The notion that adequate Vitamin D is an excellent way to prevent getting the flu was discovered by the founder of Vitamin D Council, John Cannell, Ph.D. in 2006, and further verified in 2008 (1, 2). Further studies done in 2009 have shown that individuals with lower amounts of Vitamin D present in their blood report that they get the flu and the common cold substantially more often (3).
A Vitamin D blood serum level of 50-70ng/ml is the most effective way to tell if you have enough Vitamin D in your body. Because of the fear that many people have of sun exposure and slathering on sunscreen, most of us do not get enough Vitamin D year round to prevent us from catching the flu. While getting your Vitamin D from the sun is the best and most effective way (15 minutes per day of skin exposure to the sun), a Vitamin D3 supplement is another good way to get the required levels of Vitamin D that you need to boost your immune system and prevent infection from the flu.
The RDA for Vitamin D has been recently raised due to current research that show most adults require 8,000 IUs of Vitamin D everyday in order to keep their blood serum levels at 40ng/ml, which still falls short of what appears to be required for preventing colds and the flu.
More to come on the other benefits of Vitamin D and flu prevention.
(2) http://www.virologyj.com/content/5/1/29
(3) http://archinte.ama-assn.org/cgi/content/extract/169/15/1443-a