I'm beginning this topic in light of the emerging literature on the now undeniable link between healthy vitamin d blood serum levels and susceptibility to influenza. In fact, it is so relevant to this whole forum that it deserves a whole thread unto itself. Personally, I have lot's to say on the subject and hope we can keep things to a respectful tone as per WOTAN wishes regarding his earlier reminder of the rules.
Numerous studies link Vitamin D and influenza, as well as Vitamin D and respiratory infections more generally. This vitamin up-regulates genetic expression of various endogenous antimicrobial peptides (AMP), which exhibit broad-spectrum microbicidal activity against bacteria, fungi, and viruses. Reports discussed below indicate that susceptibility to influenza is reduced with higher levels of sun exposure or vitamin D supplementation. Seasonal variation of vitamin D levels in humans can help explain the seasonality of flu epidemics.http://en.wikipedia.org/wiki/Vitamin_D_and_influenza
1) Cannell JJ, Vieth R, Umhau JC, et al. (December 2006). "Epidemic influenza and vitamin D". Epidemiol. Infect. 134 (6): 1129–40. doi:10.1017/S0950268806007175. PMID 16959053.
2) Cannell JJ, Zasloff M, Garland CF, Scragg R, Giovannucci E (2008). "On the epidemiology of influenza". Virol. J. 5: 29. doi:10.1186/1743-422X-5-29. PMID 18298852.
3) Shaman J, Kohn M (March 2009). "Absolute humidity modulates influenza survival, transmission, and seasonality". Proc. Natl. Acad. Sci. U.S.A. 106 (9): 3243–8. doi:10.1073/pnas.0806852106. PMID 19204283.
4) Smiley DF. Seasonal factors in the incidence of the acute respiratory infections. Am J Hyg 1926;6:621-6, p. 626
5) Termorshuizen F, Wijga A, Gerritsen J, Neijens HJ, van Loveren H (October 2004). "Exposure to solar ultraviolet radiation and respiratory tract symptoms in 1-year-old children". Photodermatol Photoimmunol Photomed 20 (5): 270–1. doi:10.1111/j.1600-0781.2004.00110.x. PMID 15379879.
Vitamin D can potentially play a significant role in prevention of influenza, given a well demonstrated link between vitamin D deficiency and the incidence of respiratory infections. The diagnosis and treatment of vitamin D deficiency is straight-forward, cheap and without substantial risk. Thus, physicians should make an effort to diagnose and adequately treat vitamin D deficiency to prevent influenza recurrence. Given what is known about vitamin D and influenza, many contend that the problem of vitamin D deficiency should be considered an important public health issue