Bill Faloon, Co-founder of Life Extension Foundation, recently summarized some of the science showing the critical role of Vitamin D in preventing and controlling infections due to the flu. An excerpt from his article in the January 2010 Life Extension Magazine follows:
Evidence that Vitamin D Combats Winter Infections
As daylight hours grow colder and shorter, incidences of the common cold, flu, and respiratory infections spike upwards. Scientists have identified reduced vitamin D levels in winter months as a prime suspect for this increase in infectious disease cases.
Vitamin D in all forms (sunlight, sun lamps, or supplements) reduces the incidence of respiratory infections.[24,26] Dutch children with the least sun exposure are twice as likely to develop a cough and three times more likely to develop a runny nose compared with children with the most sun exposure.
When Russian athletes were given access to a sun lamp to stimulate vitamin D synthesis in the body, there were 50% fewer respiratory infections and 300% fewer days of absence.
Children with the lowest vitamin D serum levels are 11 times more likely to develop respiratory infection. When 60,000 IU per week of vitamin D was administered (for six weeks) to children with frequent respiratory infections, the result was a complete disappearance of such infections in the following six months.
In a controlled trial of African women, a low dose (800 IU a day) of vitamin D resulted in a 3-fold reduction in cold and flu symptoms compared to those given placebo.
Influenza kills between around 36,000 Americans each year. Ensuring optimal vitamin D status could slash influenza incidence and morality.
How vitamin D boosts immune function and suppresses inflammation
Flu viruses (including swine flu) induce a massive inflammatory response that can kill the victim. In other words, it is not the virus that often kills, but the body’s hyper-reaction to the virus—in the form of uncontrolled over-production of pro-inflammatory cytokines. Vitamin D down-regulates the expression of pro-inflammatory cytokines such as tumor necrosis factor-alpha. 
As people age, they often over-express these same destructive pro-inflammatory cytokines. The result is chronic low-level inflammation that damages aging arteries, joints, and neurons[40-44]. By down-regulating excess pro-inflammatory cytokine production, vitamin D could save the lives of those stricken with acute influenza, or the dozens of inflammatory diseases that afflict millions of aging Americans each year.
Antimicrobial peptides are components of the immune system that protect against bacterial, fungal and viral infections. Secreted by immune cells throughout the body, antimicrobial peptides damage the outer lipid membrane of infectious agents (including influenza viruses), rending them vulnerable to eradication.
Recent studies confirm that vitamin D dramatically up-regulates the expression of these antimicrobial peptides in immune cells. We now have a definitive biological mechanism to explain why vitamin D confers such dramatic protection against common winter illnesses.
What are minimum vitamin D blood levels?
When blood is tested to assess vitamin D status, what is actually measured is the metabolically active 25-hydroxyvitamin D form of the vitamin in the serum.
When data emerged about vitamin D’s role in preventing disease, experts initially recommended a minimum target blood level of 30 ng/mL of 25-hydroxyvitamin D.
In recognition of findings showing reduced incidences of disease in those with higher vitamin D levels, the standard laboratory reference range for 25-hydroxyvitamin D was raised to 32-100 ng/mL.
Based on recent and conclusive published studies, the new minimum target level for optimal disease prevention is over 50 ng/mL of 25-hydroxyvitamin D.[46-51]
A startling 36%[52-55,56] of the general population has 25-hydroxyvitamin D levels below 20 ng/dL, which may represent the world’s leading cause of unnecessary disease and death.
24. Ginde AA, Mansbach JM, Camargo CA Jr. Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey. Arch Intern Med. 2009 Feb 23;169(4):384-90.
Vitamin D and the Flu: More about the Science
January 5, 2010
25. Karatekin G, Kaya A, Salihoğlu O, Balci H, Nuhoğlu A. Association of subclinical vitamin D deficiency in newborns with acute lower respiratory infection and their mothers. Eur J Clin Nutr. 2009 Apr;63(4):473-7.
26. Cannell JJ, Vieth R, Umhau JC, et al. Epidemic influenza and vitamin D. Epidemiol Infect. 2006 Dec;134(6):1129-40.
27. Alvarez-Díaz S, Valle N, García JM, et al. Cystatin D is a candidate tumor suppressor gene induced by vitamin D in human colon cancer cells. J Clin Invest. 2009 Aug;119(8):2343-58.
28. Lowe L, Hansen CM, Senaratne S, Colston KW. Mechanisms implicated in the growth regulatory effects of vitamin d compounds in breast cancer cells. Recent Results Cancer Res. 2003 164:99-110.
29. Moreno J, Krishnan AV, Peehl DM, Feldman D. Mechanisms of vitamin D-mediated growth inhibition in prostate cancer cells: inhibition of the prostaglandin pathway. Anticancer Res. 2006 Jul;26(4A):2525-30.
30. Lehmann B. The vitamin D3 pathway in human skin and its role for regulation of biological processes. Photochem Photobiol. 2005 Nov-Dec;81(6):1246-51.
31. Qiao G, Kong J, Uskokovic M, Li YC. Analogs of 1alpha,25-dihydroxyvitamin D(3) as novel inhibitors of renin biosynthesis. J Steroid Biochem Mol Biol. 2005 Jun;96(1):59-66.
32. Johansson S. Vitamin A and Osteoporosis: Experimental and Clinical Studies. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine. Uppsala University, Sweden. 2004 pg. 1392.
