Function of Vitamin C
- By:Robert Baird Baird
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. In this study runners received one of the following supplements for 21 days before competition-
500 mg vitamin C
500 mg vitamin C
400 IU vitamin E
300 mg vitamin C
300 IU vitamin E
18 mg beta-carotene
placebo.
The results from this study again showed that vitamin C had a protective effect in terms of reduced incidence of URI symptoms, but the addition of vitamin E or beta-carotene did not confer any additional benefits. Others have reported that vitamin C consumption in the range of 600 mg-1 g per day was associated with a decrease in symptoms of the common cold compared with subjects consuming placebo. However, others have reported that prophylactic administration of vitamin C (2 g/day) for an 8-week period in marine recruits was not associated with decreased incidence or duration of colds, although the vitamin C group rated their colds as being less severe. At this time, the results from several studies suggest that vitamin C supplementation may reduce the incidence and/or severity of URI during times of heavy physical stress. None of these studies explored changes in immune function that could potentially be associated with reduced incidence or severity of infection. One recent study examined the effects of vitamin C supplementation for 8 days on a range of immune responses before performing a 2.5-hour run at a high intensity. No change was observed in NK cell cytotoxicity, lymphocyte proliferative response to mitogens, granulocyte phagocytosis, or production of the cytokine IL-6 These findings may not appear to be consistent with the previous studies that observed an effect of supplementation on the incidence of infection. However, a longer period of supplementation could have been necessary (21 days, rather than 8 days). The study of immune measures did not assess antigen-specific immune responses and that the antigen-specific immune response could be altered by vitamin C supplementation, resulting in fewer infections. It is also important to note that a change or lack of change in immune parameters measured does not necessarily relate to disease outcome. Other factors may mediate disease outcome. For example, the findings from some recent studies suggest that dietary oxidative stress from vitamin deficiency changes the genome of a virus, resulting in increased virulence. Vitamin C concentration in plasma has been shown to decline following long-term endurance exercise and this short-term decrease could be associated with increased susceptibility to infection. Although the findings from the vitamin C supplementation studies in those individuals undergoing heavy physical exertion are promising, further research is needed to confirm these findings and to explore potential mechanisms mediating the change in susceptibility to infection. Also, to our knowledge the role of vitamin C supplementation in preventing infection in athletes participating in heavy resistance training or bodybuilding has not been studied.