A metabolite of leucine, roughly 5% of which is converted to HMB in the body. Sold as the calcium salt or as a free acid claimed to absorb faster.
HMB is an anti-catabolic agent with a credibility problem. The consistent finding across meta-analyses is a small effect on lean mass and strength in untrained or older people and essentially nothing in trained athletes — a pattern typical of interventions that help when the baseline is poor. That reasonable picture was disrupted by a small number of trials from one group reporting gains in trained athletes so large they exceeded anything creatine has produced, which drew formal methodological criticism in the literature. The honest reading is that HMB is a modest agent in populations losing muscle, and that the spectacular results should be set aside.
Calcium HMB monohydrate versus free acid; 3 g/day is the near-universal dose — a product without this marker may not resemble what was studied.
Also known as
HMB, Calcium HMB, HMB free acid
Regulation: Sold as a dietary supplement with no pre-market efficacy review. Not prohibited in sport. It is also an ingredient in some clinical nutrition products used in muscle wasting.
What the research used
The amounts, forms and durations published trials of HMB (beta-hydroxy beta-methylbutyrate) actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Lean mass and strength
3 g
Moderate
Calcium HMB, usually in three 1 g doses · 3–12 weeks
Almost every trial uses 3 g/day. Effects appear largest in untrained participants and in older adults, and smallest or absent in trained athletes.
Studied for Muscle preservation during bed rest or illness
3 g
Limited
HMB, often within a clinical nutrition supplement · 10 days to 6 months
Trials in older adults during bed rest and in clinical malnutrition report preserved lean mass. This is the most plausible use and the trials are still small.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
None of the timings below were directly compared in a trial. They reflect how the research happened to dose, or what the pharmacology implies — which is not the same as a timing being better.
🔀Split through the day
Not enough
Trials divide 3 g across the day, and plasma HMB after calcium HMB peaks at around two hours. No timing comparison has shown that this matters for outcomes.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
The evidence in depth
Best supported: Limited
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Some trials report reduced markers of muscle damage after intense exercise with HMB. Results are inconsistent and the largest reported effects come from the trials whose methods have been questioned.
Limited evidence. Early human work, or mostly lab and animal research.
Meta-analyses report small increases in lean mass and strength with 3 g/day, concentrated in untrained and older participants and largely absent in trained athletes. A small set of trials reporting very large effects in trained athletes has attracted published methodological criticism and should be read separately from the rest.
What may be going on: HMB appears to act mainly by attenuating muscle protein breakdown through the ubiquitin-proteasome pathway, rather than by stimulating synthesis as leucine does.
Limited evidence. Early human work, or mostly lab and animal research.
Randomised controlled trial · The journal of nutrition, health & aging · 2023 · PMID 37248756
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Worth knowing
A contested subset of the evidence
Several trials reporting extraordinary gains in trained athletes have been the subject of published methodological criticism, including implausible effect sizes and questions about data. Meta-analyses that include them look far better than those that do not.
Worth knowing
Effects concentrate in the untrained and the elderly
The consistent pattern is benefit where muscle is being lost or the training stimulus is new, and little in trained people. That is the opposite of how it is marketed.
Too much
Generally well tolerated
Safety studies at 3 g/day report no consistent adverse effects on liver, kidney or lipid markers over several weeks. Long-term data are limited.