"Women with breast cancer should be tested for vitamin D levels and offered supplements, if necessary," says researcher Sonia Li, MD, of the Mount Vernon Cancer Centre in Middlesex, England. The findings were presented at the San Antonio Breast Cancer Symposium.
Some studies have suggested a link between low vitamin levels and breast cancer risk and progression, but others have not, she says. No studies have proven cause and effect.
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 cell growth and cause cells to die, she says.
Even if it does not have a direct effect on the tumor, vitamin D is needed to maintain the bone health of women with breast cancer, Li says. That's especially important given the increasing use of aromatase inhibitors, which carry an increased risk of bone fractures, she says.
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.
More Than 50% of Women Tested Have Low Vitamin D Levels
For the study, Li and colleagues collected blood samples from 166 women with breast cancer and measured their levels of vitamin D.
When ethnicity was considered, vitamin D levels were lower in Asian women than in white or other women: an average of about 36 nmol/L vs. 61 nmol/L and 39 nmol/L, respectively.
The researchers theorized that vitamin D levels would be higher in the summer, when there are more daylight hours, but the study showed no association between vitamin D levels and seasons.
Last month, the U.S. Institute of Medicine issued updated guidelines stating that a blood level of 50 nmol/L (or 20 nanograms/milliliter) is sufficient for 97% of people.
Edith Perez, MD, director of the breast cancer program at the Mayo Clinic in Jacksonville, Fla., tells WebMD that there is not enough evidence to support routine testing of women with breast cancer.
"But if women happen to be tested and have levels below that called for in the Institute of Medicine report, we prescribe supplements," she says. "That's not just for breast cancer, but for bone health, too."
This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.