Supplements

HMB (Hydroxymethylbutyrate)

Updated · 21 sources · Editorial policy

β-Hydroxy-β-methylbutyrate (HMB) is a metabolite of the amino acid leucine. The body makes small amounts of it on its own, and it is sold as a dietary supplement, either as a calcium salt (HMB-Ca) or as a “free acid” (HMB-FA). It is marketed mainly for muscle growth, strength and recovery, and it is also an ingredient in some medical nutrition drinks for older or ill adults.

The single most important fact is that the evidence is split by population. In young adults doing resistance training, pooled trials show little or no added gain in lean mass or strength. In older adults and in people losing muscle to illness or bed rest, some meta-analyses find small benefits. One 2014 trial reporting large gains in trained men has been publicly questioned by other researchers.

This page summarizes what clinical trials, meta-analyses and regulators report. It is for education only and is not medical advice.

Property Detail
Class Leucine metabolite; dietary supplement ingredient
Forms Calcium salt (HMB-Ca); free acid (HMB-FA)
Route Oral
Dose used in most trials 3 g/day, usually split into several servings; studied range 1.5–6 g/day
Plasma half-life ≈2.4 h after 1 g HMB-Ca in one study; 2.5 h (free acid) vs 3.2 h (calcium salt) in another
Made in the body About 5% of leucine is converted to HMB
US status Sold as a dietary supplement; FDA does not approve supplements before sale
WADA status Not named on the 2026 Prohibited List

What it is

Leucine is one of the three branched-chain amino acids. In the body it is first converted to α-ketoisocaproate (KIC). Most KIC is broken down by another route, and only about 5% of leucine ends up as HMB, through the enzyme KIC dioxygenase (ISSN position stand, 2025).

Two supplement forms exist:

Form What it is Notes
HMB-Ca Calcium salt of HMB, usually in capsules or powder Every published study used this form until 2011
HMB-FA The free acid, first described in 2011 and studied as a gel Reaches the blood faster and at higher peak levels in some studies; later comparisons are mixed

The first published HMB training trials appeared in 1996. In two small trials, 1.5 or 3 g/day reduced a urinary marker of muscle protein breakdown (3-methylhistidine) and increased weight lifted or fat-free mass compared with no HMB (Nissen et al., J Appl Physiol 1996). The first trial had 41 participants over 3 weeks, and the second had 28 over 7 weeks.

Mechanism

The proposed mechanism is a dual effect on muscle protein: more synthesis and less breakdown. In a human tracer study, 3.42 g of HMB-FA (2.42 g of HMB itself) raised muscle protein synthesis by 70%, compared with 110% for the same amount of leucine. HMB also cut muscle protein breakdown by 57% (Wilkinson et al., J Physiol 2013). These are acute, single-dose measurements. They do not by themselves show that muscle grows more over months of training.

Pharmacokinetics

Two small human studies give most of the kinetic data:

Study Dose and form Time to peak Half-life
Vukovich et al., 2001, 8 men 1 g HMB (capsule) 2.0 h 2.37 h
Vukovich et al., 2001, 8 men 3 g HMB with water 1.0 h 2.38 h
Vukovich et al., 2001, same men 3 g HMB with 75 g glucose 1.94 h 2.69 h
Fuller et al., 2011, 8 adults per study 1 g HMB-Ca capsule 128 min 3.17 h
Fuller et al., 2011 Equivalent HMB-FA gel, swallowed 38 min 2.50 h

Other findings from these studies:

  • Plasma HMB returned to baseline about 9 h after a dose.
  • About 14% of a 1 g dose and 27–29% of a 3 g dose appeared in the urine. The rest was retained for further metabolism.
  • Taking HMB with glucose delayed and lowered the peak.
  • The free-acid gel reached a higher peak than the calcium salt (249 vs 131 μmol/L), and the authors reported an area under the curve almost twice as large.

The 2025 position stand of the International Society of Sports Nutrition (ISSN) notes that HMB-FA “appears to lead to increased appearance” of HMB in the blood, “though recent results are mixed” (Rathmacher et al., JISSN 2025).

Regulatory status

In the US, HMB is sold as a dietary supplement. Under the 1994 Dietary Supplement Health and Education Act, FDA does not have the authority to approve dietary supplements before they are marketed. Firms are responsible for ensuring their products are safe and correctly labeled (FDA Q&A on dietary supplements).

HMB is not named on the WADA 2026 Prohibited List.

