Cystine is a nonessential amino acid (protein building block), meaning that cystine can be made in the human body. Cystine is one of the few amino acids that contains sulfur. This allows cystine to bond in a special way and maintain the structure of proteins in the body. Cystine is a component of the antioxidant, glutathione. The body also uses cystine to produce taurine, another amino acid.

Cystine can also be converted into glucose and used as a source of energy. Cystine strengthens the protective lining of the stomach and intestines, which may help prevent damage caused by aspirin and similar drugs. In addition, cystine may play an important role in the communication between immune system cells. Cystine is rarely used as a dietary supplement. N-acetyl cystine (NAC), which contains cystine, is more commonly used as a supplement.

Only the L form of amino acids are constituents of protein. Cystine is an important part of GTF (glucose tolerance factor). Cystine or cysteine is required for the proper utilization of vitamin B6. Cystine assists in the supply of insulin to the pancreas.

It functions as an antioxidant and is a powerful aid to the body in protecting against radiation and pollution. It can help slow down the aging process, deactivate free radicals, neutralize toxins. It aids in protein synthesis and presents cellular change. It is necessary for the formation of the skin, which aids in the recovery from burns and surgical operations. Hair and skin are made up 10-14% Cystine.

According to several studies, blood levels of cystine and glutathione are low in people infected with HIV. Cystine has a role in the proper function of the immune system, so a deficiency of this amino acid may either contribute to, or result from, immune suppression associated with HIV.

No consistent adverse effects of NAC have been reported in humans. One small study found that daily amounts of 1.2 grams or more could lead to oxidative damage. Extremely large amounts of cystine, the amino acid NAC is derived from, may be toxic to nerve cells in rats.

Adequate amounts of methionine are needed in the diet, as the precursor to cystine, to prevent cystine deficiency.

The body can synthesize cystine from methionine and other building blocks. Cystine, the amino acid from which NAC is derived, is found in most high-protein foods.