HMB

What is HMB and what does it do in the body?

HMB is a metabolite of the amino acid leucine. According to NIH ODS, about 5% of the body’s leucine is converted to HMB and then into a precursor needed for cholesterol synthesis in the liver. Some experts argue that muscle cells stressed by exercise need this precursor to repair their membrane structure, and that the conversion of leucine to HMB triggers muscle protein synthesis and reduces breakdown. ^[1] The ISSN 2025 position also states that the main mechanism of action is a “dual mechanism” that increases muscle protein synthesis and suppresses breakdown, and that mTORC1 activation works independently of the leucine-sensing pathway (Sestrin2-GATOR2). ^[2]

NIH ODS states that, for 3 g of HMB per day, the amount made from leucine would require more than 600 g of high-quality protein (for example, 5 lb of beef tenderloin), so this amount can in practice be reached only with supplements. ^[1] That a mechanism is plausible does not mean there is measurable benefit in every population; the sections below separate results by population.

In which forms is HMB found?

Two forms have been studied: calcium HMB (HMB-Ca, a monohydrate calcium salt) and the calcium-free free acid (HMB-FA). According to ISSN 2025, HMB-FA appears to increase the appearance of HMB in the bloodstream compared with HMB-Ca, but recent results are mixed. ^[2] According to NIH ODS, HMB-Ca contains about 13% calcium by weight; 3 g of HMB-Ca per day adds about 400 mg of calcium to the diet. Studies comparing the effects of the two forms are insufficient. ^[1] The 2013 ISSN position also stated that there was not enough research to support whether HMB-FA is superior. ^[3]

Does HMB increase strength and muscle mass in trained athletes?

The meta-analysis by Sanchez-Martinez and colleagues combined 6 randomized controlled studies (193 participants) examining the effect of HMB supplementation on strength and body composition in trained and competitive athletes. Effect sizes were trivial and nonsignificant: bench press ES 0.00, leg press 0.09, body mass −0.01, lean mass 0.16, and fat mass −0.20 (p>0.05 for all; I² 0%); results did not change in subgroups by HMB dose, duration, training level, and dietary co-intervention. The authors concluded that HMB supplementation has no effect on strength and body composition in trained and competitive athletes. ^[4]

The 2022 meta-analysis by Holland and colleagues looked at body composition in athletes, using studies lasting at least 4 weeks. For body mass, 7 studies (208 people) were analyzed; for lean mass, 5 studies (161 people); and for fat mass, 5 studies (128 people). The estimate for lean mass was 0.30 ± 0.13 (95% CI −0.07 to 0.68; p=0.08), meaning the confidence interval included zero; the authors say HMB may have a small, positive effect on lean mass and that this appears specific to situations in which protein intake is inadequate (less than 1.6 g/kg per day). There was no significant effect for body mass (−0.02; p=0.70) and fat mass (−0.33; CI −0.96 to 0.31; p=0.22). ^[5]

There are opposing views as well: the ISSN 2025 position states that daily HMB supplementation (38 mg/kg body weight) together with exercise may improve body composition by increasing lean mass and/or reducing fat mass, may increase strength and power in untrained people, that the performance benefit in trained athletes is “mixed,” and that it increases as study duration exceeds 6 weeks. ^[2] The 2013 position was far bolder: it said that 38 mg/kg/day increased muscle hypertrophy, strength, and power in both trained and untrained populations. ^[3] The difference between the two positions is consistent with newer meta-analyses shrinking the effect.

Authors’ ties in HMB research

According to the disclosure section of the ISSN 2025 position, the first author works at a biotechnology company; the company where the author previously worked marketed HMB-Ca and HMB-FA until recently, the author is listed as an inventor on HMB-related patents, and has received a federal grant for HMB-Ca research. Several other authors reported HMB patents or consulting and grant relationships with the supplement industry. ^[2] This does not show that the findings are wrong; however, conditional wording in the text such as “may” and “can reduce,” when weighed together with the effect remaining small or nonsignificant in the meta-analyses above, gives a more cautious picture.

Does HMB help recovery after exercise?

NIH ODS states that there is a “general consensus” that HMB speeds recovery from exercise of an intensity and amount that causes muscle damage, so trained athletes would need to exert more effort than untrained people to benefit. Studies differ greatly in supplementation duration (1 day–6 weeks) and dose (1.5–6 g/day; most often 3 g), participant age (19–50), training status, and other supplements; it is therefore hard to predict how much benefit an exercising person can expect. ^[1] HMB is not on the AND/DC/ACSM evidence-based ergogenic aid list; the Australian Institute of Sport does not recommend HMB supplementation for athletes outside a research protocol or appropriate monitoring. ^[1] ISSN 2025 also says HMB may reduce muscle damage and inflammation and support recovery. ^[2]

Does HMB reduce muscle loss in older adults and patients?

