Although high calcium intake has been shown to inhibit colon cancer in animal experiments, these effects have not been seen consistently in human studies. In addition, the few studies that have demonstrated an association between calcium intake and moderate decreases in the risk of precancerous colorectal tumors have rarely evaluated the effect of calcium on different types of colorectal lesions or how other aspects of diet may alter the effects of calcium.
To explore in greater detail the chemopreventive effects of calcium supplementation, Kristin Wallace, M.S., and John A. Baron, M.D., of the Dartmouth Medical School in Lebanon, N.H., and colleagues analyzed data from 913 patients enrolled in the Calcium Polyp Prevention Study, a randomized, double-blind, placebo-controlled trial. Patients took either a 1200 mg calcium supplement or a placebo and had a follow-up colonoscopy 1 and 4 years after enrolling in the trial.
Supplemental calcium slightly decreased the risk of all types of colorectal polyps. The effect was greatest for the advanced lesions that are felt to be most strongly associated with invasive colorectal cancer. In addition, there was some evidence that a diet high in fiber and low in fat increased the preventive effect of calcium, but these results were not statistically definitive.
These findings "suggest that total calcium intakes above 1200 mg are necessary, and perhaps that high dietary fiber and modest dietary fat are required to optimize this [anticancer] effect. Additional data regarding nutrient interactions with calcium will help further refine optimum cancer protective strategies and may clarify the mechanisms by which calcium has its effects in the large bowel," the authors write.