HMB (Beta-Hydroxy Beta-Methylbutyrate)
A leucine metabolite marketed to reduce muscle protein breakdown. Evidence is strongest in catabolic or untrained/elderly populations; in trained athletes the anti-catabolic benefit is small and disputed. Extraordinary early claims did not replicate in well-controlled trials.
01 Overview
HMB is a downstream metabolite of the amino acid leucine, proposed to attenuate muscle protein breakdown and support recovery. The clearest signals appear in catabolic states, in older adults, and during periods of intense unaccustomed training or bed rest, where preserving lean mass is the endpoint.
In resistance-trained athletes the picture is far weaker: some highly favourable early trials were not reproduced by independent, better-controlled work, and meta-analyses in trained lifters show minimal effect. The free-acid form is absorbed faster than the calcium salt, though clinical superiority is not firmly established.
02 Mechanism
Attenuates muscle protein breakdown (via the ubiquitin-proteasome pathway) and may modestly stimulate protein synthesis via mTOR, reducing net catabolism during stress.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Common | 3 g/day | Oral | Usually split into 3 x 1 g doses across the day. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Reduced muscle protein breakdownAttenuates markers of muscle damage and preserves lean mass in elderly, untrained or catabolic populations. | Clearest in catabolic states | Clinical | |
| Lean-mass/strength gains in trained athletesLarge early gains in trained lifters were not replicated by well-controlled independent studies. | Minimal in most independent trials | Disputed |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Generally well toleratedAdverse effects are uncommon; safety profile in trials is clean at standard doses. | Mild | Rare | Clinical | |
| Mild gastrointestinal upsetOccasional stomach discomfort, more with the free-acid form. | Mild | Occasional | Observational |