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What Is Cysteine?

author image Michelle Kerns
Michelle Kerns writes for a variety of print and online publications and specializes in literature and science topics. She has served as a book columnist since 2008 and is a member of the National Book Critics Circle. Kerns studied English literature and neurology at UC Davis.
What Is Cysteine?
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All proteins are made up of the same basic compounds, known as amino acids. Cysteine, an amino acid that can be synthesized by the body or consumed in certain foods, is one. Cysteine has several physiological roles, though many people believe that taking additional cysteine as a dietary supplement can also benefit your health. There are, however, some instances in which taking cysteine can be dangerous. Cysteine supplementation is not known to prevent or treat any medical conditions and should be used only under the supervision of your physician.


Cysteine is an amino acid that enters the body in two ways: first, through cysteine-containing foods and second, through a metabolic pathway that converts the amino acid methionine to S-adenosyl methionine, on to homocysteine which then reacts with serine and forms cysteine.

The body's ability to produce cysteine can be affected if the diet does not contain sufficient amounts of folic acid, vitamin B6, methionine and vitamin B12. In the body, cysteine is an essential part of glutathione, an antioxidant compound, and is also used to produce the amino acid taurine as well as coenzyme A, biotin and heparin. Cysteine is a component in beta-keratin and is believed to preserve skin elasticity and to protect the lining of the digestive system.

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Cysteine is found naturally in a number of foods, including egg yolks, red peppers, garlic, onions, yogurt, grains such as wheat germ and oats, poultry and dark leafy vegetables like Brussels sprouts and broccoli. As a dietary supplement, cysteine is available both as L-cysteine hydrochloride and N-acetyl-cysteine, the latter of which is thought to be more soluble and better able to be metabolized by the body.


According to public health recommendations published by the National Academy of Science, any individual over 1 year of age should consume 25 mg of cysteine for each gram of protein consumed. According to eVitamins, however, not enough research has been conducted with supplemental cysteine to determine appropriate recommended levels. Two hundred mg of cysteine taken twice or three times a day is considered a safe amount.


The N-acetyl-cysteine form of cysteine is used in medicine as a way to increase the effectiveness of corticosteroid drugs. It can also be used to decrease the unpleasant symptoms of certain chemotherapy drugs, as a treatment for acetaminophen poisoning and as a way to prevent physiological tolerance to the chest pain medication nitroglycerin.

According to the World's Healthiest Foods website and Healthy.net, consuming cysteine both in foods and as a dietary supplement has alternative medicine benefits; it is thought to help protect against cellular damage by free radical compounds, eliminate metal ions and potentially harmful chemicals from body tissue and help treat or prevent respiratory problems such as asthma and excessive mucus buildup.


Taking large doses of cysteine -- particularly in its N-actetyl-cysteine form -- can cause allergic reactions in some people and digestive problems like diarrhea, vomiting or nausea in others. According to the World's Healthiest Foods, people who are unable to metabolize cysteine correctly are thought to be at higher risk of developing certain neurological disorders such as multiple sclerosis or Alzheimer's disease.

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