HMB

Last reviewed October 2026 · Based on 18 studies

Bottom line

HMB, a leucine byproduct, gives older adults a small gain in muscle mass and strength in some trials. The effects are modest and inconsistent, the evidence for walking and daily function is weak, and HMB added to resistance training shows little extra benefit.

What it helps with

Builds muscle mass in older adults — Benefit, evidence grade C (Limited). Several meta-analyses of mostly small trials; downgraded for inconsistency (CI crosses zero in one review), risk of bias, and muscle mass as a surrogate for function.
Preserves muscle strength — Mixed results, evidence grade C (Limited). Meta-analyses disagree: a small gain across clinical conditions, but no gain when added to resistance training; most trials at risk of bias.
Improves walking and daily physical performance — Mixed results, evidence grade C (Limited). Mixed results: a moderate SPPB gain when combined with resistance training, and an umbrella review finding no evidence of functional benefit.
Helps keep muscle during bed rest — Benefit, evidence grade C (Limited). One small RCT (19 older adults analysed); downgraded for small sample size and reliance on DXA lean mass as a surrogate.
Helps muscle recovery in critical illness — No effect, evidence grade C (Limited). Two RCTs with 378 people in total; no effect on primary outcomes. Downgraded for small total sample and open-label or single-blind design.
Helps muscle in people on dialysis — No effect, evidence grade C (Limited). One 6-month RCT of 33 people; no effect on body composition, strength or function. Downgraded for small sample.
Helps muscle in cirrhosis — No effect, evidence grade C (Limited). One pilot RCT of 21 people; no between-group difference in muscle mass or grip strength. Downgraded for small sample.
Adds to the benefits of resistance training — No effect, evidence grade B (Moderate). Meta-analysis of 13 RCTs found no added gain; downgraded once for high heterogeneity in strength results.
Can raise blood sugar — Harm, evidence grade C (Limited). Single RCT reported high blood sugar in 3 of 134 patients given HMB; trial was not designed to measure this harm.

What it is

HMB (beta-hydroxy-beta-methylbutyrate) is a metabolite of the amino acid leucine [1]. It is used as a free acid or as calcium HMB [2], and some nutrition shakes contain 3 g of HMB per serving [3].

How it might work

Researchers propose that HMB favourably affects muscle protein turnover, the balance of muscle protein being built and broken down [4]. This is a proposed mechanism, not a proven one.

What the research says

Meta-analyses of randomised trials in older adults found a small gain in muscle mass. Seven trials with 287 older adults found more muscle mass gain on HMB than on control [1]. Nine trials with 448 people found a rise in fat-free mass, with no change in fat mass [5]. An umbrella review from 2025 found a small gain in muscle mass and strength, with no change in fat mass or body weight [6].

Strength and walking results are less consistent. A 2019 review across several clinical conditions found a small strength gain [4]. A 2026 meta-analysis of 13 trials in adults aged 50 and over found that HMB added to resistance training did not improve muscle mass or strength [7]. A 2025 meta-analysis found a moderate improvement in a physical performance test when HMB was combined with resistance training, with no significant change in gait speed or appendicular lean mass [8]. An earlier umbrella review found no evidence that HMB improves physical function [9].

In a 10-day bed rest study, HMB prevented the drop in lean body mass seen in the placebo group, but only 19 people were analysed [10]. In critically ill adults, HMB did not improve the primary walking or physical performance outcomes in two trials [11][12].

Doses used in studies

Trials commonly used 3 g of HMB a day, for example 1.5 g twice daily in a 10-day bed rest study [10] and 3 g a day in an ICU trial [11]. A cirrhosis pilot used 1.48 g of calcium HMB a day [13]. Trial durations ranged from 10 days [10] to 12 months [14].

Safety and interactions

Side effects were rare in the trials. The 2022 cancer review reported no serious adverse effects directly related to the nutrition intervention [15]. In one ICU trial, high blood sugar occurred in 3 of 134 patients given HMB [12]. No upper intake limit or toxic dose was reported in the trials cited here, and no data were found on interactions with medicines, pregnancy or breastfeeding. A cirrhosis pilot reported no significant adverse events [13]. HMB did not show benefit in people on haemodialysis [16], so people with kidney disease should discuss it with their doctor first.

Caveats and open questions

Most trials are small and short, and many gave HMB combined with amino acids, vitamin D or protein, so HMB's own effect is hard to separate [15]. Several trials were at high risk of bias [8][4]. Exercise appears to matter more than HMB: the gains were largely from resistance training [2], and HMB added nothing to exercise in a 12-month trial [14]. Two registered trials in older adults are not yet reported: one tests HMB with vitamin D in people starting semaglutide (NCT07760948), and one compares HMB with a traditional Chinese formula in pre-frail elders (NCT07597850).

HMB