 :::: The problem is that big-enough, controlled clinical trials are expensive, and private industry has no reason to fund them because vitamin D is generic (in other words, no patent protection that would allow a company to charge a high enough price for investors to get their money back).   As a result, there have not been any, so there is nothing definitive to say.  The only an organization like the NIH will fund clinical trials on vitamins.   Because the NIH is spending taxpayers' money, it has to pick very very carefully.  It funded big clinical trials on Vitamin E to prevent cancer, since there was TONS of in vitro and animal showing that its antioxidant qualities might prove useful to prevent cancer in humans.  Surprisingly - shockingly - people in the intervention arm actually had a slightly higher chance of getting cancer.  Crazy.  Anyway, very recently there was a great article on clinical trials for Vitamin D in particular in the magazine called The Scientist.  Check it out  http://the-scientist.com/2012/03/01/vitamin-d-on-trial/   