The effects of vitamin C supplementation on disease states has been studied extensively but still remain controversial. Vitamin C alters the immune system, specifically lymphocyte proliferation and natural killer cell activities. The vitamin inhibits and inactivates viruses, but no clinical efficacy has been proven. Vitamin C supplementation trials have not shown that it reduces the incidence of colds, but some studies have seen a decrease in the duration and severity of colds with supplementation.
Vitamin C blocks carcinogenic processes through antioxidant activity. Epidemiological studies have shown a strong association of vitamin C supplementation with decreased risk of cancer of the oral cavity, esophagus, stomach, and pancreas. Less strong associations were found with lung, cervix, rectum, and breast cancer.
Epidemiological studies have shown mixed results of the efficacy of vitamin C for heart disease. It has been suggested that vitamin C may decrease risk of heart disease due to the inhibition of plasma LDL oxidation and vasodilatory and anticlotting activity.