Abstract Background: Native Hawaiian (NH) women experience high breast cancer (BC) incidence and mortality. While obesity is a known risk factor, the role of ethnic mixture, defined as self-reported combinations of racial or ethnic backgrounds, remains understudied. This population-based study examined whether breast cancer risk varies by ethnic mixture among NH women and whether these associations differ across BMI categories. Methods: We analyzed data from 7,700 NH women in the Multiethnic Cohort Study. Participants were categorized into ethnic mixture groups: NH-only, NH/White, NH/Chinese, NH/White/Chinese, NH/Other Asian, and NH/Other. Age-adjusted incidence rates were calculated per 1,000 person-years. Cox proportional hazards models estimated hazard ratios (HRs) and 95% confidence intervals (CIs), adjusting for age, BMI, and other factors. Results: Incidence rates were highest among NH-only and NH/White/Chinese (5.7 and 5.9 per 1,000 person-years) and lowest among NH/Other (3.6). However, ethnic mixture was not significantly associated with risk in adjusted models. Stratified analyses showed modest, non-significant risk elevations among overweight NH/Chinese (HR=1.33, 95% CI: 0.85–2.10). Among obese women, risk was consistent across all groups (HRs: 0.78–0.96), highlighting the dominant role of BMI. Overall, the association between ethnic mixture and breast cancer risk did not differ by BMI category. Conclusions: Ethnic mixture was not independently associated with BC risk; BMI remained a consistent predictor. These findings underscore the need for research integrating ancestry, body composition, and social context to address cancer disparities. Impact: Obesity, not ethnic mixture, is the dominant factor influencing BC risk in NH women.
Valdez et al. (Thu,) studied this question.