Low vitamin D levels are associated with breast cancer, too. I'm unsure whether it's the low D that makes you more prone to the cancer, or if the cancer leeches the D from your system (I think it's the first), but when I was diagnosed with breast cancer, the ladies in my online support group urged me to get my levels checked. Mine was wacky low. Normal is 30 to 50, and mine was 12. Only D3 gets my levels up and I now take 2,000 IUs/day just to keep it in the 30s. One of my nieces has the same really low D levels, but I can't convince her to take supplements. She shrugs it off with, "I'll just go to the beach more." I worry that she's setting herself up for trouble.