Beta-hydroxy-beta-methylbutyrate (HMB) is a metabolite of the amino acid leucine. It is often used as a body building supplement to help improve muscle strength by reducing protein breakdown. HMB is also added to some medical foods for patients with muscle wasting and cachexia.

Small studies were initially conducted in active and younger men with mixed results on body composition and physical performance ^{(1) (2) (3) (4) (5) (6)}. A systematic review suggests that HMB may prevent exercise-related muscle damage in healthy trained and untrained individuals, as well as muscle loss during chronic disease ⁽⁷⁾.

However, research on HMB in clinical populations is limited. Some studies in the elderly suggest HMB benefits vascular endothelial function ⁽⁸⁾, reduces muscle breakdown ⁽⁹⁾, decreases abdominal fat mass ⁽¹⁰⁾, improves functional recovery after hip fracture ⁽¹¹⁾, and reduces mortality ⁽¹²⁾. Other small studies suggest HMB may improve pulmonary function in COPD patients ⁽¹³⁾ and nitrogen balance in critically injured patients ⁽¹⁴⁾, but it did not reduce comorbidities in hemodialysis patients ⁽¹⁵⁾.

Very few studies have been conducted in cancer populations. One small study suggests an HMB-containing supplement may help prevent radiation dermatitis in head and neck cancer patients ⁽²⁰⁾. HMB is often used together with glutamine and arginine for muscle wasting and cachexia ^(16) (17), but a larger randomized trial failed to demonstrate a significant effect ⁽¹⁸⁾. Meta-analyses suggest HMB may help increase muscle and strength in various clinical conditions, but the effect size was small ⁽³¹⁾, and little to no impact was seen when HMB supplementation was added to exercise in the elderly ⁽³²⁾. The upcoming HEAL study may help elucidate any true effects of HMB in frail older adults ⁽³³⁾.