Calcium supplements for the prevention of colorectal adenomas. Calcium Polyp Prevention Study Group.

This was a multicenter, double-blind, randomized placebo-controlled trial in adults with a prior history of colorectal adenomas who were generally in good health. The calcium arm had 464 participants randomized, 409 who completed two follow-up colonoscopies, and 409 included in the main analyses at six U.S. clinical centers.

The active regimen was calcium carbonate 3 g daily, providing 1200 mg of elemental calcium, given as one tablet twice daily with meals in a double-blind design. Participants in the comparison group received identical-appearing placebo tablets.

Calcium carbonate reduced recurrent colorectal adenomas. The adjusted relative risk for any adenoma recurrence was 0.85 (95 percent confidence interval, 0.74 to 0.98; P=0.03), and among those who completed two follow-up colonoscopies it was 0.81 (95 percent confidence interval, 0.67 to 0.99; P=0.04). During the main risk period, the mean number of adenomas per patient was 0.55 with calcium versus 0.73 with placebo, and the ratio of the average number of adenomas was 0.76 (95 percent confidence interval, 0.60 to 0.96; P=0.02). The effect appeared early: during the first study interval, at least one adenoma occurred in 25 percent of calcium-treated participants versus 33 percent of placebo-treated participants. The regimen was well tolerated, with no major toxicity or substantial differences in deaths, hospitalizations, cancers, cardiac disease, stroke, gastrointestinal disease, or urinary stones.

The outcome was adenoma recurrence rather than colorectal cancer or severe dysplasia, so long-term clinical benefit remains indirect. Follow-up was limited to several years and attrition reduced the analyzable sample from the randomized calcium arm (464 randomized, 409 analyzed). The findings come from adults with prior adenomas in U.S. specialty centers, which may limit generalizability.