The role of leucine and its metabolites in protein and energy metabolism.

Leucine (Leu) is a nutritionally essential branched-chain amino acid (BCAA) in animal nutrition. It is usually one of the most abundant amino acids in high-quality protein foods. Leu increases protein synthesis through activation of the mammalian target of rapamycin (mTOR) signaling pathway in skeletal muscle, adipose tissue and placental cells. Leu promotes energy metabolism (glucose uptake, mitochondrial biogenesis, and fatty acid oxidation) to provide energy for protein synthesis, while inhibiting protein degradation. Approximately 80 % of Leu is normally used for protein synthesis, while the remainder is converted to α-ketoisocaproate (α-KIC) and β-hydroxy-β-methylbutyrate (HMB) in skeletal muscle. Therefore, it has been hypothesized that some of the functions of Leu are modulated by its metabolites. Both α-KIC and HMB have recently received considerable attention as nutritional supplements used to increase protein synthesis, inhibit protein degradation, and regulate energy homeostasis in a variety of in vitro and in vivo models. Leu and its metabolites hold great promise to enhance the growth and health of animals (including humans, birds and fish).