Abstract
BACKGROUND: The risk of breast cancer has been associated with various lifestyle factors, yet the evidence regarding how lifestyle modifications affect this risk remains limited. This study examines the relationship between changes in the Healthy Lifestyle Index (HLI) and postmenopausal breast cancer risk in women participating in the European Prospective Investigation into Cancer (EPIC). METHODS: HLI scores (ranging from 0 to 16) were computed based on smoking habits, alcohol consumption, body mass index (BMI), and physical activity levels, using data from baseline and follow-up questionnaires, which were separated by a median interval of 10 (IQR: 5.2-12.0) years. Among the 125,746 women included in the analyses, 2,175 developed breast cancer over a median follow-up period of nearly 4 (IQR: 2.9-8.4) years starting from the date of the second lifestyle questionnaire. Cox proportional hazards models were employed to estimate hazard ratios (HRs) and confidence intervals (CIs) for the relationship between changes in HLI and postmenopausal breast cancer risk, analysed both overall and by estrogen receptor (ER) status. Individual components of the HLI were also analysed, with sensitivity analyses addressing potential reverse causation by delaying the start of follow-up by 1 to 3 years. RESULTS: Each unit increase in the HLI-reflecting a healthier lifestyle-was not associated with the overall risk of postmenopausal breast cancer. Among individual components, only a one-unit increase in the BMI score, corresponding to a shift towards a healthier BMI, was inversely associated with overall (HR = 0.936; 95% CI 0.880-0.996) and ER-positive (HR = 0.930; 95% CI 0.865-1.000) postmenopausal breast cancer risks. CONCLUSIONS: Lifestyle changes, as measured by the HLI, during mid-adulthood were not significantly associated with the risk of postmenopausal breast cancer. More specifically, the results of this study suggested that a shift towards a healthier BMI may contribute to breast cancer prevention. Further research involving diverse and larger study populations and lifestyle assessments at earlier life stages could provide deeper insights.