Vitamin C and glutathione are the two primary water soluble antioxidants in cells. Vitamin C must be obtained from the diets of humans because they lack the gulonolactone oxidase enzyme necessary for its synthesis. As well as being an important antioxidant that protects cellular components from free radicals, vitamin C is also required for collagen synthesis and deficiency results in scurvy and ultimately death. The major sources of vitamin C in the diet are fruits and vegetables, although dietary supplements are becoming increasingly popular. In contrast, glutathione is synthesised intracellularly in humans, where it plays an important antioxidant role in reducing thiol groups. In addition, glutathione is required by the enzyme glutathione peroxidase for the elimination of hydrogen peroxide and organic peroxides. Glutathione also recycles vitamins C from its oxidised dehydroascorbate form, demonstrating an important link between the two compounds.
The relationship between vitamin C and glutathione was investigated in research published in the American Journal of Clinical Nutrition in 20001. In the study, 240 healthy volunteers had their levels of vitamin C and glutathione recorded. The results showed that there was a positive correlation between vitamin C and glutathione in lymphocytes, such that those individuals with high levels of vitamin C also had high levels of glutathione. The concentration of glutathione was inversely correlated with age (as was vitamin C), with glutathione decreasing around 10 to 20 % in elderly subjects compared to the young. Interestingly, the levels of glutathione and vitamin C were 25 and 38 % higher in the summer than during the winter. However, no correlation was observed between vitamin C and glutathione with vitamin E (α-tocopherol), an important fat soluble antioxidant.
Oxidative stress and associated inflammation in thought to be a major mechanism by which degenerative disease is propagated in the body. High plasma concentrations of antioxidants are thought to provide systemic protection from the damage associated with free radicals. This is one of the mechanisms by which an antioxidant rich plant diet is thought to prevent disease. Increasing dietary levels of vitamin C with supplements or through foods is known to increase intracellular levels of glutathione. This glutathione and vitamin C synergy is important because high intracellular levels of glutathione are associated with health, but supplemental glutathione is not possible because the molecule does not survive the digestive process. Supplemental vitamin C is therefore an indirect way of increasing glutathione levels. Increased fruit consumption in the summer months likely explains the seasonal variation in glutathione and vitamin C levels.
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