The findings are the latest in a growing body of evidence suggesting that current dietary guidelines for vitamin D may be too low.
But experts, including the study’s author, caution that the research should be viewed as preliminary and should not be taken as a reason to drastically increase vitamin D use with supplements in an effort to prevent cancer.
“It adds support to the really convincing evidence out there that we need to raise recommendations for vitamin D,” says Joan M. Lappe, PhD, the study’s main author. But she also stressed that the study was limited to healthy, postmenopausal white women and that the cancer findings could not be applied to other groups.
“More studies need to be done,” says Lappe, a professor of nursing and medicine at Creighton University in Omaha, Neb.
Cancer Risk Reduction
The study included findings from 1,024 women (average age 66 at the start) from Nebraska. Women in the four-year study took 1,500 milligrams of calcium supplementation either alone or with 1,100 International Units (IU) of vitamin D each day. A third group of women took placebo pills as a control.
At the end of the study, 50 women had developed non-skin cancers.
Researchers said Friday that women who took both supplements wound up with nearly 60% less risk of cancers at the end of the study compared with women who took placebo. Women who took calcium alone saw their cancer risk cut by nearly half when compared with placebo, though the result could have been due to chance.
The positive results were confined to cancer generally, the researchers found. No single malignancy -- including breast, colon, or lung cancer -- was significantly reduced in women who took supplements.
The study was designed primarily to detect vitamin D’s impact on bone health, not cancer. Lappe says more studies should be performed looking primarily at cancer in a broader range of patients, including men and minorities.
Current U.S. dietary guidelines recommend individuals under 50 get 200 IU of vitamin D per day, with double that amount recommended for adults 51 to 70.
Most vitamin D is manufactured naturally by the body through exposure to sunlight. Fatty fish and fortified milk and cereals contain moderate levels.
Supplements Not Recommended
Several experts warned against consumers increasing their use of vitamin D supplements based on the study results.
“The American Cancer Society is not recommending that individuals routinely increase their vitamin D intake based on this report,” Len Lichtenfeld, MD, the group’s deputy chief medical officer, says in a statement.
“Past experience has shown that, despite studies that suggest a particular vitamin or drug may be effective in reducing cancer risk, once that theory is subjected to a well-designed clinical trial the results frequently do not hold up,” he says.
David Grotto, a spokesman for the American Dietetic Association, says the study was “not a slam dunk” supporting vitamin D’s potential to prevent cancer.
“It does raise this as an issue to look at more intensely,” he says.
According to the Institute of Medicine's Food and Nutrition Board, the tolerable upper level of vitamin D intake is 2,000 IU in adults 19 and older. Risks of vitamin D toxicity include weakness, nausea, poor appetite, and weight loss. Sun exposure and diet are unlikely causes of vitamin D toxicity.
“People need to have a health professional guide them if they have any interest in this and certainly not self-medicate,” Grotto says.
- For more great discussions on women’s health issues, visit WebMD’s Women’s Health: Friends Talking message board.