Methionine is one of the essential amino acids, and as such is not synthesized by the human body. Methionine is one of only two amino acids that contains a sulfur group, important for methionine's role in the pathway of glutathione "charging". Methionine acts as a methyl donor via SAM (S-adenylmethionine), one of the principle sources of methyl groups in the body. Listed as a lipotropic agent in the Merck Index, methionine is able to decrease the deposition and increase the movement of fat from the liver. Long term use of methionine has been used to treat a variety of chronic liver conditions.

Methionine is found in animal products, and is important to control fat levels in the liver and the arteries. In the absence of high animal product intake, there is a danger of not getting enough methionine. This is the most common dietary deficiency seen in vegetarians.

Plant foods contain methionine as well: beans, seeds, onions, peanuts, lentils, and some grains. Methionine prevents fat build-up by assisting in the breakdown of fats; it detoxifies the digestive system; it is a powerful antioxidant; it reduces histamine levels; helps women excrete estrogen; helps to detoxify the liver.