Background: Mammographic density (MD) is strongly associated with breast cancer risk. We examined whether BMI partially explains racial and ethnic variation in MD. Methods: We used multivariable Poisson regression to estimate associations between BMI and binary MD (BI-RADS A&B versus BI-RADS C&D) among 160,804 women in the Utah mammography cohort. We estimated associations overall and within racial and ethnic subgroups and calculated population attributable risk percents (PAR%s). Results: We observed the lowest BMI and highest MD among Asian women, the highest BMI among Native Hawaiian and Pacific Islander women, and the lowest MD among American Indian and Alaska Native (AIAN) and Black women. BMI was inversely associated with MD (RRBMI>=30 v<25=0.43, 95% CI=0.42-0.44) in the full cohort, and estimates in all racial and ethnic subgroups were consistent with this strong inverse association. For women <45, although there was statistical evidence of heterogeneity in associations between BMI and MD by race and ethnicity (p=0.009), magnitudes of association were similar across groups. PAR%s for BMI and MD among women <45 were considerably higher in White women (PAR% 29.2, 95% CI=28.4-29.9) compared to all other groups with estimates ranging from PAR%Asain=17.2%, 95%CI=8.5-25.8 to PAR%Hispanic=21.5%, 95%CI=19.4-23.6. For women >=55, PAR%s for BMI and MD were highest among AIAN women (PAR% 37.5, 95% CI=28.1-46.9). Conclusions: While we observed substantial differences in the distributions of BMI and MD by race and ethnicity, associations between BMI and MD were generally similar across groups. Impact: Distributions of BMI and MD may be important contributors to breast cancer disparities.

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