Benefits
Lean mass during 10 days of bed rest in older adults
In a randomized, double-blind trial in healthy older adults, the placebo group lost about 2 kg of lean mass during 10 days of complete bed rest, while the group taking 3 g a day of HMB kept theirs (with one person excluded). Only 19 people were evaluable, physical function did not differ, and the authors called for a larger trial.
Physical function with resistance training in older adults
A meta-analysis of 10 randomized trials (596 older adults) found that adding HMB to resistance training gave a moderate improvement in a standard physical performance test and a borderline gain in handgrip strength. Gait speed, lean mass, muscle quality and fat mass did not change, and half of the trials had a high risk of bias.
Lower blood markers of exercise muscle damage
A meta-analysis of randomized trials in healthy adults found HMB lowered creatine kinase (10 studies, 324 people) and lactate dehydrogenase (8 studies, 229 people) after muscle-damaging exercise, most clearly in studies lasting 6 weeks or longer. These are blood markers, not direct measures of soreness, strength or recovery time.
Endurance performance and VO2max
A meta-analysis of 11 placebo-controlled studies (279 healthy people) found 3 g a day of HMB for 2 to 12 weeks improved endurance performance and VO2max. The effect was smaller, but still significant, after removing studies that skewed the funnel plot. A separate meta-analysis of 6 trials in trained and competitive athletes found no effect on strength or body composition.
Mechanism of action
mTORC1 activation and protein synthesis stimulation
In cell and animal research, HMB activates mTORC1 signaling, the main switch for muscle protein synthesis, partly through the PI3K/Akt pathway, which leads to phosphorylation of S6K1 and 4E-BP1 and the start of protein synthesis. How much this matters in people was not measured in the studies cited here.
Ubiquitin-proteasome pathway inhibition
In laboratory and animal studies, HMB lowers the activity of ubiquitin ligases (MuRF1 and atrogin-1) that tag muscle proteins for breakdown. This is the proposed reason HMB helps most when muscle is being lost, such as during bed rest, but the studies cited here did not measure this pathway in people.
Cholesterol synthesis via the mevalonate pathway (minor route)
A small fraction of HMB is converted to HMG-CoA and can enter the mevalonate pathway toward cholesterol synthesis. This has been proposed as a way HMB might supply cholesterol for repairing muscle-cell membranes after exercise, but it is a minor pathway and has not been shown to drive any effect in humans.
Clinical trials
Randomized, double-blind, placebo-controlled trial of HMB as the calcium salt (1.5 g twice daily, 3 g/day) versus placebo, started 5 days before 10 days of complete bed rest and continued through 8 weeks of resistance-training rehabilitation; body composition by DXA. (Deutz et al. 2013, Clin Nutr)
24 healthy older adults (20 women, 4 men) enrolled; 19 evaluable after bed rest (8 placebo, 11 HMB).
The placebo group lost 2.05 kg of lean body mass during bed rest; with one subject excluded, the HMB group changed by -0.17 kg, a significant difference between groups (p=0.02). Physical function did not differ, probably because the study was small, and the authors said the results need confirming in a larger trial.
Systematic review and meta-analysis of randomized controlled trials in healthy participants measuring creatine kinase and lactate dehydrogenase after muscle-damaging exercise. (Rahimi et al. 2018, J Am Coll Nutr)
324 participants for creatine kinase (10 studies) and 229 for lactate dehydrogenase (8 studies).
HMB lowered creatine kinase by about 60.7 U/L and lactate dehydrogenase by about 15.4 U/L, with low heterogeneity, and the effect was significant in studies lasting 6 weeks or longer. Only blood markers were pooled, not soreness, strength or performance.
Systematic review and meta-analysis of randomized controlled trials comparing resistance training plus HMB with resistance training alone. (García-Alonso et al. 2025, Nutrients)
596 older adults across 10 trials.
HMB gave a moderate improvement in Short Physical Performance Battery scores (SMD 0.54) and a borderline gain in handgrip strength (SMD 0.24, p=0.05). Gait speed, appendicular lean mass, muscle quality, fat mass and body weight did not change. Half of the trials had a high risk of bias.
Systematic review and meta-analysis of placebo-controlled human trials of HMB on endurance performance and VO2max in healthy people. (Fernández-Landa et al. 2024, J Strength Cond Res)
279 healthy participants across 11 studies.
HMB at 3 g/day for 2 to 12 weeks improved endurance performance (SMD 0.58) and VO2max (SMD 0.58). After removing studies that skewed the funnel plot, the effects were smaller but still significant (SMD 0.38 and 0.25).
Meta-analysis of randomized controlled trials of HMB on bench press, leg press, body mass, fat-free mass and fat mass in trained and competitive athletes. (Sanchez-Martinez et al. 2018, J Sci Med Sport)
193 athletes across 6 trials.
Null result. HMB had trivial, non-significant effects on every outcome (bench press, leg press, body mass, fat-free mass and fat mass), and this held across dose, duration and training level subgroups.