The modifying effect of social class on the relationship between body mass index and breast cancer incidence

Objectives. We sought to determine whether social class modifies the effect of BMI on breast cancer incidence. Methods. Participants included 5642 postmenopausal White women recruited in 1989 to CLUE II, a prospective cohort study in Washington County, Maryland. We obtained exposure data from CLUE Il and the 1990 US Census. We used survival and random-effects Cox proportional hazards analyses to determine the association of social class and BMI with breast cancer incidence. Results. Education was independently associated with increased risk of breast cancer incidence (hazard ratio [HR] = 1.06; 95% confidence interval [Cl] = 1.01, 1.11; P<.05); contextual measures of social class were not. Education modified the effect of BMI at age 21 years (HR=0.98; 95% Cl=0.97, 0.99}; area-level social class modified the effect of BMI at baseline (HR =0.97; 95% Cl=0.94, 0.99) and BMI change (HR=0.98; 95% Cl=0.95, 1.00). Subpopulation analyses that were adjusted for hormone use, parity, and breast-feeding found similar effects. Conclusions. Social class moderates the influence of body size on breast cancer incidence. Public health efforts, therefore, should advocate for policies that improve social conditions to decrease the burden of breast cancer.