33. Rohde CM, DeLuca HF. All-trans retinoic acid antagonizes the action of calciferol and its active metabolite, 1,25-dihydroxycholecalciferol, in rats. J Nutr. 2005 Jul;135(7):1647-52
34. Rohde CM, Manatt, Clagett-Dame M, DeLuca HF. Vitamin A antagonizes the action of vitamin D in rats. J Nutr. 1999 Dec;129(12):2246-50.
35. Termorshuizen F, Wijga A, Gerritsen J, Neijens HJ, van Loveren H. Exposure to solar ultraviolet radiation and respiratory tract symptoms in 1-year-old children. Photodermatol Photoimmunol Photomed. 2004 Oct;20(5):270-1.
36. Wayse V, Yousafzai A, Mogale K, Filteau S. Association of subclinical vitamin D deficiency with severe acute lower respiratory infection in Indian children under 5 y. Eur J Clin Nutr. 2004 Apr;58(4):563-7.
37. Cannell JJ, Zasloff M, Garland CF, Scragg R, Giovannucci E. On the epidemiology of influenza. Virol J 2008 5:29.
Available at: http://www.virologyj.com/content/pdf/1743-422X-5-29.pdf. Accessed September 29, 2009.
38. Available at: http://www.cdc.gov/flu/about/disease/us_flu-related_deaths.htm. Accessed October 7, 2009.
39. Peterson CA, Heffernan ME. Serum tumor necrosis factor-alpha concentrations are negatively correlated with serum 25(OH)D concentrations in healthy women. J Inflamm (Lond). 2008 Jul 24;5:10.
40. Van der Meide PH, Schellekens H. Cytokines and the immune response. Biotherapy. 1996 8(3-4):243-9.
41. Devaux B, Scholz D, Hirche A, Klövekorn WP, Schaper J. Upregulation of cell adhesion molecules and the presence of low grade inflammation in human chronic heart failure. Eur Heart J. 1997 Mar;18(3):470-9.
42. Licinio J, Wong ML. The role of inflammatory mediators in the biology of major depression: central nervous system cytokines modulate the biological substrate of depressive symptoms, regulate stress-responsive systems, and contribute to neurotoxicity and neuroprotection. Mol Psychiatry. 1999 Jul;4(4):317-27.
43. Ridker PM, Cushman M, Stampfer MJ, Tracy RP, Hennekens CH. Inflammation, aspirin, and the risk of cardiovascular disease in apparently healthy men. NEJM. 1997 Apr 3;336(14):973-9.
44. De Keyser F, Elewaut D, De Vos M, et al. Bowel inflammation and the spondyloarthropathies. Rheum Dis Clin North Am. 1998 Nov;24(4):785-813, ix-x.
45. Schauber J, Dorschner RA, Coda AB, et al. Injury enhances TLR2 function and antimicrobial peptide expression through a vitamin D-dependent mechanism. J Clin Invest. 2007 Mar;117(3):803-11.
46. Aloia JF, Patel M, Dimaano R, et al. Vitamin D intake to attain a desired serum 25-hydroxyvitamin D concentration. Am J Clin Nutr. 2008 Jun;87(6):1952-8.
47. Dawson-Hughes B, Heaney RP, Holick MF, Lips P, Meunier PJ, Vieth R. Estimates of optimal vitamin D status. Osteoporos Int. 2005 Jul;16(7):713-6.
48. Heaney RP, Armas LA, Shary JR, Bell NH, Binkley N, Hollis BW. 25-Hydroxylation of vitamin D3: relation to circulating vitamin D3 under various input conditions. Am J Clin Nutr. 2008 Jun;87(6):1738-42.
49. Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am.J Clin Nutr. 1999 May;69(5):842-56.
50. Holick MF. The role of vitamin D for bone health and fracture prevention. Curr Osteoporos Rep. 2006 Sep;4(3):96-102.
51. Tukaj C. [Adequate level of vitamin D is essential for maintaining good health]. Postepy Hig Med Dosw. 2008 Oct 9;62:502-10.
52. Holick MF. High prevalence of vitamin D inadequacy and implications for health. Mayo Clin Proc. 2006 Mar;81(3):353-73. Available at: http://www.grc.com/health/pdf/High_Prevalence_of_Vitamin_D_Inadequacy_and_Implications_for_Health.pdf. Accessed October 5, 2009.
53. Hanley DA, Davison KS. Vitamin D insufficiency in North America. J Nutr. 2005 Feb;135(2):332-7. Available at: http://jn.nutrition.org/cgi/content/full/135/2/332. Accessed October 5, 2009.
54. Yetley EA. Assessing the vitamin D status of the US population. Am J Clin Nutr. 2008 Aug;88(2):558S-564S.
55. Looker AC, Pfeiffer CM, Lacher DA, Schleicher RL, Picciano MF, Yetley EA. Serum 25-hydroxyvitamin D status of the US population: 1988-1994 compared with 2000-2004. Am J Clin Nutr. 2008 Dec;88(6):1519-27.
56. Ginde AA, Liu MC, Camargo CA Jr. Demographic differences and trends of vitamin D insufficiency in the US population, 1988-2004. Arch Intern Med. 2009 Mar 23;169(6):626-32.
For more on the extensive science behind Vitamin D, see the following:
Natural Health Science News
Vitamin D Council