A 2006 study by researchers at the University of California at San Diego, involving more than 120,000 women, showed that those women with the highest blood levels of vitamin D had a 50 percent reduced risk of breast cancer. Their study was published in the journal Nutrition Reviews.
However, a 1999 study by researchers at the University of Miami and the Northern California Cancer Center in Union City, Calif., found that vitamin D status had no effect on breast cancer mortality. Their study was published in the Journal of the National Cancer Institute.
Despite the conflicting evidence, some experts still recommend that all their patients supplement their vitamin D.
"All my patients are on Vitamin D," said Stefan Glück, clinical director of the Braman Family Breast Cancer Institute and UMSylvester Comprehensive Cancer Center at the University of Miami's Miller School of Medicine. "They should [take one vitamin D supplement] per day, or prior to sun exposure, put your sunscreen on 10 minutes after you are exposed."
According to Gluck, those 10 unprotected minutes in the sun give you about 12,000 IUs of vitamin D -- about 12 times the amount you get from taking one vitamin D supplement.
Still, some experts said they refrain from recommending that their cancer patients begin vitamin D supplementation as a preventive measure because of past conflicting evidence on the benefit of such supplementation.
"On the basis of this study, I would not recommend vitamin D supplementation," said Tim Byers, professor in the Department of Preventive Medicine and Biometrics and deputy director at the University of Colorado's Cancer Center. "There have been far too many examples in the past where preliminary studies of this type led to recommendations for supplementation that later we found to be either not helpful or harmful."
Past research has indicated that oversupplementing your vitamin D can be toxic, leading to symptoms such as nausea, vomiting, poor appetite, constipation, weakness and weight loss. More seriously, it can also raise blood levels of calcium, causing kidney stones or even heart rhythm abnormalities.
Vitamin D is found in salmon and other oily fish, and is routinely added to milk, but diet accounts for very little of the vitamin D nutrient that enters into the bloodstream. Most people get their vitamin D from dietary supplements. However, the simplest way to get your daily dose of vitamin D is from the sun's ultraviolet rays, which naturally trigger vitamin D synthesis.
Vitamin D is essential for promoting calcium absorption in the stomach and maintaining bone health, immune function and reduction of inflammation.
Overall, many cancer experts said that they are encouraged by the results of this study, though remain cautious in making any specific recommendations to their patients on supplementing their vitamin D as a method of preventing cancer or improving cancer outcomes.
"I agree with researchers for putting study in proper light -- that it's a preliminary study and needs to be confirmed by more research, and doctors should not make specific recommendations to women with breast cancer based on this study," said Len Lichtenfeld, deputy chief medical officer of the American Cancer Society.