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.
