HMB is a metabolite of the amino acid leucine. About 5% of dietary leucine is converted to HMB endogenously.
Mechanism — Reduces muscle protein breakdown via the ubiquitin-proteasome pathway. Smaller, less consistent effect on muscle protein synthesis than free leucine itself. Most useful in catabolic contexts: aging, bedrest, prolonged immobilization.
Use case — Elderly preserving muscle, recovery from injury/bedrest, prolonged training phases with caloric deficit. Typical 3 g/day divided into three doses.
Caveats — Underwhelms in healthy young athletes already optimizing protein. The HMB-FA (free acid) form is faster acting than calcium HMB. Don't expect creatine-level returns.