Both diabetes and cancer have long preoccupied public-health concerns, strained national budgets, and been associated with complications that may affect quality of life. They also share some of the same risk factors, such as age, smoking, weight gain, and a diet poor in fruits and vegetables. The earliest link between diabetes and cancer was alluded to in the 1930s,1,2 however, convincing epidemiological evidence has emerged only recently proving an association between diabetes and cancer.3

The segment of the population affected by diabetes or breast cancer is large. Type 2 diabetes, which includes 90% of all diabetes diagnoses, affects 7% of the adult population, and 15% of people over 60 years of age.4 Breast cancer will affect 1 in 9 women in their lifetime,4 and 16% to 20% of women with breast cancer have diabetes.5 The percent of breast cancer patients with previously undiagnosed or delayed-diagnosed diabetes may be as large as 30%.5

A recent meta-analysis of published studies evaluating the effect of preexisting diabetes on breast cancer outcomes found a 49% increase in the risk of death (all cause mortality) in diabetic women with breast cancer compared to nondiabetic counterparts (pooled hazard ratio [HR], 1.49; 95% CI=1.35, 1.65).6 The analysis also found a positive association between preexisting diabetes and late- or advanced-stage breast cancer disease in women; an increased risk of late-stage disease in women with diabetes (diabetes vs. nondiabetes, OR 1.17; 95% CI=1.08, 1.27) and more often with stage 3 or stage 4 than their nondiabetic counterparts (19% vs. 12% stage 3 or 4).6  

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Several factors may contribute to the increased risk of death in diabetic breast cancer patients. These include delayed cancer diagnosis, suboptimal cancer treatments, direct tumor-promoting effects of hyperinsulinemia, and adverse effects of diabetes-related comorbidities or certain antidiabetic medications.7 Visceral obesity, a shared risk factor for diabetes and breast cancer, may hamper timely breast cancer diagnosis, as early signs of breast cancer may be missed. Metabolic alterations with aberrant signaling pathways associated with diabetes may contribute to the aggressiveness of breast cancer, as well.8