What is HMB? HMB, also known as Hydroxy Methylbutyrate or hydroxymethylbutyrate, is a metabolite of the amino acid Leucine and is produced naturally in the body in small amounts. Getting more HMB naturally isn’t always easy. In the body, less than five percent of all leucine is converted to HMB. Even on a good day, protein-packed foods can leave adults coming up short. As a result, athletes and bodybuilders have recognized this and use an HMB supplement to build and repair muscle as well as improve recovery from exercise.