Calcium doesn't just build strong bones, it may fight cancer too, a study says.

Researchers at the Ponce School of Medicine in Puerto Rico reported that women who took calcium had a 40 percent lower risk of getting breast cancer, while those getting multivitamins showed a 30 percent reduction in risk. The new findings, from a study of 744 women, were presented at a recent meeting of the American Association for Cancer Research in Washington.

The data contradict results of a December 2008 trial that showed no reduction in cancer risk from vitamin supplements. The scientists attributed the calcium benefit seen in the study to its effect on what they called DNA repair capacity -- the biological process by which cells patch up damaged DNA that otherwise may cause cancer. The report suggests women may boost their cellular defenses with dietary changes and long-term use of supplements, they said.

"The importance of this finding is that now we can monitor breast-cancer risk using DNA repair capacity," said Manuel Bayona, a professor of biostatistics and epidemiology at Ponce School of Medicine and an author of the paper. "We believe that all women should be taking vitamins and supplements. Now we can tell if that regimen is really doing its work in reducing the risk."

A good analogy, Bayona said, is the management of heart disease. Just as doctors track patients' cholesterol and modify it with changes in diet or drugs, they could use blood tests to monitor the DNA repair ability of women at elevated risk for breast cancer and tweak it using dietary supplements.

The study included 278 women with breast cancer and 466 healthy controls. They were given a questionnaire that included several questions about their current and past intake of specific vitamins and minerals. The researchers also took blood samples to measure DNA repair capacity, and the cancer patients were compared with the controls on several dimensions.

The researchers said that the protective effect of calcium is due to its impact on DNA repair. Previous studies have demonstrated calcium's cancer-blocking benefit without explaining the mechanism of action. A study in June 2007 in the American Journal of Clinical Nutrition from researchers at Creighton University School of Medicine found that women taking calcium and vitamin D showed a 60 percent reduction in cancer risk.

The relationship between vitamin supplements and cancer risk is more complicated. Vitamins prevent and repair the cancer-causing damage done by free radicals, the highly reactive atoms that can wreak havoc on cells, according to Jaime Matta, a professor of pharmacology and toxicology at Ponce School of Medicine and a co-author of the study.

Some recent research suggests that vitamin supplements have no protective power against cancer. One study of 7,627 women in the Journal of the National Cancer Institute in December 2008 found that women who took vitamins C and E and beta carotene had the same risk of cancer diagnosis and death as those who didn't. Research published in the Journal of the American Medical Association in January 2009 showed that vitamin E may raise the risk of prostate cancer.

In the study, other factors found to be associated with higher rates of breast cancer were older age, a family history of breast cancer, and no breast-feeding, according to the paper.

In follow-up research, the scientists will try to find the ideal daily vitamin regimen for a woman, according to her age and other characteristics, Bayona said in a telephone interview. He plans to ask more detailed questions about participants' diets and vitamin intake and to take blood samples to confirm the levels of different nutrients in their bodies.

"People tend to be on the very low end of antioxidant and vitamin intake from veggies and fruit," Matta said. "The good thing about taking vitamins is that there's no downside in terms of risk. This opens the door to very cost-effective prevention."

The research was funded by the National Cancer Institute's Center to Reduce Health Disparities in Rockville, Md., and the National Institute of Health's Minority Biomedical Research Support program in Bethesda, Md.