Effect of breastfeeding on the risk of breast cancer: a meta-analysis of observational studies

Background: Breast cancer is the most common cancer among women worldwide, with its incidence increasing, particularly in low- and middle-income countries. Breastfeeding has been proposed as a modifiable protective factor, but conflicting evidence exists regarding the relationship between breastfeeding duration and breast cancer risk, warranting further investigation. Methods: This meta-analysis, conducted according to the PRISMA guidelines, included observational studies published in English between March 2014 and April 2024 and focused on case‒control and cohort studies. Literature searches were conducted via the EBSCO, PubMed, and Scopus databases. Studies were selected on the basis of the availability of odds ratios (ORs) or relative risks (RRs) with 95% confidence intervals (CIs), specifically analyses of the association between breastfeeding duration and breast cancer risk. Results: A total of 23 case‒control studies were included in the analysis. Never breastfeeding was associated with a significantly increased risk of breast cancer (OR: 1.40; 95% CI: 1.14, 1.72; I² = 84%). Breastfeeding for less than 12 months was associated with an even greater risk (OR: 3.59; 95% CI: 2.50, 5.18; I² = 31%) than breastfeeding for more than 11 months. Sensitivity analysis excluding three studies with inverse effects reduced heterogeneity (I² = 48%) and yielded a stronger association (OR: 1.69; 95% CI: 1.49, 1.91). Conclusion: This meta-analysis supports the protective effect of extended breastfeeding against breast cancer, particularly for durations exceeding 11 months. These findings underscore the importance of promoting breastfeeding as an accessible and cost-effective preventive strategy, particularly in resource-constrained settings.