What the research shows

Young adults doing resistance training

The pooled evidence in young, healthy lifters is weak.

Meta-analysis Trials and participants Main finding
Rowlands & Thomson, 2009 9 studies, 14 comparisons, 394 men Small lower-body strength gain in untrained men (9.9% ± 5.9%, 90% confidence limits); trivial strength effects in trained men; trivial effects on fat and fat-free mass
Jakubowski et al., 2020 11 trials analyzed, 302 participants aged 18–45 Total body mass +0.34 kg (95% CI 0.03–0.66); fat-free mass +0.29 kg (95% CI −0.01 to 0.60, p = 0.06, not significant); no significant effect on fat mass or strength

Jakubowski et al. required a dose of 3 g/day and at least 3 weeks of full-body training. They concluded that HMB does not improve body composition or strength during resistance training in young adults.

A pragmatic, double-blind trial tested off-the-shelf products in 40 resistance-trained men over 8 weeks of training (Teixeira et al., MSSE 2019). The ISSN position stand lists the HMB dose as 3 g/day. The men were randomized to HMB-FA (n = 11), HMB-Ca (n = 9), another leucine metabolite called α-HICA (n = 10), or placebo (n = 10). Muscle thickness, strength and power all rose with training, but no supplement changed any outcome compared with placebo.

The Wilson et al. 2014 free-acid trial and the dispute

One trial stands far apart from the rest. Wilson et al. randomized 24 resistance-trained men to 3 g/day of HMB-FA (three 1 g servings) or placebo during 12 weeks of training. The program included an 8-week periodized phase, a 2-week “overreaching” phase of deliberately excessive training, and a 2-week taper. Diet was controlled at 25% protein, 50% carbohydrate and 25% fat. The final analysis included 11 men on HMB-FA and 9 on placebo (Wilson et al., Eur J Appl Physiol 2014).

Outcome over 12 weeks HMB-FA Placebo
Lean body mass +7.4 ± 4.2 kg +2.1 ± 6.1 kg
Total strength (bench, squat, deadlift) +77.1 ± 18.4 kg +25.3 ± 22.0 kg
Vertical jump power +991 ± 168 W +630 ± 167 W
Creatine kinase change during overreaching −6 ± 91 IU/L +277 ± 229 IU/L

The study was funded in part by Metabolic Technologies Inc. (MTI), and three of its authors were MTI employees (PMC full text). MTI had marketed both HMB-Ca and HMB-FA until recently, according to the disclosures in the 2025 ISSN position stand.

The same study (ClinicalTrials.gov NCT01508338) also had an ATP arm and an HMB-FA plus ATP arm. These were published separately, including a 2016 paper reporting a 12.7% gain in lean body mass with the combination (Lowery et al., J Strength Cond Res 2016).

Later commentary, as published:

  • Letter from 18 researchers (2017). A group led by Stuart Phillips published a letter on the HMB-FA plus ATP paper in the same journal (Phillips et al., J Strength Cond Res 2017).
  • Discrepancies letter (2017). Gentles and Phillips noted that all three papers came from one study with one shared placebo group (Gentles & Phillips, Nutr Metab 2017). They raised three issues. First, the supplement protocols differed between the papers and the trial registration. Second, the placebo group had different numbers of participants across papers without explanation. Third, the age and relative-strength means and SDs were nearly identical across the three samples.
  • Author response (2017). Wilson replied that there were “no discrepancies” (Wilson, Nutr Metab 2017). He said the critics’ table mislabeled one paper’s baseline data, and he gave corrected values. He explained that the ATP paper used a later cohort with two extra placebo recruits. He also said all groups took the same daily regimen of three gel packs and one capsule, with active or placebo contents.
  • Meta-analysis exclusion (2020). Jakubowski et al. found that Wilson 2014 (7.4 kg fat-free mass gain) and an earlier trial (≈9.3 kg) created marked funnel-plot asymmetry. They excluded both from the pooled analysis and described such results as “not commonly reproducible.”
  • ISSN position stand (2025). The ISSN stand still included the Wilson trial. It acknowledged that the paper “has been criticized” for outcomes hard to reconcile with other nutritional and drug interventions and for its statistical reporting. In turn, it criticized Jakubowski et al. for excluding the two trials late in the analysis rather than by pre-set criteria.

Conflicts on both sides. Several authors of the 2025 ISSN position stand disclose HMB patents or past employment at MTI. Stuart Phillips co-authored both critical letters, the 2020 meta-analysis and the null 2019 trial. Taken together, the independent trials and meta-analyses do not reproduce gains of the size Wilson et al. reported.

