Methionine is one of the sulphur amino acids. It cannot be synthesized by the human body, and must be obtained from the diet. It plays an important role as a precursor for the synthesis of other sulphur amino acids (cysteine and taurine), and donates sulphur and other compounds required for the synthesis of many other substances, such as choline, creatine, adrenaline and carnitine. Without adequate daily amounts of dietary methionine the body cannot produce sufficient adrenal and other hormones.
Methionine is a component of the body’s natural painkillers encephalin and endorphin, and has sometimes been used as a painkilling treatment. It is also essential in regulating the availability of folic acid. A methionine-deficient diet can cause folic acid deficiency as this B vitamin becomes trapped in the liver in an inactive form.
The Brain Bio Center in the USA uses methionine to treat one type of schizophrenia, classified as the ‘high histamine’ type, which is associated with severe depression and suicidal tendencies. L-methionine is thought to alleviate this condition by lowering blood histamine, and in some cases of depression has been described as more effective than conventional drugs. On the other hand excessive supplementation with methionine can aggravate psychotic symptoms in other types of schizophrenia where individuals suffer from folic acid deficiency. Researchers have also used methionine supplements in parkinsonism and claim to obtain results comparable to those of conventional treatments.
Along with vitamin B6, methionine is essential for the metabolism of homocysteine. If allowed to build up in the blood, homocysteine can encourage the build-up of cholesterol deposits in arteries.
In the brain, methionine is metabolized to s-adenosyl-methionine (SAM).
Vegetarian sources: Weight for weight, brazil nuts, sunflower seeds and sesame seeds are as rich in methionine as animal proteins. Vegans should take particular care to obtain enough methionine. Pulses (including many soya products) are a poor source of this amino acid.