What does it do? Cysteine is a nonessential amino acid (protein building block), meaning that cysteine can
be made in the human body. Cysteine is one of the few amino acids that contains sulfur. This allows cysteine to bond in a special way and maintain
the structure of proteins in the body. Cysteine is a component of the antioxidant glutathione.
The body also uses cysteine to produce taurine, another
amino acid.
Cysteine can also be converted into glucose and used as a source of energy. Cysteine
strengthens the protective lining of the stomach and intestines, which may help prevent damage
caused by aspirin and similar drugs.1 In
addition, cysteine may play an important role in the communication between immune system cells.2 Cysteine is rarely used
as a dietary supplement. N-acetyl cysteine (NAC),
which contains cysteine, is more commonly used as a supplement.
Where is it found? The body can synthesize cysteine from methionine and other building blocks. Cysteine, the amino acid from which NAC is derived, is found in most
high-protein foods.
Who is likely to be deficient? According to several studies,
blood levels of cysteine and glutathione are low in
people infected with HIV.3 4
5 Cysteine 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.
How much is usually taken? Most people do not need to
supplement with cysteine. Almost nothing is known about appropriate supplemental levels, in
part because almost all clinical research has been done with N-acetyl cysteine and not cysteine itself.
Are there any side effects or interactions? 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.6 Extremely large amounts of cysteine, the amino acid NAC is derived from, may be toxic to nerve cells in
rats.7
Adequate amounts of methionine are needed in the diet,
as the precursor to cysteine, to prevent cysteine deficiency.
At the time of writing, there were no well-known drug interactions
with cysteine.
References:
1. Salim AS. Sulfhydryl-containing agents in the treatment of gastric
bleeding induced by nonsteroidal anti-inflammatory drugs. Can J Surg
1993;36:53-8.
2. Droge W, Eck HP, Gander H, Mihm S. Modulation of lymphocyte functions
and immune responses by cysteine and cysteine derivatives. Am J Med 1991;91(suppl
3C):140S-4S.
3. 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.
4. Droge W, Eck HP, Mihm S. HIV-induced cysteine deficiency and T-cell
dysfunction—a rationale for treatment with N-acetylcysteine. Immunol Today
1992;13:211–4.
5. Droge W. Cysteine and glutathione deficiency in AIDS patients: a
rationale for the treatment with N-acetyl-cysteine. Pharmacology 1993;46:61–5
[review].
6. 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.
7. Olney JW, Ho OL. Brain damage in infant mice following oral intake of
glutamate, aspartate or cysteine. Nature 1970;227:609–10 [letter].
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purposes only. It is based on scientific studies (human, animal, or in vitro),
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