Like many American women, Ivalynn Oudin has been taking supplements of calcium and vitamin D for years.
"My doctor said it would increase my bone density and help prevent hip fractures," said the 57-year-old mother of two.
But new research published today in the American Journal of Clinical Nutrition suggests that vitamin D supplements may have another benefit: a 60 percent to 77 percent lower risk of cancer.
"Our study shows that with adequate vitamin D, cancer can be prevented -- or a high incidence of it can," said Joan Lappe, a professor of nursing and medicine at Creighton University and the lead author of the study.
"This is the first study that shows, in a clinical trial, that adequate levels of vitamin D can reduce the risk of cancer."
The research may already be impacting public health policy in at least one country. According to a release posted on its Web site, the Canadian Cancer Society is planning an announcement Friday that all adults should start taking vitamin D supplements. It's the first-ever move by a major public health organization to endorse daily use of the vitamin as a cancer-prevention therapy for an entire population.
However, critics of the study say this drastic drop in cancer risk is not borne out by previous research, and the effect may be much more subtle.
Dr. Jacques Rossouw of the National Institutes of Health is one of these critics. His group conducted a study that followed 36,282 postmenopausal women for seven years to test the effects of vitamin D on colorectal cancer, pegged by the NIH as the third leading cancer killer of women in the United States.
"In our study we found absolutely no indication of an effect of calcium or vitamin D [on cancer] -- zero," he said. "And that's over a seven-year period. It was a much larger study and much a longer study."
Dr. John Milner, chief of the Nutrition Science Research Group at the National Cancer Institute, agrees that some skepticism is necessary.
"We need to put this in the context of the entire diet and lifestyle and understand why we're getting some effect," Milner said. "I don't want to minimize it, but let's see a little bit more before we start jumping into public health policies."
The research was conducted with 1,179 postmenopausal Caucasian women older than the age of 55 who were randomly assigned to receive a placebo containing no medication, a dose of calcium equivalent to about five glasses of milk per day, or both calcium and a high dose of vitamin D.
Women who were on both calcium and vitamin D had 60 percent to 77 percent fewer incidences of cancer in four years than those taking the placebo, according to the study.
Vitamins are substances that the body needs but cannot make entirely on its own. Although vitamin D can be found in oily fish, such as salmon, it is most commonly obtained by the body through exposure to sunlight; as the sun's rays hit the skin, the body converts a compound similar to cholesterol into vitamin D.
The body uses this vitamin D to maintain a healthy level of calcium in the blood and for proper bone health throughout life, according to Dr. Michael Holick, a vitamin D expert at Boston University Medical Center.
In the past 20 years, research has also suggested that vitamin D may play other roles within the body.