WHAT IS HMB POWDER? HMB is an abbreviation of Beta-Hydroxy Beta-Methylbutyrate – it’s easy to see why it gets abbreviated to HMB! HMB is an active metabolite of the Branched Chain Amino Acid, Leucine. HMB has been sold since the 1990s & has gained in popularity ever since. HMB is a popular addition to ‘all in one’ formulas, typically taken post-training. Despite being similar, HMB & Leucine are metabolised differently. HMB is produced by the liver enzyme Ketoisocaproate.