And many of the 512 breast cancer patients participating in the research, published Thursday in the American Society of Clinical Oncology, had inadequate vitamin D to begin with. Some 37.5 percent of the women were "deficient" in vitamin D and 38.5 percent had "insufficient" levels of the vitamin, which is considered key to bone health. But investigators stopped short of recommending taking vitamin D supplements to reduce the risk of developing breast cancer or dying from it, saying more research needs to be done. "We were concerned to find that vitamin D deficiency was so common in women diagnosed with breast cancer and that very low vitamin D levels adversely affected patient outcome," said Pamela Goodwi n of the University of Toronto, lead author of the study. "Our results need to be replicated in other clinical studies," she cautioned. "These data indicate an association between vitamin D and breast cancer outcome, but we can't say at this time if it is causal." Researchers studied 512 women with a median age of 50 diagnosed with breast cancer in Toronto between 1989 and 1995. The women were followed until 2006, over a median period of 11.6 years. Just 24 percent had adequate blood levels of vitamin D at the time of their diagnosis. According to Goodwin, a normal level of vitamin D is 80 to 120 nanomoles per liter (nmol/L) of blood. Less than 50 nmol/L is considered deficient. In the group studied, 83 percent of those with adequate levels of vitamin D had not experienced metastases 10 years on, and 85 percent were still alive. By contrast, 69 percent of women with low levels of vitamin D had not seen their cancer recur, and 74 percent were still alive, 10 years later. Women deficient in vitamin D were more likely to develop breast cancer before the onset of menopause, to be overweight and to have high levels of insulin in their blood, the researchers said. Their cancers were also more likely to be aggressive, they said. Previous studies have shown a link between vitamin D and other types of cancer, notably prostate and colon cancer, as well as cardiovascular disease, Goodwin noted. The chief source for vitamin D is sun exposure, since the ultraviolet rays of the sun trigger vitamin D synthesis within the human body. Considered key to bone health, it is naturally present in very few foods, fish, beef liver and egg yolks among them. But it is added to many foods in the United States, including milk and breakfast cereals, according to the National Institutes of Health.