Breast cancer patients with lower levels of vitamin D were far more likely to die and far more likely to have their cancer spread than women with normal levels, Canadian researchers reported on Thursday.

Women deficient in the "sunshine vitamin" when they were diagnosed with breast cancer were 94 percent more likely to have their cancer spread and were 73 percent more likely to die than women with adequate vitamin D levels, the researchers said.

More than three-quarters of women with breast cancer had a vitamin D deficiency, the researchers reported to an upcoming meeting of the American Society of Clinical Oncology.

"The women with the lowest vitamin D levels had the highest risk of death from breast cancer," Dr. Richard Schilsky, of the University of Chicago and president-elect of ASCO, told Reuters in an interview.

"We are seeing an association. It is possible that vitamin D is simply a marker for healthy lifestyle. We don't think that is the case," said Dr. Pamela Goodwin of Mount Sinai Hospital in Toronto, who led the study.

The study adds to a growing body of evidence that vitamin D, made when sunlight hits the skin and used to fortify many foods including milk, is important for preventing chronic diseases. It is key to maintaining strong bones.

Goodwin's team studied 512 women with a mean age of 50 with newly diagnosed breast cancer treated at three University of Toronto hospitals between 1989 and 1995. They were followed until 2006, a median of just under 12 years.

Only 24 percent of the patients had adequate levels of vitamin D when they were diagnosed with cancer.

EARLIER, MORE AGGRESSIVE CANCER

Goodwin said optimal levels are considered to be somewhere between 80 and 110 or 120 nmol/L (nanomoles per liter) of blood, although there are no standard guidelines. They considered anything less than 50 nmol/L to be deficient.

After 10 years, cancer did not spread or come back in 83 percent of women with normal levels and 85 percent of these women were still alive. But just 69 percent of women with low vitamin D levels were cancer-free 10 years later and just 74 percent were still alive.

Women with low vitamin D levels were also likely to have developed cancer before reaching menopause, had higher body mass indexes - a measure of being overweight - had higher insulin levels and had more aggressive tumors.

"It is something that is going to make us think a lot harder about what the relationship is between vitamin D and breast cancer," Schilsky said. "Vitamin D may turn out to be far more important in cancer than we realized."

Associations have also been found between vitamin D and prostate and colon cancer, they noted. And Goodwin said low vitamin D levels had also been linked with heart disease.

Goodwin noted that her study had only found an association. A randomized trial in which some women were assigned to take vitamin D and others took placebos would be needed to prove that it was in fact a lack of the vitamin causing the disease.

"It is really hard to know what a woman's vitamin D level is now," Goodwin said in a telephone interview.

"That's why we have taken the fallback position to say at least for breast cancers patients, they should talk to their physician and get their vitamin D level checked," she added.

Both Goodwin and Schilsky stressed that it would be important to avoid taking too much vitamin D, which can be toxic. "We don't want to see women taking 10,000 IU a day," Goodwin said.