Drinking at least a glass of milk a day may lower the risk of colorectal cancer, say researchers who pooled some of the world's largest studies on the long-believed link.

Calcium, from milk or other sources, has long been thought to play a role in preventing colorectal cancer, the nation's second-leading cancer killer.

Studies show high calcium intake reduces the occurrence of polyps that can turn cancerous. However, diet-tracking studies stopped short of finding final proof of a truly lowered cancer risk.

To better define that link, scientists at Boston's Brigham and Women's Hospital analyzed 10 studies that together tracked nutrient consumption of more than half-a-million people, nearly 5,000 of whom eventually got colorectal cancer.

People who consumed 6 to 8 ounces of milk a day had a 12% lower risk of later developing colorectal cancer than those who drank less than two glasses a week, the researchers reported in the Journal of the National Cancer Institute.

With more than a glass a day, the risk reduction was 15%.

Other dairy foods did not show a statistically significant relationship.

A total calcium intake -- from diet plus calcium supplements -- of 1,000mg a day was protective, too. The researchers calculated that if all study participants had consumed that much, the women may have suffered 15% fewer cases of colorectal cancer and the men 10% fewer.

Vitamin D, commonly added to milk, also is thought to play a role, either alone or because it helps the body absorb calcium. The study could not tease out vitamin D's role, but it found the biggest protective effect with the highest doses of both nutrients.

The study was funded by the National Institutes of Health and National Colorectal Cancer Research Alliance.