Researchers at Sherbrooke University in Quebec report that vitamin C supplementation is highly effective in increasing glutathione concentration in blood plasma and, more specifically, in lymphocytes (white blood cells). Glutathione is the body's most important internal antioxidant, i.e. its main defense against damage and disease caused by free radical reactions. Vitamin C is the main water-soluble dietary antioxidant. Many diseases have been associated with a vitamin C deficiency among them cardiovascular disease, cancer, cataracts, hypertension, diabetes, hepatitis, HIV, and cystic fibrosis. Their study involved 48 healthy men and women between the ages of 25 and 64 years. All participants had low initial levels of plasma ascorbate (vitamin C) averaging 19.5 micromol/L. The plasma level of ascorbate was found to correlate linearly with that of the lymphocyte level.

All participants were given 1 or 2 500 mg tablets of vitamin C for a 13-week period followed by a 13-week period on a placebo. At the end of the first 13-week period vitamin C levels in lymphocytes had increased by an average of 51%. This increase was accompanied by an 18% in lymphocyte glutathione level and was independent of whether the participants had supplemented with 500 mg or 1000 mg of vitamin C (ascorbic acid) per day. The levels of both vitamin C and glutathione returned to normal after 13 weeks of no supplementation. The researchers conclude that ascorbate (vitamin C) spares glutathione by "getting at the free radicals first" and secondly by converting spent (oxidized) glutathione back to the active (reduced) form.
Lenton, Kevin J., et al. Vitamin C augments lymphocyte glutathione in subjects with ascorbate deficiency. American Journal of Clinical Nutrition, Vol. 77, January 2003, pp. 189-95