Methionine is of concern mainly because it is the limiting, or the least abundant, amino acid (see the following section on Food, Protein, and Complementarity) in many foods, particularly low in most legumes, soybeans, and peanuts. Though it is higher in dairy foods, eggs, fish, and meats, it is still present in lower concentration in many of these foods than are the other essential amino acids. Those eating vegetarian diets can obtain a fairly good proportion of methionine in the protein content of many nuts and seeds, as well as corn, rice, and other grains, which are naturally lower in tryptophan and lysine.
Methionine is one of the sulfur-containing amino acids (cysteine and cystine are others) and is important for many bodily functions. Through its supply of sulfur, it helps prevent problems of the skin and nails. It acts as a lipotropic agent (others are inositol and choline) to prevent excess fat buildup in the liver and the body, is helpful in relieving or preventing fatigue, and may be useful in some cases of allergy because it reduces histamine release. It also may help lower an elevated serum copper level. Methionine works as an antioxidant (free radical deactivator) through conversion to L-cysteine to help neutralize toxins. However, L-cysteine is used more often than methionine as an antixoidant because it seems to be better tolerated and has a wider range of protection.