Cysteine

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.

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.

According to several studies, blood levels of cysteine and glutathione are low in people infected with HIV.^{1, 2, 3} 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.

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

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.⁴ Extremely large amounts of cysteine, the amino acid NAC is derived from, may be toxic to nerve cells in rats.⁵