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 (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.