Pulmonary infections and the influence of the sun have been important elements of human experience for millennia. As human populations moved to more extreme latitudes, skin pigmentations that had been advantageous in equatorial regions proved much less so. One major reason for the change in fitness could possibly relate to Vitamin D metabolism. Vitamin D, several related secosteroid hormones involved in the regulation of perhaps 1000 genes, has been of increasing interest for a wide variety of medical conditions.

A meta-analysis of interventional studies has suggested the possibility that Vitamin D may decrease all-cause mortality, although most data on the benefits of supplementation remain suggestive rather than conclusive., The ubiquity of diseases associated with low Vitamin D levels suggest the possibility that low Vitamin D may be a marker of low sunlight exposure, physical inactivity and excess adiposity rather than a reliable therapeutic target. Nevertheless, Vitamin D is mechanistically involved in immunity, including respiratory infections. A large, retrospective nutritional survey suggested an inverse association between Vitamin D levels and the probability of recent respiratory tract infection, even after controlling for season (though not for latitude or other surrogates for sun exposure). Similar results, confounded by smoking and physical inactivity, were observed in Finnish military recruits ...