HMB (β-Hydroxy β-Methylbutyrate) is an endogenous metabolite of leucine consumption, being derived from its parent amino acid through two steps of enzymatic conversion. In total, approximately 5% of leucine by weight of consumption is converted into HMB.
Once converted, HMB has been demonstrated to possess several interesting effects. In cellular assays, HMB was shown to increase the rate of skeletal protein synthesis through the same mechanism as its parent compound, leucine – the all-important mTOR-P13K pathway that regulates the majority of muscle protein synthesis in vivo. This mechanism has also translated into practical benefits to human muscle, as various trials have demonstrated HMB to slow the rate of skeletal muscle breakdown, increase power output over both acute (short) and chronic (long-term) trials, as well as mitigating muscle damage during high-intensity exercise.