Ethnic Mixture and Body Mass Index as Modifiers of Breast Cancer Risk among Native Hawaiian Women: Insights from the Multiethnic Cohort

BACKGROUND: Native Hawaiian (NH) women experience high breast cancer incidence and mortality. Although 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 body mass index (BMI) categories. METHODS: We analyzed data from 7,700 NH women in the Multiethnic Cohort Study. Participants were categorized into ethnic mixture groups as 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 (HR) and 95% confidence intervals (CI), 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, nonsignificant 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 breast cancer 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 breast cancer risk in NH women.