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Vitamin D May Not Prevent Breast Cancer

Study Shows No Breast Cancer Benefit From Vitamin D Supplements in Postmenopausal Women, but More Research Needed
By
WebMD Health News
Reviewed by Louise Chang, MD

calcium_vitamin_d_breast_cancer.jpg

Nov. 11, 2008 -- Vitamin D supplements, taken at a dose of 400 international units per day, may not help prevent breast cancer in women after menopause, a new study shows.

But the study doesn't shut the door on vitamin D for breast cancer prevention, notes an editorial published online with the study in today's Journal of the National Cancer Institute.

Questions remain about the dose of vitamin D and when (or if) it should be taken to lower breast cancer risk, according to the study and editorial.

The body makes vitamin D when it gets enough exposure to sunshine. Vitamin D is also found in supplements and some foods.

In May, a Canadian study showed that vitamin D deficiency often accompanies breast cancer. And some other studies, but not all, have linked higher vitamin D intake to less likelihood of breast cancer.

But those studies were snapshots in time -- they didn't directly test vitamin D for breast cancer prevention. The new study tackled that task.

Breast Cancer Prevention Study

The new findings come from more than 36,200 U.S. women enrolled in the Women's Health initiative, a long-term health study.

The women were split into two groups. One group was assigned to take 1000 milligrams of calcium and 400 international units of vitamin D supplements per day. The other group got placebo pills.

All of the women were also free to take vitamin D supplements for personal use, no matter what group they were in.

The women were followed for seven years, on average. During that time, similar numbers of women in each group developed breast cancer; the supplements didn't seem to make a difference in that.

The details: 528 women assigned to take calcium and vitamin D supplements developed invasive breast cancer, compared to 546 in the placebo group. That difference could have been due to chance.

Still, some questions remain.

Lingering Questions

Here are some of the questions that remain about vitamin D and breast cancer, according to the researchers and editorialists:

  • Was the vitamin D dose too low to make a difference?
  • Would it matter if vitamin D supplementation started before menopause?
  • Did personal use of vitamin D supplements in the placebo group affect the results?
  • What, if any, impact did the calcium supplements have?

"Although further study of relationships among calcium plus vitamin D supplement use and breast cancer can be considered, current evidence does not support their use in any dose to reduce breast cancer," write the researchers, who included Rowan Chlebowski, MD, PhD, of Los Angeles Biomedical Research Institute at the Harbor-UCLA Medical Center.

But "the potential health benefits of vitamin D and calcium may yet still have a bright future," and further studies are needed, note the editorialists, who included graduate Corey Speers and Professor Powel Brown, MD, PhD, of Baylor College of Medicine in Houston.

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