When protein is in food the food, your digestive system breaks it down into amino acids, and your body uses these to manufacture hundreds of new proteins needed by all your cells. Two amino acids, called cysteine and methionine, are unusual because they contain the element sulfur, which plays an important role in determining the shape of new proteins by helping these compounds fold. Many common foods are good sources of both these amino acids.
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Meat and Poultry
Animal-based foods are generally good sources of both methionine and cysteine. For example, a serving of lean beef loin that weighs about 6 ounces provides 1,270 milligrams of methionine and 470 milligrams of cysteine, while a similar-sized serving of a pork chop that's trimmed of fat contains about 1,145 milligrams of methionine and 460 milligrams of cysteine. Other meats such as lamb and game meats are also good sources. Poultry is also rich in both amino acids, with 1 cup of roasted and chopped chicken breast meat providing about 1,200 milligrams of methionine and 550 milligrams of cysteine.
Fish and Seafood
Most fish and certain types of seafood are also good sources of methionine and cysteine. Examples include tuna, with about 1,250 milligrams of methionine and 450 milligrams of cysteine in 1 cup of light tuna meat, and salmon, which provides about 1,030 milligrams of methionine and 370 milligrams of cysteine in a 5-ounce serving. Other common types of fish, such as herring, trout, haddock and bluefish, are also good sources of both amino acids. Seafood that's rich in both compounds includes Alaskan king crab, with meat from 1 leg containing about 730 milligrams of methionine and 290 milligrams of cysteine, lobster, with 1 cup of meat providing 690 milligrams of methionine and 300 milligrams of cysteine, and shrimp, which contains 565 milligrams and 230 milligrams of methionine and cysteine, respectively, in a 3-ounce serving.
Eggs and Dairy Products
Dairy products are also good sources of both amino acids. These include different types of cheese; Swiss provides 1,035 milligrams and 380 milligrams of methionine and cysteine, respectively, in 1 cup of diced cheese, while 1 cup of provolone contains 900 milligrams of methionine and 150 milligrams of cysteine. Other cheeses, such as cheddar, Parmesan and mozzarella, are also rich in both compounds. Milk provides both amino acids in slightly lesser amounts. For example, 1 cup of nonfat milk provides about 200 milligrams of methionine and 50 milligrams of cysteine. Eggs are also a good source, with about 190 milligrams of methionine and 135 milligrams of cysteine in 1 large cooked egg.
If you follow a vegetarian diet, some types of plant-based foods are also good sources of methionine and cysteine. Many types of nuts provide these nutrients, including Brazil nuts, which have about 65 milligrams of methionine and 245 milligrams of cysteine in 1/2 cup. Other examples include spirulina, a seaweed-based food, with about 650 milligrams of methionine and 370 milligrams of cysteine in 1/2 cup, and soybeans, with 1,000 milligrams of methionine and 1,200 milligrams of cysteine in a 1-cup serving.