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

Breast cancer and diabetes mellitus frequently coexist and share metabolic risk factors, with important implications for cancer risk, treatment, and survivorship. This review synthesizes epidemiologic, mechanistic, and clinical evidence, focusing on type 2 diabetes mellitus (T2DM), the subtype most consistently associated with breast cancer. Meta-analyses indicate that women with T2DM have a 20%-27% higher breast cancer risk than women without diabetes, particularly after menopause (RR 1.20, 95% CI 1.12-1.28; SRR 1.27, 95% CI 1.16-1.39). Potential mechanisms include hyperinsulinemia, insulin-like growth factor-1 signaling, inflammation, obesity-related metabolic dysfunction, and altered sex hormone metabolism. Among patients with breast cancer, diabetes is associated with more advanced disease at diagnosis, increased treatment-related toxicity, and infectious complications. Meta-analyses also demonstrate higher all-cause mortality among patients with diabetes (HR 1.49, 95% CI 1.35-1.65; adjusted HR 1.51, 95% CI 1.34-1.70). Conversely, breast cancer survivors face a modestly elevated risk of developing T2DM, particularly following endocrine therapy or chemotherapy. These bidirectional associations support integrating metabolic assessment and diabetes management into oncology care. Future research should prioritize prospective subtype-specific studies, standardized glucose monitoring during systemic therapy, trials of integrated care, and evaluation of newer antidiabetic agents. These priorities may clarify causality and guide evidence-based interventions across the cancer care continuum.