The 2015 meta-analysis by Wu and colleagues combined 7 RCTs (fixed-effect model) covering 147 older adults using HMB and 140 in the control group, aged 65 and over: the increase in muscle mass was greater in the HMB groups (SMD 0.352; 95% CI 0.11–0.594; p=0.004); there was no difference in fat mass. The authors say additional studies are needed for definitive effects on muscle strength and physical function. ^[6]

The 2019 meta-analysis by Courel-Ibáñez and colleagues, on the other hand, reported that in 10 RCTs covering 384 participants aged 50–80, HMB added to exercise had “very little or no” effect in improving body composition, muscle strength, or physical performance compared with exercise alone. A knowledge gap was noted regarding its effect on preserving cognitive function. ^[7] The two reviews answer different questions: the first addresses the change in muscle mass with HMB in people over 65, the second the contribution of adding HMB to exercise.

The 2019 meta-analysis by Bear and colleagues, published in Am J Clin Nutr, combined 15 RCTs (2,137 patients) evaluating HMB or HMB-containing products in clinical conditions with muscle loss and weakness. SMD 0.25 for muscle mass (95% CI −0.00 to 0.50; p=0.05; I² 58%) and SMD 0.31 for muscle strength (95% CI 0.12–0.50; p=0.001; I² 0%) were found; effects were small, and no effect was detected for body weight and other measures. No study was rated low risk in all bias categories; the analysis covers HMB-containing products, not HMB alone. ^[8]

Is HMB safe?

According to NIH ODS, a safety review of nine studies reported that HMB at 3 g per day for 3–8 weeks was well tolerated in both young (ages 18–47) and older (ages 62–81) women and men, exercising or not; no adverse effects were found in blood biochemistry, hematology, and mood measures. In 37 untrained young men (ages 18–29) doing resistance training, about 3–6 g of HMB per day did not change hematologic, liver, or kidney function parameters. Safety and efficacy have not been studied in adolescents. ^[1] The ISSN 2025 position states that chronic HMB-Ca and HMB-FA use appears safe in humans for up to at least one year and does not adversely affect glucose tolerance and insulin sensitivity. ^[2] NIH ODS, on the other hand, says there is no expert consensus on the value and safety of using HMB for several months or longer. ^[1]

How should the HMB evidence be read?

NIH ODS emphasizes that studies examining HMB over twenty years differ greatly in duration (1 day–6 weeks), dose, participant age, training status, and use of additional supplements. ^[1] A population-based reading is therefore needed instead of a single “Is HMB effective?” question: in trained athletes, meta-analyses have found no effect; in untrained and older groups, small, inconsistent benefits have been reported; and in clinical disease, the effect is small and most studies carry a risk of bias. Because the result in the clinical meta-analysis covers HMB-containing products and not only HMB, the effect cannot be attributed to a single component. ^{[8] [4] [6] [7]}

HMB supplements in Türkiye

Approval from the Ministry of Agriculture and Forestry is mandatory for every dietary supplement to be marketed in Türkiye. ^[9] In the Restricted Substances List for Dietary Supplements (last updated September 21, 2026), the beta-hydroxy beta-methylbutyrate (HMB) row is of chemical synthesis origin and carries an upper limit of 3 g/day in the age 11 and over column (published October 3, 2025). Calcium HMB (Ca-HMB) is a separate row: chemical synthesis, “3 g/day, in an amount equivalent to HMB” in the age 11 and over column (published February 19, 2026). In both rows, no value is entered in the 4–10 year columns. ^[10]

The 3 g/day limit in the list is of the same magnitude as the amount most often used in the studies reported by NIH ODS; however, a study protocol and a product limit in Türkiye are different contexts. ^{[1] [10]}

Who should be cautious?

• Adolescents: NIH ODS states that HMB’s safety profile (and efficacy) has not been studied in adolescents. ^[1]
• People who need to limit calcium intake: HMB-Ca contains about 13% calcium; 3 g per day adds about 400 mg of calcium. The free acid form contains no calcium. ^[1]
• Competitive athletes: NIH ODS states that products marketed for bodybuilding can be adulterated with undeclared steroids and prescription drugs, which can lead to doping sanctions. ^[1]
• Ages 4–10: The Restricted Substances List has not entered a value for this age group. ^[10]

Evidence table: claim, population, and limits

This guide does not recommend personal doses or treatment; in situations such as chronic disease, regular medication use, and adolescence, the decision depends on individual clinical assessment.