Cysteine is a nonessential sulfur-containing amino acid. Sufficient amounts of cysteine are provided to our bodies from either the essential amino acid methionine, or the nonessential amino acid serine via transsulfuration reactions. Being an unstable organic compound, cysteine may also be derived from the amino cystine in times of increased physiological need. Cysteine does not need to be obtained from diet because of its relative abundance and distribution throughout the body.
Cysteine is present in nearly all sources of protein, with minute amounts also existing in body fluids and plants. The highest concentrations are found in alpha-keratin.  Alpha-keratin is a fibrous protein located throughout the body in various structural tissues. It is a principal constituent of digestive enzymes, fingernails, skin and hair.
Cysteine may slo be used for energy expenditure in humans. In times of elongated physical activities, cysteine is often converted into glucose. It is also of great importance in the production of taurine, another amino acid.
N-acetyl cysteine (NAC) is the most popular modified form of cysteine to be included for use in dietary supplements. In conjunction with L-cysteine, NAC has been recognized in the metabolization of various biochemicals in the body. These include; biotin, coenzyme A, heparin, lipoid acid, and perhaps of greatest importance, the antioxidant glutathione.
Cysteine may be considered conditionally essential in preterm infants. Healthcare providers often recommend a complete amino acid supplement, containing adequate amounts of cysteine, to be administered to these newborns.
A food table for cysteine has been omitted due to its abundance in nearly all protein sources. It is also readily converted from other organic compounds in times of inadequate diet and/or stress. Cysteine is found in a variety of protein sources including; ricotta, cottage cheese, yogurt, pork, meats, poultry, wheat germ, granola, and oat flakes. The average diet consumes approximately 1 gram of L-cysteine per day.
N-acetyl-L-cysteine (NAC), a modified form of cysteine, has been used in a multitude of clinical studies. Cysteine shows vast potential in various therapeutic treatments for individuals suffering from specific diseases and conditions. The most important of these ailments may be heart disease. Research suggests that supplemental NAC, in combination with nitroglycerin, may assist in alleviating symptoms associated with heart and coronary artery diseases. [2, 3] The complications of heart disease such as chest pain and heart attack, have shown dramatic improvement with added NAC supplementation. In addition, the combination of both NAC and nitroglycerin shows greater potential than when taken alone.
Respiratory illness may also be dramatically improved by supplementing N-acetyl-L-cysteine. NAC may improve acute and chronic lung conditions by dissolving the problematic mucus that is coupled with these disorders. Examples include; chronic bronchitis, asthma, cystic fibrosis, and emphysema. Because of its antioxidant properties, supplemental NAC may also be used as a preventative measure against lung cancer. [4, 5]
Although not conclusive in humans, NAC may inhibit the progression of ARDS, or acute respiratory distress syndrome, by boosting glutathione levels in the body.  It is theorized that this condition is a direct result of free radical damage upon the cells of lung tissue. It is often times characterized as a hurried and progressive failure of the lungs.
Although it does not replace the importance of conventional medicine, the supplementation of cysteine in persons suffering from HIV/AIDS has also shown great promise. One particular study supplemented 2400 milligrams of N-acety-L-cysteine with other vitamins, minerals, and amino acids. After a twelve week period, patients following the supplement regimen gained significantly more weight than their counterparts receiving placebo. This suggest that NAC may be an effective addition to the standard medical care received by HIV/AIDS patients. 
Acetaminophen (i.e. paracetamol) poisoning is often treated with NAC. [8, 9] N-acety-L-cysteine is often used to reduce the damage that is associated with an overdose in the liver and kidneys. Alcoholics may be at an increased risk, as alcohol dramatically enhances the liver and kidney’s susceptibility to damage.
More research is required, but NAC supplementation may also contribute to the improvement of symptoms associated with auto immune disorders; particularly Sjogren’s syndrome.  It may also assist in the cognitive functioning of persons suffering from Alzheimer’s disease, and aid in the prevention of cataracts and macular degeneration. 
Because cysteine is considered a nonessential amino acid, a Recommended Daily Allowance has not been established. However, the U.S. National Academy of Sciences recommends that healthy people achieve .36 grams of highly bioavailable protein for each pound of bodyweight - equaling 0.8 grams of protein, per kilogram of bodyweight.
Common dosages of NAC used to treat symptoms of specific health conditions:
|Respiratory Illness||200 milligrams, twice per day|
|Antioxidant properties||500 milligrams per day|
|General Health||500 milligrams per day|
|HIV/AIDS||Varying dosages: as high as 4,000 milligrams per day in some adults|
There has been no consistent data showing toxicity in humans. Diabetics should not use supplemental cysteine because this amino acid harbors the ability to inactivate insulin.
Studies in animals have shown that excessive and prolonged dosages have resulted in oxidative damage, massive damage to cortical neurons, permanent retinal dystrophy, hyperactivity, and brain atrophy. [15, 16]
Inadequate intakes of cysteine are extremely rare due to its nonessential nature and accessibility in all proteins. However, persons suffering from HIV/AIDS are especially at risk for the development of a deficiency. These persons usually exhibit dangerously low levels of cysteine and glutathione. [13, 14] Cysteine remains a fundamental component of the immune system. A deficiency of cysteine may coincide with the immune suppression accompanying HIV/AIDS, and may ultimately disrupt immunological integrity.
Lower levels of cysteine may also be attributed to cervical dysplasia in women. Cervical dysplasia is characterized by changes in the uterus marked by abnormalities in the tissue structure. Severity ranges from mild to severe, and can be cancerous.
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