Some epidemiologic studies have reported a reduction in colorectal cancer in persons reporting a relatively high intake of calcium and vitamin D. The risk of recurrent polyps also may be decreased. The present randomized, double-blind, placebo-controlled trial enrolled 36,282 postmenopausal women 50 to 79 years of age from 40 centers taking part in the Women's Health Initiative. Approximately half the women received 500 mg elemental calcium as calcium carbonate plus 200 IU vitamin D3 twice a day, whereas the others received a matching placebo. The average follow up was 7 years. At least 70% of women in the trial took half or more of their assigned medication through year 6 of the trial. Women in the supplement and placebo groups had their bowels examined at similar frequencies throughout the trial.
Analysis of 322 pathologically confirmed invasive colorectal cancers showed no substantial effect of calcium/vitamin D supplementation on the cancer risk (hazard ratio [HR], 1.08; 95% confidence interval [CI], 0.86-1.34). Similar results were obtained when omitting women in both groups who had a history of colorectal cancer. Allowing for personal use of calcium supplements, reported by just over half the women, left the findings essentially unchanged. Sites of cancer and their location, histologic features, grade, and stage were similar in the supplement and placebo groups. The HR for death from colorectal cancer in the supplement group was 0.82 (95% CI, 0.52-1.29), but there were not enough events to make the comparison meaningful. Supplementation could not be related to any significant risk or benefit with regard to major disease outcomes, including cancer and cardiovascular disease. Polyps were reported by similar numbers of women in the 2 groups. In general, supplementation was well tolerated.
In this randomized trial, daily supplements of calcium carbonate and vitamin D3 did not influence the risk of colorectal cancer over a 7-year period. Supplementation may protect to some degree against bone fracture, but the present findings do not support its use to prevent colorectal cancer.