Association of lifestyle factors with breast cancer incidence: An overview of systematic reviews.

Objective: Heterogeneity in the evidence of association between lifestyle factors and breast cancer (BC) incidence hampers initiatives to modify BC risk. This overview aims to synthesise evidence from systematic reviews (SRs) to inform lifestyle-related modifications for BC prevention. Methods: We systematically searched (MEDLINE, EMBASE, and CINAHL) from January 2013 to August 2023 for SRs of the association between lifestyle factors [alcohol consumption, physical activity (PA), body mass index (BMI), smoking, breastfeeding, oral contraception (OC), hormone replacement therapy (HRT), and sedentary behavior (SB)] and BC incidence. A narrative data synthesis was performed. Results: Sixty-six SRs met the eligibility criteria. Evidence from 40 SRs indicated consistent associations between the risk of BC and postmenopausal BMI increase (relative risk increase: 2%-21%), use of HRT (risk increase: 23%-33%), smoking (risk increase: 4%-86%), and alcohol consumption (risk increase: 4%-61%). Additionally, evidence from 23 SRs suggested protective associations with PA (risk decrease: 10%-39%), breastfeeding (risk decrease: 9%-53%), and healthy lifestyle scores (protective about 20%-26%). However, inconsistent and/or statistically non-significant associations were found between BC incidence and premenopausal BMI increase [relative risk (RR): 0.78-1.08], SB (RR: 1.01-1.20), and OC use [odds ratio (OR): 1.01-1.35]. Conclusions: This overview identifies lifestyle factors associated with BC incidence, highlighting both harmful and protective factors. Our summary findings can support information and interventions related to modifying these factors, including limiting alcohol and smoking, or avoiding postmenopausal BMI increase and HRT.