Background: Calcium has been hypothesized to reduce the risk of colon cancer, and in a recent randomized trial, calcium supplementation was associated with reduction in the risk of recurrent colorectal adenomas. We examined the association between calcium intake and colon cancer risk in two prospective cohorts, the Nurses' Health Study (NHS) and the Health Professionals Follow-up Study (HPFS).
Methods: Our study population included 87 998 women in NHS and 47 344 men in HPFS who, at baseline (1980 for NHS and 1986 for HPFS), completed a food frequency questionnaire and provided information on medical history and lifestyle factors. Dietary information was updated at least every 4 years. During the follow-up period (1980 to May 31, 1996 for the NHS cohort; 1986 to January 31, 1996 for the HPFS cohort), 626 and 399 colon cancer cases were identified in women and men, respectively. Pooled logistic regression was used to estimate relative risks (RRs), and all statistical tests were two-sided.
Results: In women and men considered together, we found an inverse association between higher total calcium intake (>1250 mg/day versus < or =500 mg/day) and distal colon cancer (women: multivariate RR = 0.73, 95% confidence interval [CI] = 0.41 to 1.27; men: RR = 0.58, 95% CI = 0.32 to 1.05; pooled RR = 0.65, 95% CI = 0.43 to 0.98). No such association was found for proximal colon cancer (women: RR = 1.28, 95% CI = 0.75 to 2.16; men: RR = 0.92, 95% CI = 0.45 to 1.87; pooled RR = 1.14, 95% CI = 0.72 to 1.81). The incremental benefit of additional calcium intake beyond approximately 700 mg/day appeared to be minimal.
Conclusions: Higher calcium intake is associated with a reduced risk of distal colon cancer. The observed risk pattern was consistent with a threshold effect, suggesting that calcium intake beyond moderate levels may not be associated with a further risk reduction. Future investigations on this association should concentrate on specific cancer subsites and on the dose-response relationship.