Women who had a vitamin D deficiency when they were diagnosed with breast cancer were 94% more likely to have their cancer metastasize and 73% more likely to die within 10 years, Canadian researchers reported Thursday.
The team also found that only 24% of the women in its study had what are normally considered adequate levels of vitamin D at the time of the diagnosis.
The study represents "the first time that vitamin D has been linked to breast cancer progression," said Dr. Pamela Goodwin of Mount Sinai Hospital in Toronto, who led the study.
The results are "very provocative," said Dr. Joanne Mortimer, a breast cancer specialist at City of Hope Comprehensive Cancer Center in Duarte, who was not involved in the study.
"There is some evidence that some of the drugs we use to treat breast cancer, such as aromatase inhibitors, need vitamin D to be activated and metabolized."
Some women who take the drugs get joint aches, Mortimer said, and when they are put on vitamin D, "they get better."
Experts cautioned, however, that it was too soon to recommend vitamin D supplements as a general treatment for breast cancer.
"We have no idea whether correcting a vitamin D deficiency will in any way alter these outcomes," said Dr. Julie Gralow of the University of Washington, who was not involved in the study.
The study was released by the American Society for Clinical Oncology. It will be presented next month in Chicago at the society's annual meeting.
Separately, a team from UC San Diego reported Thursday in the Breast Journal that global breast cancer incidence was linked to the amount of sunlight received annually. The skin makes vitamin D when exposed to sunlight.
The vitamin is also found in certain foods, but in a less effective form than the vitamin D manufactured by the body.
Earlier studies have suggested that vitamin D may prevent prostate and colon cancer. In laboratory dishes and animals, the chemical blocks the formation of new blood vessels feeding tumors and interferes with the growth of abnormal cells.
About 215,000 cases of breast cancer are diagnosed in the United States each year and 41,000 women die from it.
In the Canadian study, sponsored by the Breast Cancer Research Foundation, Goodwin and her colleagues studied 512 Toronto women who were diagnosed with breast cancer between 1989 and 1995, using stored blood samples taken at the time of diagnosis.
