Calcium supplements can change cancer-prone cells of the colon into normal cells, scientists report.

The research at Memorial Sloan-Kettering Cancer Center in New York is part of growing evidence that calcium may help prevent colon cancer, which kills about half its victims within five years.

Earlier research had established a statistical connection, showing that in one large group, those who drank more milk over a 19-year span got less colon cancer.

The new study, published in The New England Journal of Medicine, for the first time suggests a direct cause-effect relationship.

Sloan Kettering's Dr. Martin Lipkin and Harold Newmark examined the cells lining the colon, or large intestine, in 10 people from families with a history of the disease.

Before treatment, the 10 people had cells characteristic of those with high risk of developing colon cancer, the scientists report.

But after taking the calcium supplements for two to three months, the cells changed to resemble those of people at low risk for the disease, biopsies showed.

The 10 took 1,250 milligrams a day of calcium in the form of calcium carbonate, about 1.5 times the recommended dietary allowance.

It remains to be seen whether this effect will actually prevent people from getting cancer. Lipkin now plans a larger, long-term study to see whether colon cancer can be prevented in high-risk populations.