Cysteine is a nonessential sulfur-containing amino acid. Sufficient amounts of cysteine are provided to our bodies from either the dietary essential amino acid methionine, or the nonessential amino acid serine via a transsulfuration reaction. Being an unstable organic compound, cysteine may also be derived from the amino cystine in times of need. Cysteine does not need to be obtained from dietary sources due to its relatively high bioavailability.
Cysteine is present in nearly all sources of protein, with minute amounts 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.
Because of its abundance, cysteine may be used for energy expenditure in humans. In times of elongated physical activities, cysteine is often converted into glucose. It is also of equal importance in the production of taurine; an amino acid that provides for a host of specialized processes within the human body.
N-acetyl cysteine (NAC) is the most popular modified form of cysteine to be used in dietary supplementation. In conjunction with L-cysteine, NAC has been recognized in the metabolization of various biochemicals in the body. These biochemicals include; biotin, coenzyme A, heparin, lipoid acid, and perhaps of greatest importance is the antioxidant glutathione.
Cysteine may be considered conditionally essential in preterm infants. Healthcare providers often recommend a complete amino acid supplement, containing cysteine, to be administered to this population of 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 foods with high protein content, 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.
It is also recommended that adequate levels of selenium, Vitamin E, and B-vitamins compounds be consumed, as they provide for cysteine synthesis within the body and aid in the enhancement of many physiological activities.
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 specific diseases and conditions. The most important of these ailments may lie within the realms of heart-related 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 typically seen in heart disease patients, such as chest pain, and heart attack, have shown dramatic improvement with this added supplementation. The combination of both NAC and nitroglycerin shows greater potential than when taken alone.
Respiratory illness may also be dramatically improved by supplemental 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.
Supplementation with cysteine in persons suffering from HIV/AIDS has also shown great promise. Although it does not replace the importance of conventional medicines, one particular study supplemented 2400 milligrams of N-acetylcysteine 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. NAC may be an effective addition to the standard medical care received by HIV/AIDS patients. 
Acetaminophen (paracetamol) poisoning is often treated with NAC. [8, 9] N-acetylcysteine is often administered to reduce the damage that is associated with an overdose to 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 may assist 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. Listed below are the Recommended Daily Allowances (RDAs) for the majority of amino acids. Cysteine is a sulfur-containing (S-) amino acid and its recommended daily dosage is listed below:
|Requirement - mg. per kg. of body weight|
|Amino acid||Infant 3 - 6 mo.||Child 10 - 12 yr.||Adults|
|Histidine||33||not known||not known|
|S-containing amino acids||45||22||10|
|Aromatic amino acids||132||22||16|
In addition to the Recommended Daily Allowance, Recommended adult dosages of NAC also exist 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 protein sources. However, immunocompromised patients (i.e. 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 result directly from the immune suppression that accompanies HIV/AIDS, and may also disrupt one’s 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.
1. Balch, Phyllis A., James F. “Amino Acids.” Prescription for Nutritional Healing. Ed. Amy C. Tecklenberg. New York, NY: Penguin Putnam Inc., 3rd Ed. 2000. 42-53.
2. Pizzuli, L, Hagendorff A, Zirbes M, Jung W, Luderitz B. N-acetylcysteine attenuates nitroglycerin tolerance in patients with angina pectoris and normal left ventricular function. Am J Cardiol. 1997;79:28-33.
3. Marchetti G, Lodola E, Licciardello L, Colombo A. Use of N-acetylcysteine in the management of coronary artery diseases. Cardiologia. Jul 1999; 44(7): 633-637.
4. Jackson IM, et al. Efficacy and tolerability of oral acetylcysteine (Fabrol) in chronic bronchitis: a double-blind placebo controlled study. J Int Med Res. 1984;12(3):198-206.
5. Van Zandwijk N. N-acetylcysteine for lung cancer prevention. Chest. 1995;107(5):1437-1441.
6. Domenighetti G, Suter PM, Schaller MD, Ritz R, Perret C. Treatment with N-acetylcysteine during acute respiratory distress syndrome: a randomized double-blind, placebo-controlled study. J Crit Care. 1997:12(4):177-182.
7. Muller F, Svardal AM, Nordoy I, Berge RK, Aukrust P, Froland SS. Virological and immunological effects of antioxidant treatment in patients with HIV infection. Eur J Clin Invest. 2000;30(10): 905-914.
8. Perry HE, Shannon MW. Efficacy of oral versus intravenous N-acetylcysteine in acetaminophen overdose; results of an open label, clinical trial. J Pediatr. Jan 1998;132(1): 149-152.
9. Smilkstein MJ, Knapp GL, Kulig KW, Rumack BH. Efficacy of oral N-acetylcysteine in the treatment of acetaminophen overdose. Analysis of the national multicenter study (1976 to 1985). N Engl J Med. Dec 15 1988;319(24): 1557-1562.
10. Walters MT, et al. A double-blind, cross-over, study of oral N-acetylcysteine in Sjogren’s syndrome. Scand J Rheumatol Suppl. 1986; 61: 253-258.
11. Adair JC, Knoefel JE, Morgan N. Controlled trial of N-actylcysteine for patients with probable Alzheimer’s disease. Neurology. 2001;57(8): 1515-1517.
12. Zest for life information page. “RDA of amino acids.” (1999-2003) http://www.pdrhealth.com/drug_info/nmdrugprofiles/herbaldrugs/101840.shtml (14 Sept. 2004).
13. Eck HP, Gander H, Hartmann M, et al. Low concentrations of acid-soluble thiol (cysteine) in the blood plasma of HIV-1 infected patients. Biol Chem Hoppe Seyler. 1989;370:101-8.
14. Droge W. Cysteine and glutathione deficiency in AIDS patients: a rationale for the treatment with N-acetyl-L-cysteine. Pharmacology. 1993;46:61-5 [review].
15. Kleinveld HA, Demacker PNM, Stalenhoef AFH. Failure of N-acetylcysteine to reduce low-density lipoprotein oxidizability in healthy subjects. Eur J Clin Pharmacol. 1992;639-42.
16. Olney JW, Ho OL. Brain damage in infant mice following oral intake of glutamate, aspartate or cysteine. Nature 1970; 227: 609-10 [letter].