Because humans obtain most vitamin D from sun exposure and not from diet, a varying percentage of the population is vitamin D deficient, at any time, during any season, at any latitude, although the percentage is higher in the winter, in the aged, in the obese, in the sun-deprived, in the dark-skinned, and in more poleward populations [40, 41]. However, seasonal variation of vitamin D levels even occur around the equator [42] and widespread vitamin D deficiency can occur at equatorial latitudes [43], probably due to sun avoidance [44], rainy seasons [45], and air pollution...
"It is worth noting that one animal study indicated vitamin D, when added to the diet of rats, prevented influenza but a subsequent paper reported it did not [83, 84]. Young et al also reported that a Japanese researcher, Midzuno, was able to reproduce influenza in rats simply by maintaining them on diets deficient in vitamin D, apparently part of Japan's World War II biological weapons research. (The American CIA confiscated Midzuno's papers after the war.) As vitamin D does not upregulate AMPs in murine mammals, it is unclear what these studies mean. If researchers can identify an influenza susceptible species in which vitamin D increases expression of AMPs, it would be useful to know if vitamin D deficiency promotes the pathology of influenza."
"Over the last 20 years, why has influenza mortality in the aged not declined with increasing vaccination rates?
Given that influenza vaccines effectively improve adaptive immunity, the most likely explanation is that the innate immunity of the aged declined over the last 20 years due to medical and governmental warnings to avoid the sun. While the young usually ignore such advice, the elderly often follow it [87, 88]."
"In 1992, Hope-Simpson predicted that, "understanding the mechanism (of the seasonal stimulus) may be of critical value in designing prophylaxis against the disease." Twenty-five years later, Aloia and Li-Ng found 2,000 IU of vitamin D per day abolished the seasonality of influenza and dramatically reduced its self-reported incidence [25]."
