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 some time ago have shown that vitamin C supplementation can improve certain aspects of the immune response, such as the proliferative response of lymphocytes, the function of neutrophils, and the hypersensitivity response, although not all immunity parameters improve. It has been suggested that vitamin C and vitamin E supplements together may have a greater effect on the immune response than vitamin C alone, but more research on this topic is warranted. Although vitamin C supplementation has been shown to improve 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. Additionally, some recent evidence suggests that populations experiencing a lot of physical stress (running a marathon) may benefit from vitamin C supplementation.

Immune effects and exercise

The results of some studies suggest that supplementation with vitamin C may be beneficial in reducing the incidence or severity of upper respiratory infection after prolonged and intense exercise. Several epidemiological studies have indicated that the risk of developing an upper respiratory infection (URI) increases within a week or two after a prolonged exercise session such as running a marathon. However, runners who took a 600 mg vitamin C supplement for 21 days prior to competing in an ultramarathon reported fewer symptoms and a shorter duration of URI than runners who took a placebo. There was no difference in the incidence of URI symptoms before the race, suggesting that perhaps the benefits of vitamin C supplementation are more pronounced during periods of high physical stress. The same group of researchers examined whether different antioxidant preparations may provide additional benefits regarding URI reduction in the post-competition period. In this study, runners received one of the following supplements for 21 days prior to competition:

500 mg of vitamin C

500 mg of vitamin C

400 IU of vitamin E

300 mg of vitamin C

300 IU of vitamin E

18 mg beta carotene

placebo.

The results of this study again showed that vitamin C had a protective effect in terms of reducing the incidence of URI symptoms, but the addition of vitamin E or beta-carotene did not confer any additional benefit. Others have reported that consumption of vitamin C in the range of 600 mg-1 g per day was associated with a decrease in symptoms of the common cold compared to subjects who consumed placebo. However, others have reported that prophylactic administration of vitamin C (2 g / day) over an 8-week period in marine recruits was not associated with a lower incidence or duration of colds, although the vitamin C group rated their colds as less severe. At this time, the results of several studies suggest that vitamin C supplementation can reduce the incidence and / or severity of URI during times of high physical stress.

None of these studies explored changes in immune function that could potentially be associated with a lower incidence or severity of infection. A recent study examined the effects of 8-day vitamin C supplementation on a variety of immune responses prior to a 2.5-hour high-intensity run. No changes were observed in NK cell cytotoxicity, proliferative response of lymphocytes to mitogens, phagocytosis of granulocytes, or production of the cytokine IL-6. These findings may not appear consistent with previous studies that observed an effect of supplementation on the incidence of infection. However, a longer period of supplementation (21 days, instead of 8 days) might have been necessary. The immune measures study 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 measured immune parameters is not necessarily related to the outcome of the disease. Other factors can mediate the outcome of the disease.

For example, the findings of some recent studies suggest that oxidative stress from the vitamin deficiency diet changes the genome of a virus, resulting in increased virulence. Plasma vitamin C concentration has been shown to decrease after long-term endurance exercise, and this short-term decrease could be associated with increased susceptibility to infection. Although the findings from studies of vitamin C supplementation in those individuals who undergo intense physical exertion are promising, more research is needed to confirm these findings and explore the possible mechanisms that mediate the change in susceptibility to infection. Furthermore, to our knowledge, the role of vitamin C supplementation in preventing infections in athletes participating in heavy resistance training or bodybuilding has not been studied.