Take vitamin D to reduce your risk of infection

January 15, 2013
A study published in the American Journal of Clinical Nutrition in 2010 showed that giving just 1200 i.u. of vitamin D to school age children resulted in a 42% reduction in the flu compared to those children not receiving vitamin D. Also the children not receiving vitamin D had a six-fold increase in asthma attacks. 
Other studies have shown that vitamin D decreases the risk of upper respiratory infections, asthma, and tuberculosis.  Additionally, there are small studies that suggest vitamin D may play a beneficial role in HIV patients by increasing certain white blood cell counts and decreasing premature death.
Vitamin D apparently activates an antimicrobial peptide called cathelicidin which has a direct regulatory mechanism on the immune system by activating T-cells to produce substances that kill bacteria. 

References:
Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010 May;91(5):1255-60.

Vitamin D as an inducer of cathelicidin antimicrobial peptide expression: past, present and future. Curr Opin Allergy Clin Immunol.2009 Jun;9(3):202-7.

Vitamin D, innate immunity and upper respiratory tract infection. J Steroid Biochem Mol Biol. 2010 Jul;121(1-2):234-8.
A potential role for vitamin D on HIV infection? Nutr Rev. 2006 May;64(5 Pt 1):226-33.