Older adults

Results in older adults are mixed and depend on whether participants were also exercising.

Meta-analysis Population Finding
Wu et al., 2015 7 RCTs, 287 adults aged 65 and older Greater muscle mass with HMB (standardized mean difference 0.35, 95% CI 0.11–0.59); no fat mass effect
Bear et al., 2019 15 RCTs, 2,137 patients with muscle-wasting conditions; HMB alone or in combined supplements Muscle mass SMD 0.25 (95% CI −0.00 to 0.50); strength SMD 0.31 (95% CI 0.12–0.50); no study at low risk of bias in all categories
Wang et al., 2026 13 RCTs, 561 adults aged 50 and older, all doing resistance training No benefit for muscle mass (SMD 0.05, 95% CI −0.10 to 0.20), strength or fat mass

Bear et al. called the effect sizes “small.” Wang et al. concluded that HMB cannot be recommended as a routine add-on for older adults who are able to do structured exercise.

Bed rest and hospital nutrition

In a randomized, double-blind trial, healthy adults aged 60–76 took 3 g/day of HMB-Ca or placebo during 10 days of complete bed rest (Deutz et al., Clin Nutr 2013). Of the 24 enrolled, 19 were evaluable. The placebo group lost 2.05 kg of lean body mass. The HMB group’s change was −0.17 kg, a figure calculated after excluding one participant. Physical function did not differ, possibly because the study was small.

The NOURISH trial randomized 652 malnourished adults aged 65 and older, hospitalized for heart failure, heart attack, pneumonia or COPD (Deutz et al., Clin Nutr 2016). They received either a high-protein oral supplement containing HMB or a placebo supplement, two servings a day.

Outcome at 90 days HMB-containing supplement Placebo
Death or non-elective readmission (primary endpoint) 26.8% 31.1% (not significant)
Death 4.8% 9.7% (RR 0.49, 95% CI 0.27–0.90)

The NOURISH supplement was a high-protein formula, so the trial cannot isolate the effect of HMB. Several of the authors were affiliated with Abbott.

Side effects and risks

General safety

The early safety data come from a pooled analysis of nine studies in which people took 3 g/day of HMB for 3–8 weeks (Nissen et al., J Nutr 2000). HMB did not adversely affect blood chemistry, blood counts or a 32-item tolerance questionnaire. Compared with placebo, it lowered:

  • total cholesterol, by 5.8%;
  • LDL cholesterol, by 7.3%;
  • systolic blood pressure, by 4.4 mm Hg.

The 2025 ISSN position stand states that both HMB-Ca and HMB-FA appear safe for up to at least one year. It also reports no negative effects on glucose tolerance or insulin sensitivity.

Gaps in the evidence

  • Most trials are small, short (weeks to months) and in healthy adults.
  • The trials summarized here did not study pregnancy, breastfeeding, children or people with kidney or liver disease.
  • As with any supplement, FDA does not approve products before sale or routinely analyze their contents.

Key takeaways

  • A leucine metabolite sold as a supplement. The body converts about 5% of leucine to HMB. Supplements come as a calcium salt or a free acid, almost always studied at 3 g/day.
  • Short half-life. HMB peaks within about 40 minutes to 2 hours, depending on form, and has a plasma half-life of about 2.4–3.2 hours.
  • Little effect in young lifters. Meta-analyses find no significant gain in fat-free mass or strength from adding HMB to resistance training in young adults.
  • The large free-acid results are disputed. The 2014 Wilson et al. trial reported 7.4 kg of lean mass gain in 12 weeks. It was industry-funded and publicly questioned, and independent trials have not reproduced it.
  • Possible small benefit when muscle is being lost. Some meta-analyses find small gains in muscle mass or strength in older or ill adults. Benefit was absent when older adults were already doing resistance training.
  • Well tolerated in trials. No consistent adverse effects have been reported at 3 g/day, but most safety data come from short studies.

