Six RCTs were included in the review. Three studies were judged to be of good quality and the others were judged to be of low quality.
For patients with familial adenomatous polyposis, one RCT (n=28 participants) showed no significant difference on the incidence of adenomas between the supplemental calcium and placebo groups.
For patients with a history of adenomas, supplemental calcium was associated with a significant reduction in risk of diagnosis of any type of adenoma compared with placebo (RR 0.80, 95% CI 0.69 to 0.94; three RCTs, n=1,279). There was no difference between treatment groups in risk of diagnosis with advanced adenoma (RR 0.77, 95% CI 0.50 to 1.17; two RCTs, n=1,186). There was no significant heterogeneity for these outcomes. Sensitivity analyses did not materially alter the results.
For patients with no increased baseline risk of colorectal cancer, there was no significant difference on the rate of colorectal cancer between the calcium (with/without vitamin D) and placebo groups (RR 0.62, 95% CI 0.11 to 3.40; two RCTs, n=37,016). A high degree of heterogeneity was observed for this outcome (I2=58%).
Four RCTs reported no adverse events or complications at doses of 1,000mg to 1,500mg per day. Two RCTs with higher doses observed adverse events. One RCT administered a dose of 2,000mg per day and reported a significant increase in all adverse events compared with the placebo group (26 out of 176 versus 12 out of 178, p=0.04).