Women with vitamin D deficiency when their breast cancer is diagnosed appear to be at significantly higher risk for metastases and death compared with women who have adequate blood levels, according to a presentation at the annual meeting of the American Society of Clinical Oncology.

Specifically, women with vitamin D deficiency were 94 percent more likely to develop metastatic disease and 73 percent more likely to die. The significance of low vitamin D levels was compounded by the finding that more than a third (37.5 percent) of women with breast cancer had vitamin D levels that were classified as deficient and another 38.5 percent were classified as having insufficient levels of vitamin D).

"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. Our results need to be replicated in other clinical studies," explained lead author Pamela Goodwin, MD, professor of medicine at the University of Toronto and holder of the Marvelle Koffler Chair in Breast Research at the Samuel Lunenfeld Research Institute, Mount Sinai Hospital.

"These data indicate an association between vitamin D and breast cancer outcome, but we can't say at this time if it is causal."

In the current study, Goodwin and her Canadian colleagues examined the relationship between vitamin D levels in the blood, incidence of breast cancer metastases and overall survival in 512 women diagnosed with breast cancer between 1989 and 1995. Women were prospectively followed until 2006, for a median follow-up of 11.6 years.

Researchers found that only 24 percent of patients had adequate levels of vitamin D when diagnosed with cancer. Women deficient in vitamin D (less than 50 nmol/L) were more likely to have high-grade cancers. After 10 years, 83 percent of women with adequate levels (more than 72 nmol/L) remained free of metastases and 85 percent were still alive, compared with only 69 percent and 74 percent, respectively, of women with vitamin D deficiency. Most of these deaths were attributed to breast cancer.

If these observations are confirmed in a second, similar study including other women with breast cancer, which is already ongoing, Goodwin recommends a new randomized clinical trial examining the effects of raising blood levels of vitamin D on outcomes in women with breast cancer.