β-Hydroxy β-methylbutyrate (HMB) is a metabolite of the amino acid leucine and is commonly marketed as a dietary supplement for athletes and individuals engaged in resistance or endurance exercise. Scientific research over the past two decades has examined HMB's potential to enhance muscle mass, strength, and recovery. Multiple randomized controlled trials and meta-analyses indicate that HMB supplementation can modestly reduce muscle protein breakdown and promote muscle mass gains, especially in untrained individuals beginning resistance training or in older adults. There is also evidence supporting its ability to reduce exercise-induced muscle damage and improve recovery, with some studies noting decreases in muscle soreness and markers of muscle injury (e.g., creatine kinase levels).

The proposed mechanisms of action include the promotion of muscle protein synthesis via the mTOR pathway, as well as reducing muscle protein breakdown by inhibiting the ubiquitin-proteasome pathway. While the effects in well-trained athletes are less pronounced, HMB is generally recognized as safe and is not prohibited by major sports organizations. A 2018 meta-analysis in the Journal of Cachexia, Sarcopenia and Muscle concluded that HMB has a small but significant positive effect on muscle strength and mass, particularly during periods of muscle wasting or intensive training. However, not all studies show clear benefits, and the magnitude of effect is often modest. Thus, while the evidence is not unanimous, it is sufficient to scientifically justify the use of HMB as an athletic and exercise aid.