Exercise is well known to cause generation of free radicals and subsequent damage to cellular components including DNA. If not countered by the natural antioxidant defense systems of the body, this damage can cause suboptimal adaptations to training. Greek researchers investigated the effects of cysteine supplementation on exercise-induced markers of muscle damage, antioxidant status, and DNA damage. Blood was taken before and after exercise before and after 30 days of supplementation with L-cysteine (500 mg/day). Results indicated that supplementation with L-cysteine resulted in significantly increased antioxidant status and a lower exercise-induced elevation in two markers of muscle damage as well as a marker of DNA damage. The findings provide evidence for using L-cysteine to boost the body's antioxidant defense system and reduce the damage caused by free radicals, including disruption of muscle membrane and DNA.

Tsakiris S, Parthimos T, Parthimos N, Tsakiris T, Schulpis KH. The beneficial effect of l-cysteine supplementation on DNA oxidation induced by forced training. Pharmacol Res. 2006 Apr;53(4):386-90. Epub 2006 Mar 3.