Methionine (L-methionine) CasNo.: 59-51-8 is a nutritional supplement as well as an essential amino acid found in food. Methionine is required for normal growth and repair of body tissues; it cannot be made by the body, but must be obtained from the diet; thus, it is considered an “essential” amino acid. There are two types of methionine—L-methionine (which is naturally-occurring) and D-methionine. Each contains the same chemical make-up, but the molecules are mirror images. A mixture of the two is called DL-methionine.