Dairy and other calcium-rich foods might help protect against some types of cancer, a study of nearly half a million older men and women suggests.

Yikyung Park of the U.S. National Cancer Institute in Bethesda, Md., and colleagues gave food questionnaires to 492,810 participants and checked their medical records for cases of cancer.

Their findings appear in Monday's Archives of Internal Medicine.

They found more than 50,000 cancers among the participants, who were between the ages of 50 and 71 when the study started in the mid-1990s and who were followed for an average of seven years.

A high calcium intake was associated with a lower risk of colorectal and other cancers of the digestive system, the researchers found.

The effect was particularly strong among women. Those in the top one-fifth for consuming calcium (1,881 milligrams per day) had a 23 per cent lower risk than those in the bottom one-fifth (494 milligrams per day).

The one-fifth of men who consumed the most calcium through food and supplements (about 1,530 milligrams per day) had a 16 per cent lower risk of the digestive types of cancer than the one-fifth who consumed the least (526 milligrams per day).

The risk reduction peaked at about 1,300 milligrams, and anything greater than that didn't seem to make any more difference.

Adults can get the recommended amount of calcium from drinking four cups of milk or calcium-fortified orange juice.

Calcium and dairy food intake was not found to affect the incidence of prostate cancer, breast cancer or cancer in any other system besides the digestive system.

The study's authors called for more research to confirm the findings.

The study provides evidence that calcium might help keep cells from becoming cancerous, said University of North Carolina nutrition expert John Anderson, who was not involved in the study.

Research has shown that calcium might help reduce abnormal growth and induce normal turnover among cells in the gastrointestinal tract.

Participants who ate the most calcium tended to be healthier overall than the others.

The results were impressive, but participants might have been healthier and wealthier than the average U.S. population, so it's not clear whether the results apply more widely, said Patricia Sheean, a preventive medicine instructor at Northwestern University.

With files from the Associated Press