Researchers in Japan recently studied the effect of vitamin D3 supplements (1,200 IU per day from December through March) on the incidence of seasonal influenza A in school children. Influenza A infection occurred in 18.6% of children in a placebo group versus 10.8% of children who received the supplement – a 42% reduction in risk among those taking the supplement. The reduction was more prominent among children who had not been taking other vitamin D supplements. Influenza infection was not reduced among a subgroup of asthmatic children but those who became infected were significantly less likely to have an asthmatic attack if they received vitamin D than if they had not. Supplementation did not affect the incidence of influenza B (which is less common than influenza A and is not seasonal).
A review of medical studies published from 1950 to 2009 that looked, among other variables, at vitamin D intake and asthma suggested that vitamin D deficiency may be linked to airway inflammation, decreased lung function and poor asthma control. The researchers conducting the review hypothesized that vitamin D supplementation may lead to improved asthma control, although this cannot be established as many of the studies were not specifically designed to test the effects of vitamin D supplementation on patients with asthma.
