Methionine is an essential amino acid, which means that your body must have it to function normally but cannot synthesize it on its own. Methionine is a sulfur-containing amino acid, closely related to cysteine, and both are building blocks of protein, as are most amino acids. Both methionine and cysteine play key roles in protecting and promoting the health of your connective tissue, joints, skin, hair and nails. They also have potent detoxification properties, helping your body to excrete heavy metals. Another benefit of these sulfur-containing amino acids is their ability to reduce the formation of ammonia in urine, thus protecting against bladder irritation.
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Of methionine’s many functions in the body, one of the most significant is its role in the methylation process. Some of the methionine that your body derives from dietary sources combines with adenosine triphosphate, an energy-producing compound found in every cell in the body, to form S-adenosyl-L-methionine, or SAMe. This latter compound is involved in a number of biochemical reactions in the body, including methylation, a process in which a single carbon unit, or methyl group, is added to another molecule.
Although SAMe is not the only methyl donor in the body, it is by far the most effective. Methylation is essential to the production of various body components, most notably brain chemicals, and it also plays a key role in the body’s detoxification processes. SAMe is also required for the body’s synthesis of sulfur-containing compounds, such as glutathione and chondroitin sulfate, a cartilage component.
Generally speaking, animal-based foods are the best source of methionine. The significantly higher levels of the amino acid in animal foods is the cause for some concern about the possibility of methionine deficiency among those who follow a vegan diet or otherwise sharply restrict their consumption of animal foods. Among animal-based foods, fish is particularly high in the amino acid. Skipjack tuna, chum salmon, bluefin tuna, yellowfin tuna, halibut and mackerel all contain plenty of methionine, offering 690 to 2,200 milligrams of the amino acid in every 100-gram serving. Other methionine-rich animal foods include eggs, milk, chicken, turkey and pork.
Tofu, a soy derivative, tops the list of plant-based foods that contain relatively high levels of methionine. For every 100 grams of raw edamame you consume, you’ll get 780 milligrams of methionine. Other plant foods rich in the amino acid include raw sweet corn, raw fava beans, spinach, broccoli, garlic, mustard greens, green peas, cauliflower, bamboo shoots, soybean sprouts, asparagus, butter lettuce and okra.
A Cautionary Note
As with many otherwise beneficial substances, too much methionine might be a bad thing, according to a study conducted by researchers at Temple University’s School of Medicine. Their animal study found that a diet that contains excessive amounts of methionine can elevate the body’s levels of homocysteine, a nonprotein amino acid. In earlier studies, excessive levels of homocysteine have been linked to an increased risk of Alzheimer’s disease. The recommended daily intake of methionine for adults ranges from 1.2 to 2.2 grams, depending on age and weight. Temple researchers published their findings in a 2010 issue of “Current Alzheimer Research.”