Preexisting Type 2 Diabetes and Survival among Patients with Colorectal Cancer

Background: Type 2 diabetes increases risk of developing colorectal cancer, but the association of preexisting diabetes with colorectal cancer survival remains unclear. Methods: We analyzed survival by diabetes status at cancer diagnosis among 4,038 patients with colorectal cancer from two prospective U.S. cohorts. Cox proportional hazards regression was used to calculate HRs and 95% confidence intervals (CI) for overall and cause-specific mortality, with adjustment for tumor characteristics and lifestyle factors. Results: In the first 5 years after colorectal cancer diagnosis, diabetes was associated with a modest increase in overall mortality in women (HR, 1.22; 95% CI, 1.00–1.49), but not in men (HR, 0.83; 95% CI, 0.62–1.12; P heterogeneity by sex = 0.04). Beyond 5 years, diabetes was associated with substantially increased overall mortality with no evidence of sex heterogeneity; in women and men combined, the HRs were 1.45 (95% CI, 1.09–1.93) during >5–10 years and 2.58 (95% CI, 1.91–3.50) during >10 years. Compared with those without diabetes, patients with colorectal cancer and diabetes had increased mortality from other malignancies (HR, 1.78; 95% CI, 1.18–2.67) and cardiovascular disease (HR, 1.93; 95% CI, 1.29–2.91). Only women with diabetes for more than 10 years had increased mortality from colorectal cancer (HR, 1.33; 95% CI, 1.01–1.76). Conclusions: Among patients with colorectal cancer, preexisting diabetes was associated with increased risk of long-term mortality, particularly from other malignancies and cardiovascular disease. Impact: Our findings highlight the importance of cardioprotection and cancer prevention to colorectal cancer survivors with diabetes.