Vitamin D was not helpful in reducing the number of upper respiratory tract infections such as colds or flu. The New Zealand researchers recruited more than 300 healthy adults to get either vitamin D or placebo monthly. The first two doses were 200,000 international units each, and subsequent doses were 100,000 IU for the rest of the 18-month study. This supplied more than five times the US RDA for vitamin D. This dose ensured that those getting vitamin D pills had blood levels of at least 48 ng/ml. There was, unfortunately, no statistical difference in the number of colds and flu infections between the two groups. There were also no side effects.
[Journal of the American Medical Association, Oct. 3, 2012]