Higher levels of vitamin D can arrest the growth of cancerous cells, it seems.
Scientists have found breast cancer patients with low levels of vitamin D were much more likely to die of the disease or have it spread than patients getting enough of the nutrient.
Those who were deficient in the sunshine vitamin were twice more likely to suffer recurrence.
Those who had sufficient blood levels of the vitamin at the time they were first diagnosed fared much better, Canadian researchers said.
Study leader Dr. Pamela Goodwin of Mount Sinai Hospital in Toronto said, "It's the first time that vitamin D has been linked to breast cancer progression."
The skin makes vitamin D from ultraviolet light. Too much sunlight can raise the risk of skin cancer, but small amounts — 15 minutes or so a few times a week without sunscreen — may be beneficial, many doctors believe.
While the vitamin is found in certain foods and supplements, most don't contain the best form, D-3, and have only a modest effect on blood levels of the nutrient. That's what matters, the Canadian study found.
But people shouldn't start downing supplements, she warned. Experts don't agree 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 Dr. Julie Gralow, a cancer specialist at the University of Washington in Seattle.
The study was released Thursday by the American Society of Clinical Oncology and will be presented at the group's annual meeting later this month.
Lots of 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 anti-cancer effects.
Other evidence: 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 diagnosed with early-stage breast cancer.
A decade later, 83 percent of those who had had adequate vitamin D blood levels were alive without extensive spread of their cancer, versus 79 percent of those whose vitamin D levels were insufficient and 69 percent of those who were deficient, as defined by widely used medical standards for measuring intake.
One red flag: The few women with the very highest levels of vitamin D seemed to have worse survival.
Though the study was too small and those results were 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.
The federal government says up to 2,000 international units of vitamin D a day seems OK. Taking 800 units per day will, on average, raise blood levels to the middle of the range that seems best for bone and general health, Goodwin said.
Vitamin D is in salmon and other oily fish, and milk is routinely fortified with it, but dietary sources account for little of the amount of D circulating in the blood, experts say.
"It's very hard to make a recommendation" because how much difference a supplement makes depends on someone's baseline level, which also can be affected by sunlight, skin type and time of year, she explained.
Doctors do suggest breast cancer patients get their vitamin D levels checked to see whether they are deficient. The simple blood test is available in many hospitals and labs for about $25, Goodwin said.
Dr. Nancy Davidson, a Johns Hopkins University cancer specialist who is president of the oncology society, said those tests are growing in popularity, even in ordinary medical care.
"Rightly or wrongly, I'm increasingly seeing physicians who are measuring this," she said.
The Canadian study was paid for by the Breast Cancer Research Foundation in New York, established by cosmetics magnate Evelyn Lauder.
"It's a very provocative paper. It's confirmatory of a tremendous amount of evidence that vitamin D is an important component of health," said Dr. Larry Norton, chief of breast cancer programs at Memorial Sloan-Kettering Cancer Center in New York and a medical adviser to the foundation.
Breast cancer is the most common cancer in women. About 184,450 cases and 40,930 deaths from the disease are expected in the United States this year, news agency AP reports.