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HMB (Beta-Hydroxy Beta-Methylbutyrate)

Also known as: HMB, Beta-Hydroxy Beta-Methylbutyrate, HMB-FA, HMB free acid, Ca-HMB, calcium HMB, leucine metabolite

Evidence under review. — Not yet rated

Leucine metabolite that may support muscle preservation and reduce cortisol during fasting or illness.

  • What it does

    HMB is a natural byproduct of the amino acid leucine, found in small amounts in food and produced in the body. The provided studies suggest it may help preserve muscle mass and improve body...

  • Evidence quality

    Evidence base hasn't been formally rated yet. See research below.

  • Clinical dose

    3 g daily (based on available study data)

See full breakdown ↓ Methodology →

What the Science Says

HMB is a natural byproduct of the amino acid leucine, found in small amounts in food and produced in the body. The provided studies suggest it may help preserve muscle mass and improve body composition when combined with exercise rehabilitation in people with lung disease, and it may reduce cortisol levels during short-term fasting. It comes in two forms — a calcium salt (Ca-HMB) and a free acid (HMB-FA) — with the free acid form absorbing faster and reaching higher blood concentrations.

What It Doesn't Do

Not proven to build muscle in healthy, well-nourished adults based on the provided studies. No evidence from these papers that it boosts athletic performance or speeds recovery in trained athletes. The cortisol-lowering effect was only seen during a 24-hour fast — don't expect it to work as a general stress reducer. Animal and fish studies don't confirm human benefits.

Evidence-Based Benefits

Combined with pulmonary rehab, HMB-enriched supplements improved lean mass and bone density in bronchiectasis patients.

Weak Evidence

Effective at: No established dose from provided studies

HMB free acid reduced the cortisol awakening response during a 24-hour fast in a small crossover trial.

Weak Evidence

Effective at: 3 g/day

The free acid form of HMB absorbs faster and reaches higher blood levels than the calcium salt capsule form.

Weak Evidence

Effective at: No established dose from provided studies

Absorption & Bioavailability

Good for HMB-FA (free acid) form — reaches peak plasma levels 3x faster and 76% higher peak concentration than calcium salt capsules. Ca-HMB is still absorbed but more slowly. Both forms absorb better when taken in capsule form versus mixed in water.

Red Flags to Watch For

  • Most dramatic claims about HMB come from industry-funded research not represented in these provided papers — be skeptical of bold muscle-building marketing.
  • The free acid form (HMB-FA) is often marketed as significantly superior, but differences may matter less when mixed in water rather than taken as capsules.
  • The clinical trial in bronchiectasis patients used HMB as part of a multi-ingredient nutritional supplement — it's impossible to isolate HMB's contribution alone.
  • Animal and fish studies (piglets, rainbow trout) in the provided data cannot be directly applied to human supplementation outcomes.
  • Only 11 participants completed the fasting/cortisol study — results should be considered very preliminary.

Research Sources

  • PubMed
  • NIH DSLD

This information is for educational purposes only and is not medical advice. Always consult a healthcare professional before starting any supplement regimen. Last updated: 2026-05-25