Sources

  1. Rathmacher JA, Pitchford LM, Stout JR, et al. International society of sports nutrition position stand: β-hydroxy-β-methylbutyrate (HMB). J Int Soc Sports Nutr. 2025;22(1):2434734. PubMed.
  2. Nissen S, Sharp R, Ray M, et al. Effect of leucine metabolite beta-hydroxy-beta-methylbutyrate on muscle metabolism during resistance-exercise training. J Appl Physiol. 1996;81(5):2095–2104. PubMed.
  3. Wilkinson DJ, Hossain T, Hill DS, et al. Effects of leucine and its metabolite β-hydroxy-β-methylbutyrate on human skeletal muscle protein metabolism. J Physiol. 2013;591(11):2911–2923. PubMed.
  4. Vukovich MD, Slater G, Macchi MB, et al. beta-hydroxy-beta-methylbutyrate (HMB) kinetics and the influence of glucose ingestion in humans. J Nutr Biochem. 2001;12(11):631–639. PubMed.
  5. Fuller JC Jr, Sharp RL, Angus HF, et al. Free acid gel form of β-hydroxy-β-methylbutyrate (HMB) improves HMB clearance from plasma in human subjects compared with the calcium HMB salt. Br J Nutr. 2011;105(3):367–372. PubMed.
  6. US Food and Drug Administration. Questions and answers on dietary supplements. FDA.
  7. World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.
  8. Rowlands DS, Thomson JS. Effects of beta-hydroxy-beta-methylbutyrate supplementation during resistance training on strength, body composition, and muscle damage in trained and untrained young men: a meta-analysis. J Strength Cond Res. 2009;23(3):836–846. PubMed.
  9. Jakubowski JS, Nunes EA, Teixeira FJ, et al. Supplementation with the leucine metabolite β-hydroxy-β-methylbutyrate (HMB) does not improve resistance exercise-induced changes in body composition or strength in young subjects: a systematic review and meta-analysis. Nutrients. 2020;12(5):1523. PubMed.
  10. Teixeira FJ, Matias CN, Monteiro CP, et al. Leucine metabolites do not enhance training-induced performance or muscle thickness. Med Sci Sports Exerc. 2019;51(1):56–64. PubMed.
  11. Wilson JM, Lowery RP, Joy JM, et al. The effects of 12 weeks of beta-hydroxy-beta-methylbutyrate free acid supplementation on muscle mass, strength, and power in resistance-trained individuals: a randomized, double-blind, placebo-controlled study. Eur J Appl Physiol. 2014;114(6):1217–1227. PubMed; PMC.
  12. Lowery RP, Joy JM, Rathmacher JA, et al. Interaction of beta-hydroxy-beta-methylbutyrate free acid and adenosine triphosphate on muscle mass, strength, and power in resistance trained individuals. J Strength Cond Res. 2016;30(7):1843–1854. PubMed.
  13. Phillips SM, Aragon AA, Arciero PJ, et al. Changes in body composition and performance with supplemental HMB-FA+ATP. J Strength Cond Res. 2017;31(5):e71–e72. PubMed.
  14. Gentles JA, Phillips SM. Discrepancies in publications related to HMB-FA and ATP supplementation. Nutr Metab (Lond). 2017;14:42. PubMed.
  15. Wilson JM. Response to discrepancies in publications related to HMB-FA and ATP supplementation. Nutr Metab (Lond). 2017;14:74. PubMed.
  16. Wu H, Xia Y, Jiang J, et al. Effect of beta-hydroxy-beta-methylbutyrate supplementation on muscle loss in older adults: a systematic review and meta-analysis. Arch Gerontol Geriatr. 2015;61(2):168–175. PubMed.
  17. Bear DE, Langan A, Dimidi E, et al. β-Hydroxy-β-methylbutyrate and its impact on skeletal muscle mass and physical function in clinical practice: a systematic review and meta-analysis. Am J Clin Nutr. 2019;109(4):1119–1132. PubMed.
  18. Wang G, Jawed I, Tufail M, et al. Efficacy of HMB supplementation as an adjunct to resistance training in older adults: a comprehensive meta-analysis. Age Ageing. 2026;55(3):afag073. PubMed.
  19. Deutz NE, Pereira SL, Hays NP, et al. Effect of β-hydroxy-β-methylbutyrate (HMB) on lean body mass during 10 days of bed rest in older adults. Clin Nutr. 2013;32(5):704–712. PubMed.
  20. Deutz NE, Matheson EM, Matarese LE, et al. Readmission and mortality in malnourished, older, hospitalized adults treated with a specialized oral nutritional supplement: a randomized clinical trial. Clin Nutr. 2016;35(1):18–26. PubMed.
  21. Nissen S, Sharp RL, Panton L, et al. beta-hydroxy-beta-methylbutyrate (HMB) supplementation in humans is safe and may decrease cardiovascular risk factors. J Nutr. 2000;130(8):1937–1945. PubMed.