A provocative study this year by Canadian researchers found that women with breast cancer who were deficient in bone-building vitamin D had twice the risk of having their cancer recur or progress over 10 years compared with women who had sufficient levels of the vitamin; the D-deficient patients also had a 73% greater risk of dying from their disease. Current guidelines recommend 200 IU of vitamin D each day for women up to age 50, and progressively more for older women. (Many doctors feel that's too low, however, and say 800 IU daily is better, while the government says taking up to 2,000 IU a day is safe). Still, oncologists are not comfortable prescribing supplements for breast cancer patients. It is unclear how much vitamin D adults really need or whether supplements would have any benefit for women already diagnosed with cancer. Further, most supplements do not contain the vitamin's most potent form, D-3, which is what the body naturally produces when skin is exposed to ultraviolet sunlight. It's also worth noting that women in the Canadian study who had the highest levels of vitamin D also had worse survival rates, which suggests that there may be a limit to the vitamin's value or a dose-specific benefit.