In a new study, women with vitamin D deficiency at the time of breast cancer diagnosis were 94% more likely to experience cancer spread and 73% more likely to die over the next 10 years, compared to women with adequate vitamin D levels.
More than 1 in 3 women studied had a vitamin D deficiency.
The study is the first to suggest a link between vitamin D deficiency and breast cancer progression, but it doesn't prove cause and effect. And it's too soon to recommend that all women with breast cancer start taking supplements to improve their outlook, says study head Pamela Goodwin, MD, professor of medicine at the University of Toronto.
The findings are scheduled to be reported at the annual meeting of the American Society of Clinical Oncology in Chicago.
Vitamin D Puts Brakes on Breast Cancer
Vitamin D is found in some foods, especially milk and fortified cereals, and is made by the body after exposure to sunlight. It is necessary for bone health, and some studies suggest that it may protect women from developing breast cancer in the first place.
From a biological point of view, it makes sense that vitamin D would put the brakes on breast cancer development and spread, Goodwin says.
"Breast cancer cells have vitamin D receptors, and when these receptors are activated by vitamin D, it triggers a series of molecular changes that can slow down cell growth, cause cells to die, and make the cancer less aggressive," she says.
For the new study, Goodwin and colleagues measured vitamin D levels in the blood of 512 women diagnosed with breast cancer in Toronto between 1989 and 1995. They were followed for a median of 12 years.
Only 24% had adequate levels of vitamin D when they were diagnosed with cancer. A total of 37.5% were deficient in vitamin D. The other 38.5% had insufficient levels of vitamin D.
Of note, Goodwin says, is that women with vitamin D deficiency were more likely to have aggressive cancers than those with sufficient levels.
Vitamin D Deficiency Linked to Poor Breast Cancer Prognosis
After 10 years, 83% of women with adequate levels were alive without signs of cancer spread (metastasis) vs. only 69% of women with vitamin D deficiency. Most of the deaths were from breast cancer.
For women with insufficient levels of vitamin D, there was a slightly increased risk of cancer spread compared to women with sufficient levels, but the difference was so small it could have been due to chance. "And their risk of death was the same," Goodwin says. "So the majority of the [negative] effect is in women with a deficiency."
But there was a point above which there seemed to be too much of a good thing, Goodwin says. If vitamin D blood levels were too high, the risk of dying appeared to rise, although the number of women with very high levels was so small that the finding could be due to chance.
"Our concern is that women may think, if some is good, more must be better, and increase vitamin D intake beyond what's optimal," she says.
So just what is optimal? A reading of 80 to 120 nanomoles per liter, according to Goodwin. That range has also been shown to be optimal for bone and heart health, she says.
Vitamin D Testing
If you don't know your vitamin D level, you're not alone: Doctors don't routinely order it as part of the blood tests taken for an annual physical, Goodwin says.
Julie Gralow, MD, chairwoman of ASCO's Cancer Communications Committee and associate professor of medicine at the University of Washington in Seattle, suggests that women with breast cancer take the initiative and ask about having their vitamin D levels measured.
"We now have a reliable test, and we know to safely correct deficiencies," she tells WebMD.
Goodwin says there is evidence to suggest that for women with a deficiency, taking 800 international units (IU) of vitamin D a day can raise levels to the optimal range.
For healthy people without a deficiency, current recommendations call for people between ages 0-50 to get 200 IU of vitamin D daily, with 400 IU recommended for those between the ages of 51 and 70. After age 70, 600 IU of vitamin D are recommended each day.
Goodwin and colleagues are now trying to confirm the findings in a second, similar study; results are due by the end of the year. If confirmed, the next step will be a study to determine whether giving supplements actually lowers the risk of recurrence and death in women with breast cancer.
The research was funded by the Breast Cancer Research Foundation.