HMB (β-Hydroxy-β-Methylbutyrate)

What HMB is

β-Hydroxy-β-methylbutyrate is formed in the body during the breakdown of leucine, one of the three branched-chain amino acids. As a supplement it is usually sold as a calcium salt, less often as the free acid (HMB-FA), which is said to enter the blood faster.

In the BK-Score, human evidence stands at 7 out of 10 points: there are many controlled studies and several meta-analyses. Well studied does not mean effective here, though – depending on the group, the studies show very different results.

How it is supposed to work

HMB is supposed to stimulate muscle building and at the same time slow the breakdown of muscle protein, similar to leucine but in smaller amounts. From this, advertising derives more muscle mass, more strength and faster recovery after hard training.

The idea is plausible, but it assumes that muscle is actually limited by too much breakdown. In healthy trained people who eat enough protein, this is apparently rarely the case. In illness, bed rest or muscle wasting, things may look different.

What the studies show

Trained people and competitive athletes: no effect

Sanchez-Martinez 2018 analyzed 6 randomized studies with 193 trained or competitively experienced athletes. Bench press, leg press, body weight, lean mass and fat mass did not change with HMB (all effect sizes between −0.20 and 0.16, all not significant), and without any heterogeneity between the studies (I² = 0 %). This also held when analyzed separately by dose, duration, training status and diet. Rowlands 2009 had already found only trivial strength differences in trained people.

Young adults in resistance training: only more body weight

Jakubowski 2020 pooled 11 studies with 302 participants aged 18 to 45. HMB increased body weight slightly, but neither lean mass nor strength, and did not reduce fat mass. The authors conclude that HMB is not to be recommended for muscle building and strength in resistance training.

Older and sick people: small effects, but not on top of training

In clinical groups with muscle wasting, Bear 2019 found small effects on strength and muscle mass across 15 RCTs; not a single study had a low risk of bias in all domains, and many products contained other nutrients besides HMB. In people aged 50 and over who did resistance training, by contrast, HMB provided no added benefit for muscle mass, fat mass or strength in 13 RCTs with 561 participants (Wang 2026).

The Wilson studies: big numbers, open questions

Wilson 2014 reported that after 12 weeks with HMB free acid, trained people gained 77.1 kg in total strength compared with 25.3 kg under placebo, and 7.4 kg of lean mass compared with 2.1 kg. Such gains would be extraordinarily large for trained people and contradict the meta-analyses. In 2017, Gentles and Phillips pointed to inconsistencies between several publications from the group, for example in the numbers and characteristics of participants in the placebo groups. Wilson replied that there were no inconsistencies, only a misunderstanding. To this day the results have not been independently confirmed.

Where the data stop

• Muscle building in trained people. This is exactly what HMB is most heavily advertised for, and exactly where the meta-analyses show no effect.
• Added benefit on top of training in older age. People aged 50 and over who did resistance training gained neither muscle mass nor strength with HMB in 13 RCTs (Wang 2026).
• The role of HMB in combination products. Many clinical studies tested oral nutritional supplements containing HMB and other nutrients; what share is attributable to HMB remains open (Bear 2019).
• Long-term safety. The safety data come mostly from studies lasting weeks to a few months.

Status, approval and legal

In Germany, HMB, usually as a calcium salt, is sold as a dietary supplement. There is no official maximum amount from EFSA or the BfR. In 2011, EFSA examined health claims submitted for HMB, including less muscle damage, more lean mass, more strength, more endurance and faster recovery. There is no health claim for HMB authorized in the EU; statements about muscle building or strength are therefore not permitted as health claims. HMB is not on the World Anti-Doping Agency’s Prohibited List.

Safety

In the analysis of 9 studies with 3 g of HMB per day over 3 to 8 weeks, with men and women, younger and older people, there were no unfavorable changes in blood values, organ function or well-being (Nissen 2000). These data do not extend beyond a few weeks, and long-term safety studies are lacking. In the BK-Score, safety data therefore stand at 6 out of 10. Anyone who has kidney or liver disease, is pregnant or is breastfeeding should clarify its use with a physician. This text is information and does not replace medical advice.

Sources

• Rowlands DS, Thomson JS, J Strength Cond Res 2009 – meta-analysis, 9 studies, trained and untrained people (PMID 19387395)
• Sanchez-Martinez J et al., J Sci Med Sport 2018 – meta-analysis, 6 RCTs, trained athletes (PMID 29249685)
• Jakubowski JS et al., Nutrients 2020 – meta-analysis, young adults in resistance training (PMID 32456217)
• Bear DE et al., Am J Clin Nutr 2019 – meta-analysis, 15 RCTs, clinical groups with muscle wasting (PMID 30982854)
• Wang G et al., Age Ageing 2026 – meta-analysis, 13 RCTs, HMB plus resistance training from age 50 (PMID 41934514)
• Wilson JM et al., Eur J Appl Physiol 2014 – 12 weeks of HMB free acid in trained people (PMID 24599749)
• Gentles JA, Phillips SM, Nutr Metab 2017 – inconsistencies in publications on HMB-FA and ATP (PMID 28680453)
• Wilson JM, Nutr Metab 2017 – reply to the criticism (PMID 29209404)
• Nissen S et al., J Nutr 2000 – safety data from 9 studies with 3 g HMB per day (PMID 10917905)
• EFSA 2011 – assessment of health claims related to HMB (EFSA Journal 2011;9(6):2227)