More study needed on vitamin D-ovarian cancer link

NEW YORK (Reuters Health) - There isn't enough evidence to back or debunk the claim that vitamin D can help reduce the risk of ovarian cancer, despite several recent studies making this claim, the authors of a new review of the scientific literature conclude.

"Based on the current evidence, it is premature to make any definitive claims for or against the role of vitamin D in ovarian cancer," Dr. Dr. Linda S. Cook of the University of New Mexico in Albuquerque and her colleagues conclude.

Nevertheless, they add, because an association between vitamin D and the disease is "biologically plausible," and there were problems with the studies that they reviewed, "this is an area worthy of further primary research."

Several review articles -- evaluations of original research on a particular topic -- have reported that vitamin D guards against ovarian cancer or the risk of dying from the disease, Cook and her team note in the American Journal of Obstetrics & Gynecology.

To investigate the quality of the evidence for this claim, they searched the medical literature and identified 20 studies for review. Ten were ecological studies, meaning they used an environmental measure of vitamin D levels, such as latitude (sun exposure is higher the closer one is to the equator) or direct measurements of ultraviolet light exposure; six were case-control studies, comparing vitamin D exposure in people with ovarian cancer and people without the disease; and four were cohort studies, in which a group of people were followed over time and their risk of developing ovarian cancer was compared to their vitamin D exposure.

The researchers identified several flaws in the studies, including women having low vitamin D intakes that often didn't reach recommended daily amounts for the nutrient, and the use of "food frequency" questionnaires to measure vitamin D intake, a "less than optimal" strategy.

The best evidence, they say, came from a study in which investigators tested the levels of vitamin D in women's blood before they were diagnosed with ovarian cancer. This study found no evidence for an overall association between vitamin D levels and risk of the disease, although overweight women did have a lower risk of the disease as their vitamin D levels increased - "a finding that needs to be confirmed in other studies," the researchers say.

"A simple increase in vitamin D exposure, if truly associated with lowered risk, would be an important intervention strategy for reducing ovarian cancer occurrence," Cook and her colleagues conclude. "This hypothesis, however, needs to be tested with rigorous epidemiologic studies."

SOURCE: American Journal of Obstetrics & Gynecology, March 2010.