Methionine

Methionine is one of the essential amino acids (building blocks of protein), meaning that it cannot be produced by the body, and must be provided by the diet. It supplies sulfur and other compounds required by the body for normal metabolism and growth. Methionine also belongs to a group of compounds called lipotropics, or chemicals that help the liver process fats (lipids). Others in this group include choline, inositol, and betaine (trimethylglycine).

People with AIDS have low levels of methionine. Some researchers suggest this may explain some aspects of the disease process, especially the deterioration that occurs in the nervous system that can cause symptoms, including dementia. A preliminary study has suggested that methionine (6 grams per day) may improve memory recall in people with AIDS-related nervous system degeneration.

Other preliminary studies have suggested that methionine (5 grams per day) may help treat some symptoms of Parkinsonís disease. However, another form of methionine, S-adenosylmethionine, or SAMe, may worsen the symptoms of Parkinsonís disease and should be avoided until more is known.

Methionine (2 grams per day) in combination with several antioxidants, reduced pain and recurrences of attacks of pancreatitis in a small but well-controlled trial.

Most people consume plenty of methionine through a typical diet. Lower intakes during pregnancy have been associated with neural tube defects in newborns, but the significance of this is not yet clear.

Animal studies suggest that diets high in methionine, in the presence of B-vitamin deficiencies, may increase the risk for atherosclerosis (hardening of the arteries) by increasing blood levels of cholesterol and a compound called homocysteine. This idea has not yet been tested in humans. Excessive methionine intake, together with inadequate intake of folic acid, vitamin B6, and vitamin B12, can increase the conversion of methionine to homocysteineóa substance linked to heart disease and stroke. Even in the absence of a deficiency of folic acid, B6, or B12, megadoses of methionine (7 grams per day) have been found to cause elevations in blood levels of homocysteine. Whether such an increase would create a significant hazard for humans taking supplemental methionine has not been established. Supplementation of up to 2 grams of methionine daily for long periods of time has not been reported to cause any serious side effects.

Meat, fish, and dairy products are all good sources of methionine. Vegetarians can obtain methionine from whole grains, but beans are a relatively poor source of this amino acid.