The fact that cases of the common cold and influenza occur seasonally is well known. Explanations for this fact from the scientific literature in the past have never provided evidence to fully explain the reason behind this pattern of infection, which generally abruptly ends during warmer weather in the springtime. Amazingly, little attention has been spent examining the role played by the sun in the seasonal oscillation of influenza and common cold epidemics. Recent research shows that the vitamin D requirements have been vastly underestimated, with the result that a large percentage of the populations of high latitude countries fall into vitamin D insufficiency during the winter months. This marginal vitamin D deficiency is now known to adversely affect the immune system, which could leave the individual at increased risk of infections. Indeed, evidence is now accumulating to suggests that vitamin D is correlated with a reduced risk of upper respiratory tract infections.
For example, researchers1 have investigated the effects of vitamin D supplements on the incidence of seasonal influenza A in school children. The children in the treatment group received 1200 IU/d of vitamin D3 supplements from December to March, while the control group received a placebo. The trial design was double-blind and the primary outcome was the incidence of influenza A diagnosed by detection of the influenza antigen using a nasopharyngeal swab. The results showed that in the treatment group only 18 out of 167 children were diagnosed with influenza A, compared to 31 out of 167 in the control group. Interestingly the benefit of vitamin D was most prominent in those children not already taking a vitamin D supplement. In children already diagnosed with asthma, asthma attacks occurred in only 2 children in the vitamin D group, compared to 12 children in the control group.
The mechanism by which vitamin D reduces the risk of influenza is not know. However a number of possible theories may explain these findings. The vitamin D metabolites 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25(OH)2D] are known to be needed for the production of antimicrobial peptides (e.g. defensin) in white blood and epithelial cells, and so seasonal insufficiency of vitamin D may reduce the number or effectiveness of these proteins. The antimicrobial peptide defensin is known to block the membrane fusion of the haemogglutinin molecule on influenza, and so supplementation with vitamin D may prevent seasonal falls in protection from the influenza virus. Although the vitamin D supplements did not protect again the influenza B virus in this study, this may be related to the inhibitory effects of vitamin D on cytokine release, the pattern of release being different between influenza A and influenza B.
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