A Puerto Rican study funded by the National Cancer Institute found a 40% reduction in the incidence of breast cancer in women who take calcium supplements. Previous studies have indicated that calcium helps prevent colon cancer as well. Calcium helps with DNA repair. To get the maximum benefit, I suggest that you take at least 500 mg of calcium daily that is balanced with magnesium, add a multivitamin, and top it off with about 5000iu of Vitamin D. A family history of breast cancer, long term use of a synthetic progestin, and no history of breast feeding are other important risk factors.
See Bloomberg News, 4/20/2010.