Breast-cancer patients with low levels of Vitamin D were more likely to die of the disease or have it spread than were patients who got enough of the nutrient, researchers reported yesterday.
Although not conclusive, the findings add to evidence that the "sunshine vitamin" has anticancer benefits - and are sure to renew arguments about whether a little more sunshine is a good thing.
The skin makes Vitamin D from ultraviolet light. Too much sun can raise the risk of skin cancer, but many doctors also believe that small amounts - 15 minutes or so a few times a week without sunscreen - may be beneficial.
While the vitamin is found in certain foods and supplements, most do not contain the best form, D-3, and have only a modest effect on levels of the nutrient in the blood. That is what matters, the Canadian study found.
Only 24 percent of women in the study had sufficient blood levels of Vitamin D when they were first found to have breast cancer. Those who were deficient were nearly twice as likely to have their cancer recur or spread over the next 10 years and 73 percent more likely to die of the disease.
"It's the first time that Vitamin D has been linked to breast-cancer progression," said the study's leader, Pamela Goodwin of Mount Sinai Hospital in Toronto.
But she warned people not to start downing supplements. Experts disagree on how much Vitamin D people need or the best way to get it - and too much can be harmful. They also don't know whether getting more Vitamin D can help when someone already has cancer.
"We have no idea whether correcting a Vitamin D deficiency will in any way alter these outcomes," said Julie Gralow, a cancer specialist at the University of Washington in Seattle.
The study, released by the American Society of Clinical Oncology, will be presented at the group's annual meeting later this month.
Much earlier research suggests Vitamin D may help prevent prostate, breast and especially colon cancer. In lab and animal tests, Vitamin D stifles abnormal cell growth, curbs formation of blood vessels that feed tumors, and has many other anticancer effects. People who live in Northern regions of the world have higher cancer rates than those living closer to the equator, possibly because of less sunshine and Vitamin D.
The Canadian researchers wanted to see whether it made a difference in survival. They took blood from 512 women at three University of Toronto hospitals between 1989 and 1995, when the women were first found to have early-stage breast cancer.
Although the study was too small and the results not conclusive, "there may be an optimal level of Vitamin D in women with breast cancer, and it may be possible to take too much," Goodwin said.