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Flow and functions of BCAAs in the body.The concentrations of free BCAAs in the amino acid (AA) pool are regulated by the balance of supply and catabolism of the BCAAs (bold arrows). Branched-chain α-keto acid dehydrogenase complex (BCKDC) is subject to regulation of BCAA catabolism (oxidation). BCAAs function as not only building blocks of the body protein but also regulators of protein, glucose and energy metabolism, and brain function.Branched-chain amino acids (BCAAs: leucine, isoleucine, and valine) are essential amino acids for humans and play an important role as the building blocks of proteins. Recent studies have disclosed that free BCAAs in the tissue amino acid pool function not only as substrates for protein synthesis, but also as regulators of protein and energy metabolism. Furthermore, BCAAs are actively used as an amino group donor to synthesize glutamate in the brain. These functions of BCAAs are closely related to human health. This review summarizes the recent findings concerning physiological and pathological roles of free BCAAs in the metabolism and neurological functions.