Breast Cancer and Diabetes Mellitus: Epidemiology, Pathophysiology, and Clinical Outcomes

Background : Breast cancer and diabetes mellitus are highly prevalent conditions that frequently coexist and share overlapping metabolic risk factors. Their interaction has important clinical implications for cancer risk, treatment, and survivorship. This review synthesizes current evidence on the epidemiologic associations, underlying mechanisms, and clinical outcomes linking these conditions. Findings The strongest and most clinically relevant evidence concerns type 2 diabetes mellitus (T2DM), which accounts for most diabetes cases and is the subtype most consistently linked to breast cancer risk and outcomes. Compared with women without diabetes, women with T2DM have an approximately 20–27% higher risk of developing breast cancer in meta-analyses (RR 1.20, 95% CI 1.12–1.28; SRR 1.27, 95% CI 1.16–1.39), particularly after menopause. Proposed mechanisms include hyperinsulinemia, insulin-like growth factor-1 signaling, chronic inflammation, obesity-related metabolic dysfunction, and altered sex hormone metabolism. Among patients with breast cancer, coexisting diabetes is associated with more advanced stage at diagnosis, greater treatment-related toxicity, and higher infection-related complications compared with no diabetes. Systematic reviews and meta-analyses report a 49–51% higher all-cause mortality risk among breast cancer patients with diabetes compared with those without diabetes (HR 1.49, 95% CI 1.35–1.65; pooled adjusted HR 1.51, 95% CI 1.34–1.70). Breast cancer survivors also have a modestly increased risk of subsequent T2DM compared with cancer-free women, particularly after endocrine therapy and chemotherapy. Conclusion : The coexistence of diabetes and breast cancer is associated with increased morbidity and mortality, driven by shared metabolic pathways and treatment-related factors. Integrated care approaches incorporating metabolic management into oncologic care are essential to improve outcomes. Future Directions Future research should prioritize prospective subtype-specific studies, standardized glucose monitoring during systemic therapy, randomized or pragmatic trials of integrated oncology-diabetes care, and careful evaluation of newer antidiabetic agents as metabolic therapies.