During periods of increased volume and/or intensity of training, the body can easily become chronically inflamed and immuno-suppressed, thereby having negative implications on training adaptations.
(GSH) is one of the most important cellular antioxidant defense systems. Suboptimal cellular levels of glutathione contribute to excessive oxidative stress that can impair immune responses and recovery. Because glutathione is formed from three amino acids — cysteine, glutamate, and glycine — supplementing with these precursors may offer some protection from the stress of high level exercise.
Japanese researchers provided subjects with two amino acids,
(which readily converts to glutamate), during a period of high level resistance training. Subjects ingested cystine (700 mg/day) and theanine (280 mg/day) or a placebo during two weeks of resistance training that involved training daily during the second week. They focused on immune measures, specifically the natural killer cells which are a marker of innate immunity. After two weeks of training this marker of immune function decreased by more than 30 percent in the placebo group, whereas it was maintained in the cytine+theanine group. These results highlight the potential for providing increased amounts of cystine to protect against decreases in immune function associated with intensified training regimens. Whey is a naturally rich source of cysteine (3-4 times higher than soy), which may explain why it has been shown to enhance recovery from exercise.
Reference: Kawada S, Kobayashi K, Ohtani M, Fukusaki C. Cystine and theanine supplementation restores high-intensity resistance exercise-induced attenuation of natural killer cell activity in well-trained men. J Strength Cond Res. 2010 Mar;24(3):846-51.