What Are BCAAs? Branched-chain amino acids (BCAAs) are a group of three essential amino acids: leucine, isoleucine and valine. They are the building blocks of protein, which helps your body build muscle, repair muscle damage, and regulate immune function, among other things. While there are 21 amino acids in total, nine are essential—essential, because your body can’t produce them, but you need them to live. BCAA supplements are commonly taken