A [ Branched-chain amino acid, BCAA ] An amino acid having aliphatic side-chains with a branch. The three proteinogenic branched-chained amino acids are 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... More. For humans, [ Valine ] A branched-chain amino acid. For humans, valine is one of the nine essential amino acids.... More is one of the nine [ Essential Amino Acid ] An amino acid considered and essential nutrient. That is, that an organism cannot endogenously synthesise and, therefore, must be acquired from dietary sources. For humans, there are nine amino acids that are considered essential: Histidine (H) Isoleucine (I) Leucine (L) Lysine (K) Methionine (M) Phenylalanine (F) Threonine (T) Tryptophan (W) Valine (V) See: Non-Essential Amino... More.