BACKGROUND: Consistent evidence supports a reduction in breast cancer risk with a high healthy lifestyle index (HLI) score
however, this relationship has not been well studied in multiethnic populations. METHODS: Within the Multiethnic Cohort study, we followed 65,561 African American, Japanese American, Latina, Native Hawaiian, and White postmenopausal women for incident invasive breast cancer (n=4,555, mean 19.2 years). The HLI summed seven components with higher scores assigned to healthier behaviors: diet quality, physical activity, sedentary behavior, smoking status, alcohol consumption, body mass index and sleep duration. Multivariable Cox proportional hazards models estimated adjusted hazard ratios (aHR) and 95% confidence intervals (CI) for associations between the HLI score (continuous, tertiles (T)) and breast cancer risk overall and stratified by race and ethnicity and hormone receptor status. Multiplicative interaction by race and ethnicity (P-int) and heterogeneity of effect by hormone receptor status (P-het) assessed by the Wald test. RESULTS: Higher HLI scores were associated with reduced postmenopausal breast cancer risk (aHRcont:0.95 [95% CI:0.94-0.97], P<
0.0001
aHRT2vsT1:0.92 [95% CI:0.85-0.99], aHRT3vsT1: 0.81 [95% CI:0.75-0.87], P-trend<
0.01) with similar risk reductions observed across racial and ethnic groups (P-trend≤0.05
P-int=0.96). Similar findings were observed with hormone receptor-positive breast cancer (overall: P-trend<
0.01
P-int=0.90)
no significant associations were observed with hormone receptor-negative breast cancer (P-trend >
0.05
P-int=0.64
P-het=0.79). CONCLUSIONS: Higher HLI scores are associated with breast cancer risk reductions overall, by race and ethnicity and hormone receptor status. IMPACT: Engaging in healthy lifestyle behaviors may reduce breast cancer risk among a multiethnic population of postmenopausal women.