Although vitamin C has long been touted as a preventive measure against the common cold, the available evidence from several large-scale randomized controlled trials does not support this claim. Several studies conducted awhile ago have shown that vitamin C supplementation can enhance certain aspects of immune response such as lymphocyte proliferative response, neutrophil function, and hypersensitivity response, although not all parameters of immunity are enhanced It has been suggested that vitamin C and vitamin E supplementation together may have a greater effect on immune responsiveness than vitamin C alone, but further research on this topic is warranted. Although supplementation of vitamin C has been shown to enhance certain aspects of immune function, this does not appear to result in a reduced incidence of the common cold in humans or a reduced incidence of influenza in an animal model. However, a recent review of placebo-controlled trials suggests that vitamin C may decrease the duration or severity of the common cold in certain populations such as children Also, some recent evidence suggests that populations undergoing heavy physical stress (running a marathon) may benefit from vitamin C supplementation.Immune Effects and Exercise
The results from a few studies suggest that vitamin C supplementation may be beneficial in reducing the incidence or severity of upper respiratory infection following strenuous, prolonged exercise. Several epidemiological studies have indicated that the risk of developing an upper respiratory infection (URl) is increased in the week or two following a session of prolonged exercise such as running a marathon. However, runners who consumed a 600-mg supplement of vitamin C for 21 days before competing in an ultramarathon race reported fewer symptoms and a shorter duration of URI than the runners who consumed placebo. There was no difference in the incidence of symptoms of URI before the race, suggesting that perhaps the benefits of vitamin C supplementation are more pronounced during periods of heavy physical stress. The same group of investigators examined whether different antioxidans preparations may provide additional benefits in regards to reduced URI in the postcompetition period.
