Although several modifiable risk factors have been independently associated with risk of breast cancer, few studies have investigated their joint association with breast cancer risk. Using a healthy lifestyle index (HLI) score, we assessed the association of a combination of selected modifiable risk factors (diet, alcohol, physical activity, BMI, and smoking) with risk of invasive breast cancer in the Women's Health Initiative (WHI). This study comprised 131,833 postmenopausal women, of whom 8,168 had breast cancer, who were enrolled in the WHI Observational Study or the WHI clinical trials. Cox proportional hazards regression was used to estimate the HRs and 95% confidence intervals (CI) for the association of the score with the risk of developing breast cancer overall and according to specific breast cancer clinicopathologic characteristics. There was a 4% reduction in the risk of breast cancer per unit increase in the HLI score. Compared with those with an HLI score in the lowest quintile level, those in the highest quintile level had 30%, 37%, and 30% lower risk for overall, ER þ /PR þ , and HER2 þ breast cancer, respectively (HR ¼ 0.70; 95% CI, 0.64–0.76; 0.63, 0.57–0.69; and 0.70; 0.55–0.90, respectively). We also observed inverse associations between the score and risk of breast cancer irrespective of nodal status, tumor grade, and stage of the disease. Most individual lifestyle factors were independently associated with the risk of breast cancer. Our findings support the view that promoting healthy lifestyle practices may be beneficial with respect to lowering risk of breast cancer among postmenopausal women.
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