Low sun exposure and lack of vitamin D
Breast Cancer Risks: Page 7
While sun exposure has long been considered a risk factor for skin cancer, the opposite appears to be true for breast cancer. Research published in the fall of 2007 revealed a surprise: Sun exposure appears to decrease the risk of advanced breast cancer. Women with high sun exposure (defined for the study as having darkened skin on the forehead) had half the risk of developing advanced breast cancer -- cancer that has spread beyond the breast -- as women with low sun exposure.
Researchers explain this phenomenon by the fact that sun exposure increases vitamin D production, and there's increasing evidence that vitamin D is important in slowing the growth of breast cancer cells.