Previous studies of the association between body mass index (BMI) and colorectal cancer among women found increased risk only among women who were premenopausal or used hormone replacement therapy (HRT). These results led to the hypothesis that BMI may increase risk more strongly among women with higher levels of circulating estrogen. We examined the association between BMI and colorectal cancer incidence by use of HRT, and number of postmenopausal years without HRT, a measure of duration of exposure to lower levels of estrogen.
During follow-up from 1992-2003, 814 incident colorectal cancer cases were identified among 73,842 postmenopausal women in the Cancer Prevention Study-II Nutrition Cohort. Information on BMI and other risk factors was obtained from questionnaires completed at enrollment in 1992-1993 and during follow-up. Hazard ratios were calculated using proportional hazards modeling.
The association between BMI and colorectal cancer incidence was similar among never-users of HRT (hazard ratio 1.13, 95% confidence interval 1.02-1.25 per 5-unit increase in BMI) and current-users of HRT (hazard ratio 1.08, 95% confidence interval 0.92-1.27 per 5-unit increase in BMI). Among women not currently using HRT, the association between BMI and colorectal cancer did not differ by postmenopausal years without HRT.
Our results do not support the hypothesis that BMI increases risk of colorectal cancer more strongly among women with higher levels of estrogen.