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Cystine is a sulfur-containing amino acid that helps form healthy skin, hair, bones, and connective tissue. It is also required to make glutathione, one of the natural antioxidants that fight free-radical damage (glutathione detoxifies the liver). Each molecule of cystine is made from two molecules of cysteine, another sulfur-containing amino acid that, as its name suggests, is very similar to cystine; in fact, cystine is generally regarded as the more stable form of cysteine, although both amino acids can be converted into the other as needed.

Cystine works closely with glutathione to remove toxins from the liver. In fact, cysteine is often used in emergency rooms to treat acetaminophen overdoses, which are known to cause liver damage. It also protects the liver and brain against toxins absorbed from alcohol and cigarette use, and may be effective in preventing hangovers.

There’s a lot of cystine in alpha-keratin, the protein that makes up the fingernails, skin, and hair. For this reason cysteine supplements are sometimes marketed as anti-aging products that claim to stimulate the formation of collagen and improve skin elasticity. Cysteine supplementation may also help speed the healing of burns and wounds, and improve joint flexibility in those with rheumatoid arthritis.

Because it promotes the production of the antioxidant glutathione, cysteine supplementation may be useful to those suffering from conditions that have been linked to free-radical oxidative damage, such as hardening of the arteries and cancer. It may also help treat emphysema, tuberculosis, and bronchitis—cysteine has been shown to help break down mucus in the respiratory tract.

Cystine is a nonessential amino acid, which means the body manufactures its own supply of this substance. However, foods such as eggs, meat, dairy products, and whole grains are also good sources of cystine.

Cysteine is more easily absorbed by the body than cystine, so most supplements contain cysteine rather than cystine. In addition, too much cystine in the body can cause cystinosis, a rare disease that can cause cystine crystals to form in the body and produce bladder or kidney stones. This side effect has not been associated with cysteine; however, as previously mentioned, cysteine is unstable, and is often converted to cystine in the body. To avoid the conversion of cysteine to potentially harmful amounts of cystine, it is advised that you take vitamin C supplements or consume citrus fruits along with cysteine supplements. Cystine cannot be used by the body without adequate amounts of vitamin B6, vitamin B12, and folic acid, so you’ll want to make sure you get the right amount of these supplements as well. B vitamins are also needed to help the body get rid of homocysteine, a waste product that is formed when the body uses cysteine. Homocysteine has been linked to inflammation in the arteries that results in heart disease.

If you want to try cysteine supplements, try N-acetyl-cysteine (NAC), which is made naturally from cysteine, and is better absorbed than other cysteine or cystine supplements.

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