Low Levels of Vitamin D Spell Trouble for Breast Cancer Patients

"Fat tissue acts as a trap for vitamin D," Goodwin explained. "Levels were also lower in younger women, which was a bit of a surprise, until we realized older women were taking more supplements."

The risk of breast cancer spreading was almost double in women with deficient levels of vitamin D at diagnosis, compared with women whose levels were healthy, the researchers said.

The risk of dying of breast cancer was 75 percent higher in women with too-low levels of vitamin D versus women in the optimal range, they added.

However, there was no survival difference between women with healthy levels and insufficient levels. The data also suggested that there may be a small increased risk of metastasis or death if vitamin D levels are high, but not a statistically significant effect, Goodwin said.

"This suggests that there's a healthy level for vitamin D and, if you are deficient, you have an increased risk of metastasis, but if you go above [a certain point], your risk of death goes up again," Goodwin said.

Although it's not a firm conclusion, Goodwin added, the finding "is raising a red flag. It's probably not safe to take very high levels and get vitamin D levels up really high."

And the findings are consistent with another study which found that men with prostate cancer who took vitamin D died earlier than those taking a placebo.

More information

The American Society of Clinical Oncology has more information on breast cancer.

SOURCES: Pamela Goodwin, M.D., medical oncologist, Mount Sinai Hospital, University of Toronto; May 6 teleconference with Nancy E. Davidson, M.D., director, breast cancer program, Johns Hopkins Kimmel Cancer Center, Baltimore, and president, American Society of Clinical Oncology