"This new data makes it increasingly likely that calcium will have a protective effect on cancer itself, not just on polyps", said Dr. John Baron, from the Dartmouth-Hitchcock Medical Center in New Hampshire.
Until now, studies on animals have suggested that high calcium intake may reduce the risk of colon cancer, but this effect has not been seen consistently in human studies.
A research group from Dartmouth, led by John Baron and Kirstin Wallace also felt that studies demonstrating an association between calcium intake and moderate decreases in the risk of precancerous colorectal tumors had rarely evaluated the effect of calcium on different types of colorectal lesions or how diet in general may alter the effects of calcium.
The group thus enrolled 913 patients in a randomized, double-blind, placebo-controlled trial. Patients took either a 1200 mg calcium supplement or a placebo, followed by a colonoscopy one and four years after the beginning of the trial.
The researchers found that a calcium supplement slightly decreased the risk of all types of colorectal polyps and that the effect was greatest for advanced lesions most strongly associated with invasive colorectal cancer. There was also some evidence that a diet high in fiber and low in fat increased the preventive effect of calcium.
"All types of polyps occurred less frequently among the calcium treatment patients than among the placebo patients," said the group. "Calcium use reduced the risk of all polyps by 14 percent and the risk of advanced polyps by 35 percent."
They concluded that a total calcium intake above 1200 mg was necessary to gain the optimal results.
"Calcium is not a sexy drug, and to some people it might seem implausible that something as simple and familiar as calcium might lower the risk of a major cancer," said Baron.
Arthur Schatzkin and Ulrike Peters of the National Cancer Institute in Bethesda offered a word of caution, highlighting that this study does not yet prove a causal relation between calcium intake and colorectal cancer. But, "studies are now in place with the potential to provide a compelling case that a nutritional factor (calcium) can alter the occurrence of malignant disease (colorectal cancer)," they added in a statement.