Seasonal oscillation of influenza is prominent, its epidemic is explosive, and it ends abruptly. To explain this peculiar pattern, Cannell et al (1) hypothesized that the seasonal oscillation of serum vitamin D concentrations, which was recently discovered to up-regulate innate immunity, may affect the epidemic pattern of influenza. Vitamin D is mostly obtained from sun exposure; thus, serum vitamin D concentrations can be affected by season. In fact, serum concentrations of vitamin D have been shown to decrease in winter, the season when influenza occurs, to concentrations half those during the summer (1). In a post hoc analysis of side effect questions asked during a randomized controlled trial performed to determine whether vitamin D could prevent osteoporosis (2), cold and flu symptoms were reported 3 times less often in the vitamin D group than in the placebo group (3). However, although the authors conducted an additional randomized trial in 162 healthy adults, they could not reconfirm the benefit of vitamin D supplementation for the prevention of symptomatic upper respiratory tract infections (4). On the other hand, a significant inverse association between serum vitamin D intake and recent upper respiratory tract infections was seen in the third National Health and Nutrition Examination Survey (5). However, no rigorously designed clinical trials have evaluated the relation between vitamin D and physician-diagnosed influenza or delineated the necessary changes to prepare for an influenza pandemic (6). We conducted a randomized, double-blind, placebo-controlled trial comparing vitamin D3 supplements with placebo in schoolchildren to elucidate whether preventive intake of vitamin D supplements during winter and early spring seasons can reduce the incidence of seasonal influenza A.
In this randomized clinical trial, daily supplementation with 1200 IU vitamin D3 in school children between December and March showed a significant preventive effect against influenza A, although no significant difference was observed for influenza B. A 10-d course of postexposure prophylaxis with zanamivir or oseltamivir resulted in only an 8% decrease in the incidence of symptomatic influenza in children (7). In contrast, daily dietary probiotic supplementation was a safe effective way to reduce fever and other symptoms in small children (8). Moreover, a significant preventive effect of a product containing echinacea, propolis, and vitamin C on the incidence of respiratory tract infections was observed in children (9).
In conclusion, our study suggests that vitamin D3 supplementation during the winter season may reduce the incidence of influenza A. This effect was prominent in specific subgroups of schoolchildren. Moreover, asthma attacks were also prevented by vitamin D3 supplementation.
© 2010 American Society for Nutrition