Physical Activity in Relation to Mammographic Density in the Dutch Prospect-European Prospective Investigation into Cancer and Nutrition Cohort

Background: Evidence accumulates that physical inactivity is one of the few modifiable risk factors for breast cancer. The mechanism through which physical inactivity affects breast cancer risk is not clear. The study aim was to investigate the association between physical activity and breast density because mammographic density is strongly associated with breast cancer risk. Methods: We did a cross-sectional study in 620 women, of ages 49 to 68 years and participants of the Dutch Prospect-European Prospective Investigation into Cancer and Nutri-tion cohort. A self-administered questionnaire was used to obtain information on duration and intensity of physical activity (recreational, household, and occupational) during the year preceding study recruitment. A total activity index (inactive, moderately inactive, moderately active, and active) was estimated by combining all activity types. Percent and absolute breast density were determined on screening mammograms using a computer-aided method. Multivariate linear regression was used to examine the association between physical activity and breast density. Results: Mean percent density was 35.3 % [95 % confidence interval (95 % CI), 31.8-38.8] for the inactive category compared with 36.1 % (95 % CI, 33.0-39.2) for the activ