Colon cancer is the third most common cancer in men and women. The American Cancer Society estimates that colorectal cancer accounted for about 11% of new cases of cancer reported in 1998, and for about 10% of all cancer deaths in the US. People who have had colorectal polyps -- benign tumors that can develop into cancer -- have less risk of developing recurrent polyps if they take calcium supplements each day I don't think there's any reason to think that the results wouldn't also apply in people who have not been diagnosed as having a polyp. The patients were assigned to take either two calcium tablets per day or two placebo tablets. Each commercially available calcium tablet contained 1.5 grams of calcium carbonate, equivalent to 600 milligrams of elemental calcium.

The New England Journal of Medicine January 14, 1999;340:101-107

COMMENT: Yet another benefit of calcium supplements. My concern though is that there are many types of calcium supplements available. Calcium carbonate probably benefits most people as the majority of people are to acidic and the carbonate helps to neutralize it. It is important to also recognize that 90% of people do not generate enough hydrochloric acid to digest their food. If one takes calcium carbonate with or slightly before meals the already decreased acid in most people will be neutralized thus further compromising digestion. If one use calcium carbonate one should take it 30-60 minutes before meals or 60-120 minutes after meals to minimize this complication. It is also important to balance calcium with magnesium to prevent an imbalance between these two minerals.