L-methionine is a protein amino acid. It is classified as an essential amino acid for humans and therefore must be supplied in the diet. According to the Food and Agriculture Organization of the United Nations (FAO) and World Health Organization (WHO), recommended daily L-methionine intake is 13 mg per kg or about one gram daily for adults. Actual intake is higher. This is principally derived from dietary proteins. Rich sources of L-methionine include cheeses, eggs, fish, meat and poultry. L-methionine is also found in fruits and vegetables, but not as abundantly. Small amounts of free L-methionine occur in vegetables, vegetable juices and fermented foods.In addition to its role as a precursor in protein synthesis, L-methionine participates in a wide range of biochemical reactions, including the production of S-adenosylmethionine (SAM or SAMe), L-cysteine, glutathione, taurine and sulfate. SAM itself, as a methyl donor (see SAMe), is involved in the synthesis of creatine, epinephrine, melatonin and the polyamines spermine and spermidine, among several other substances.

L-methionine is also a glycogenic amino acid and may participate in the formation of D-glucose and glycogen. The ability of L-methionine to reduce the liver-toxic effects of such hepatotoxins as acetaminophen and methotrexate has led to the suggestion that methionine should be added to acetaminophen products. However, there is some recent research suggesting that elevated L-methionine intake may promote intestinal carcinogenesis. This is unclear. Further, one of the metabolites of L-methionine, L-homocysteine, has been implicated as a significant factor in coronary heart disease and other vascular diseases.

L-methionine is a sulfur-containing amino acid that is minimally soluble in water. Its molecular formula is C5H11NO2S, and its molecular weight is 149.21 daltons. L-methionine is also known as 2-amino-4-(methylthio)butyric acid, alpha-amino-gamma-methylmercaptobutyric acid, (S)-2-amino-4-(methylthio)butanoic acid and gamma-methylthio-alpha-aminobutyric acid. It is abbreviated as Met and its one-letter abbreviation is M. The terms L-methionine and methionine are used interchangeably. The D-stereoisomer, D-methionine, does not possess biological activity with regard to protein synthesis and the biochemical reactions mentioned above. However, D-methionine, as well as L-methionine, may possess antioxidant activity.

Methionine is an essential amino acid. Dietary source of sulfur and methyl groups. Important for growth, healthy nails and skin and the synthesis of taurine, cysteine, phosphatidylcholine (lecithin), bile, carnitine and endorphins. It is an antioxidant nutrient and lipotropic agent which promotes the physiological utilization of fat. This amino acid is a principle supplier of sulfur, which inactivates free radicals. Adequate methionine prevents disorders of the hair, skin and nails; helps lower cholesterol levels by increasing the liver's production of lecithin; reduces liver fat and protects the kidneys. Methionine is a natural chelating agent for heavy metals and helps detoxify the body of these metals. It regulates the formation of ammonia and creates ammonia-free urine which reduces bladder irritation. It also influences hair follicles and prevents brittle hair.