Interesting, this Vit D thing always gave me doubt, it was just a feeling nothing from experience or scientific at all. From that link:
Let's move on to the study so trumpeted by Adams, though. The specific study, Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial, did indeed come from Creighton University and did indeed study 1,179 women age 55 and older in Nebraska. The strength of the study was primarily that it was randomized and double-blinded. That's good. Even so, it had significant weaknesses. Perhaps the most significant weakness is that cancer was a secondary outcome of this study, not its primary endpoint. The study was designed to determine whether vitamin D plus calcium supplementation was superior to calcium supplementation alone in presenting osteoporosis. It's always problematic to look at secondary endpoints like this. In this case, the experimental groups included calcium and vitamin D placebos, calcium supplementation plus vitamin D placebo, and calcium supplementation plus vitamin D supplementation. There was no vitamin D alone group. Another weakness in the study is that it only covered four years, enough time for osteoporosis but probably not enough time for a good look at cancer.