HMB Hydroxymethylbutyrate

non-nutrient/non-botanical2-ethylhydracrylate

What is it

HMB (beta-hydroxy-beta-methylbutyrate) is a metabolite of the essential amino acid leucine. It is widely used as a supplement to reduce muscle protein breakdown and support muscle preservation during training, aging, or catabolic states.

How it works

HMB is produced naturally in the body when leucine is metabolized: leucine is converted to alpha-ketoisocaproate (KIC), and a small fraction of KIC (approximately 5%) is then converted to HMB. Supplemental HMB delivers HMB directly without requiring large amounts of leucine. HMB's main mechanism appears to be reducing muscle protein breakdown by inhibiting the ubiquitin-proteasome and autophagy-lysosomal pathways, the two major systems responsible for protein degradation. It may also support cell membrane integrity through effects on cholesterol synthesis (since HMB is a precursor to HMG-CoA, which feeds into the cholesterol/coenzyme Q10 pathway), and modestly stimulate protein synthesis via mTOR. The net effect is preservation of lean mass during periods of high catabolic stress: intense training, aging, illness, and caloric restriction. Benefits in well-trained athletes who already meet protein needs are often modest, while benefits in older adults, untrained individuals starting exercise, or catabolic clinical states tend to be more apparent.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Muscle preservation during exercise / training

Grade B

Good evidence

HMB at 3 g/day reduces exercise-induced muscle damage and may support lean mass gains, particularly in less-trained individuals or during periods of high training stress. Effects in well-trained athletes are smaller.

Sarcopenia / muscle preservation in older adults

Grade B

Good evidence

HMB combined with adequate protein and resistance training helps preserve muscle mass and function in older adults. Multiple trials support modest improvements in strength, lean mass, and physical performance.

Bedrest / immobilization (muscle preservation)

Grade B

Good evidence

HMB reduces muscle loss during periods of bed rest or immobilization, with reasonable evidence in older adults. Useful in post-surgical recovery and hospitalization scenarios.

Cancer cachexia / catabolic illness

Grade C

Moderate evidence

HMB (often combined with arginine and glutamine) has been studied as part of nutritional support in cancer cachexia and HIV-associated wasting, with modest evidence of improved lean mass and clinical outcomes.

Lean mass preservation during weight loss

Grade C

Moderate evidence

Some evidence suggests HMB may help preserve lean mass during caloric restriction, particularly when combined with resistance training. Effect size is modest.

2 commercial forms

Calcium HMB (CaHMB)

The traditional form; absorption is reasonable, with peak levels at 1-2 hours.

The form used in most published clinical trials. Inexpensive and widely available.

HMB free acid (HMB-FA)

More rapid and complete absorption, with higher peak plasma levels and area under the curve compared to CaHMB.

Marketed as more bioavailable. Some evidence supports better acute kinetics; long-term clinical superiority over CaHMB is debated.

Dosage

Standard supplement dose is 3 g/day, typically split into three 1 g doses. For best effect, take consistently for at least 2 weeks before benefits become measurable. HMB-FA (free acid form) may be more rapidly absorbed and is sometimes dosed at 1-2 g per dose. There is no formal UL; 6 g/day has been tested without adverse effects.

When and how to take it

HMB is typically taken in three divided doses across the day. Pre- or post-workout dosing is common for athletes. For elderly or those targeting muscle preservation, spread doses throughout the day with meals. Effects build over 2-4 weeks of consistent use; HMB is not an acute-effect supplement.

Food sources

Food	Amount	%DV
Catfish (3 oz)	Trace amounts	—
Grapefruit (1 medium)	Trace amounts	—
Alfalfa	Modest amounts	—

Safety

HMB is generally well-tolerated. Long-term studies (up to a year) show no significant adverse effects. Mild GI upset is uncommon. The compound is naturally produced in the body and present in small amounts in foods like catfish, grapefruit, and alfalfa.

Who should be cautious

Pregnant and lactating women should avoid supplementation due to insufficient safety data. Use cautiously in liver or kidney disease. Children and adolescents should use only with clinician guidance. Otherwise generally safe in healthy adults.

Interactions

Few significant drug interactions documented. May theoretically interact with statins given its position upstream in the cholesterol synthesis pathway, though no clinical issues have been documented. May modestly affect cholesterol metabolism. Combine well with leucine, creatine, and protein for muscle support.

Frequently asked questions

Is HMB the same as leucine?⌄

HMB is a metabolite of leucine. The body converts a small percentage of leucine into HMB. Supplementing HMB directly delivers the active metabolite, which would otherwise require very large leucine doses.

How much HMB do I need?⌄

Standard dose is 3 g/day, split into three 1 g doses. Effects build over 2-4 weeks of consistent use.

Does HMB build muscle?⌄

It primarily reduces muscle protein breakdown rather than driving large gains in protein synthesis. Effects are most apparent in untrained individuals, older adults, or during catabolic conditions. Well-trained athletes often see smaller benefits.

Is HMB worth it for older adults?⌄

Reasonable evidence supports HMB plus protein and resistance training for combating sarcopenia. It may be particularly useful during periods of bedrest or recovery.

Are there side effects?⌄

HMB is well-tolerated with minimal side effects in studies. Long-term safety (up to a year) appears acceptable. Avoid in pregnancy due to lack of safety data.

References

HMB review — Journal of the International Society of Sports Nutrition link

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.