A prospective study of bowel motility and related factors on breast cancer risk

Background: Estrogen is an established risk factor for breast cancer. Greater bowel motility has been associated with increased estrogen excretion and lower serum estrogen levels, suggesting that it may influence breast cancer risk. However, only one other epidemiologic study thus far, to our knowledge, has examined the relation between bowel motility and breast cancer risk. Methods: We prospectively examined whether bowel motility, measured by self-reported frequency of bowel movements, and related factors (constipation, laxative use, water consumption, and dietary fiber intake) were associated with incidence of breast cancer among 28,586 postmenopausal women, ages 50 to 76 years, in the Vitamins and Lifestyle study. Cox proportional hazards models were used to estimate multivariate-adjusted relative risks (RR) and 95% confidence intervals (95% CI). From 2000 to 2005, 507 incident invasive breast cancers among the cohort were identified. Results: Women with very frequent (>= 3/d) bowel movements had a 46% decreased risk compared with 1/d women (RR, 0.54; 95% CI, 0.31-0.92), but the test for linear trend was not significant (P-trend = 0.41). Constipation was nonsignificantly associated with increased risk (RR, 1.30 for >= 1/wk versus <1/y; 95% Cl, 0.87-1.95). No statistically significant associations were observed for the other study exposures: 10-year chemical laxative use, 10-year use of fiber laxatives, water consumption, and dietary fiber intake. Conclusion: This study adds limited support to the hypothesis that increased bowel motility lowers breast cancer risk.