A branched-chain amino acid (BCAA) is an amino acid having aliphatic side-chains with a branch (a central carbon atom bound to three or more carbon atoms). Among the proteinogenic amino acids, there are three BCAAs: leucine, isoleucine and valine. Non-proteinogenic BCAAs include 2-aminoisobutyric acid.

The three proteinogenic BCAAs are among the nine essential amino acids for humans, accounting for 35% of the essential amino acids in muscle proteins and 40% of the preformed amino acids required by mammals.[2] Synthesis for BCAAs occurs in all locations of plants, within the plastids of the cell, as determined by presence of mRNAs which encode for enzymes in the metabolic pathway.

BCAAs fill several metabolic and physiologic roles. Metabolically, BCAAs promote protein synthesis and turnover, signaling pathways, and metabolism of glucose.[4][5] Oxidation of BCAAs may increase fatty acid oxidation and play a role in obesity. Physiologically, BCAAs take on roles in the immune system and in brain function. BCAAs are broken down effectively by dehydrogenase and decarboxylase enzymes expressed by immune cells, and are required for lymphocyte growth and proliferation and cytotoxic T lymphocyte activity.[4] Lastly, BCAAs share the same transport protein into the brain with aromatic amino acids (Trp, Tyr, and Phe). Once in the brain BCAAs may have a role in protein synthesis, synthesis of neurotransmitters, and production of energy.