The amino acid composition of a protein can tell you a lot about it’s health implications. For example a protein source high in sulfur amino acids (methionine and cysteine) can affect methionine metabolism and, ultimately, health. Cysteine is a conditionally essential amino acid, because it can be synthesized endogenously from methionine. Thus, when a dietary protein provides enough methionine beyond the need for cellular protein synthesis, the remaining surplus can be used to synthesize cysteine. Therefore, when a dietary protein has a cysteine concentration in balance with methionine, the need to utilize methionine for its synthesis is reduced. This is often referred to as the methionine-sparing effect of cysteine.