This Mercola article is a great summary of the vitamin D link to cancer.
An optimal vitamin D level is critically important in minimizing your cancer risk; a study of menopausal women showed that maintaining vitamin D serum levels of 40ng/ml cut overall cancer risk by 77 percent.
GrassrootsHealth founder Carole Baggerly believes 90 percent of ordinary breast cancer is related to vitamin D deficiency; in fact, breast cancer has been described as a “vitamin D deficiency syndrome”.
Vitamin D can stop breast cancer cells from spreading by replenishing E-cadherin, one of the glue-like components giving structure to those cells.
In addition to being a strong cancer preventative, vitamin D is crucial for pregnant women and their babies, lowering risk for preterm birth, low birth weight, and C-section; 80 percent of pregnant women have inadequate vitamin D levels.
The most important factor is having your vitamin D serum level tested every 6 months, as people vary widely in their response to ultraviolet exposure or oral supplementation; your serum level should be kept between 50 and 70 ng/ml for optimal health.