HMB added nothing on top of resistance training

Resistance training has strong evidence behind it for protecting muscle and strength with age, which sets a high bar for any supplement claiming to add to it. HMB, a metabolite of leucine, has long been promoted as an anti-catabolic aid and appears in products aimed at older adults. The open question is not whether HMB does anything in a test tube, but whether it delivers measurable extra body composition or strength gains when someone is already lifting. That is a practical question about cost and expectation as much as physiology.

The reviewers searched PubMed, Scopus, Clinicaltrials.gov and Cochrane for randomised controlled trials combining HMB supplementation with resistance training in adults aged 50 years and older. Thirteen randomised controlled trials involving 561 participants were included. The primary outcomes examined were fat mass, muscle mass and muscle strength. Meta-analyses used robust variance estimation with small-sample corrections, and risk of bias was assessed with the Cochrane RoB 2.0 tool, with publication bias also examined.

None of the three main outcomes favoured HMB. Fat mass gave an SMD of 0.24 (95% CI -0.01 to 0.49), muscle mass 0.05 (95% CI -0.10 to 0.20) and muscle strength 0.04 (95% CI -0.72 to 0.63) — all with confidence intervals crossing no effect. Heterogeneity was low for the body composition outcomes but high for the strength measures, meaning the strength trials disagreed with each other more than the composition trials did. No evidence of publication bias was detected, so the null result is unlikely to be an artefact of missing studies. The authors' conclusion is that HMB cannot be recommended as a routine adjunct for those able to undertake structured exercise.

This tightens a picture that has been mixed for years, with earlier enthusiasm for HMB resting largely on smaller studies and on populations losing muscle for other reasons. By restricting to trials where resistance training was present in both arms, the analysis isolates what HMB adds rather than what training does. The wide confidence interval and high heterogeneity for strength leaves room for specific subgroups or protocols to behave differently. It also does not address people who cannot train, a group where the supplement question is quite separate.

For an older adult already doing structured resistance work, these pooled data give little reason to expect HMB to improve muscle, strength or fat mass on top of the training. The absence of publication bias and the low heterogeneity in composition outcomes make the null relatively convincing for those measures. Strength results were noisier, so that door is not fully shut, but nothing here supports routine use. The training stimulus itself remains the part with the robust evidence.