by attenuating muscle protein breakdown, which is the dominant process during energy restriction — but the clinical trials are small and the effect sizes are modest. Treat it as a reasonable adjunct during a structured cut, not a transformation.

Form discussion: HMB calcium salt (Ca-HMB) is what every major trial has used. HMB free acid (HMB-FA) was developed to produce a faster, higher plasma peak, but the more recent head-to-head pharmacokinetic work (the 2024 Amino Acids paper) actually found Ca-HMB delivered comparable or superior systemic bioavailability, and no clinical-endpoint trial has shown HMB-FA outperforming Ca-HMB on any outcome. Treat the free acid premium as marketing.

Practical bottom line: 3g/day of calcium HMB, split into 3 × 1g with meals, for at least 2-4 weeks. Worth trying if you are over 60 and concerned about sarcopenia, recovering from bed rest or hospitalization, beginning a structured resistance program for the first time, or running a serious caloric deficit. Not worth bothering with if you are an experienced lifter eating adequate protein — the marginal return is too small to justify the cost.