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HMB (Beta-Hydroxy Beta-Methylbutyrate)

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

Evidence under review. — Not yet rated

Leucine metabolite that may support muscle preservation and body composition, especially in clinical populations.

  • What it does

    HMB is a natural byproduct of the amino acid leucine, found in small amounts in foods like citrus and catfish. 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 foods like citrus and catfish. The provided studies suggest it may help preserve muscle mass and improve body composition when combined with exercise, particularly in people with respiratory disease or muscle-wasting conditions. At 3 g/day, it also appears to modestly influence cortisol responses during fasting, though it did not reduce muscle protein breakdown in that context.

What It Doesn't Do

Not proven to prevent muscle breakdown during fasting in healthy adults. No evidence from these studies that it builds significant muscle on its own without exercise. Don't expect dramatic strength gains from HMB alone. Animal and fish studies don't translate directly to human benefits.

Evidence-Based Benefits

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

Weak Evidence

Effective at: One can/day of HMB-enriched oral nutritional supplement for 12 weeks

Supporting studies (click to view on PubMed):

PMID 26522923

Adding HMB to pulmonary rehabilitation improved handgrip strength more than rehab alone in lung disease patients.

Weak Evidence

Effective at: One can/day of HMB-enriched oral nutritional supplement for 12 weeks

Supporting studies (click to view on PubMed):

PMID 26522923

HMB free acid (3 g/day) reduced the cortisol awakening response during a 24-hour fast.

Weak Evidence

Effective at: 3 g/day HMB-FA

Supporting studies (click to view on PubMed):

PMID 29508695

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

Weak Evidence

Effective at: 3 g single dose

Supporting studies (click to view on PubMed):

PMID 26373270

Absorption & Bioavailability

Good — the free acid form (HMB-FA) absorbs faster and reaches higher peak plasma levels than the calcium salt form (Ca-HMB). HMB-FA in capsules showed 76% higher peak plasma concentration and absorbed in one-third the time compared to Ca-HMB capsules.

Red Flags to Watch For

  • Most compelling data comes from a small clinical trial (n=30) in a specific disease population (bronchiectasis), not healthy adults
  • One of the indexed papers (PMID 33552898) is completely unrelated to HMB in humans — data quality from the source is inconsistent
  • Animal studies (piglets, rainbow trout) are included in the evidence base and cannot be directly applied to human supplementation
  • The fasting study found no reduction in actual muscle protein breakdown despite cortisol changes — a key marketing claim that isn't supported
  • 256 registered supplement products exist, suggesting heavy commercial interest that may outpace the actual evidence

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