Background: Patients with type 2 diabetes mellitus (DM) are at increased risk for colorectal adenomas and cancer because of endogenous hyperinsulinemia. Exogenous insulin therapy has been associated with higher colorectal cancer incidence. The aim of this study was to evaluate the association between exogenous insulin therapy and adenoma formation, accounting for duration of therapy and location and stage of the adenoma. Methods: We conducted a cross-sectional study of patients with type 2 diabetes between the ages of 50 and 80 years who completed full colonoscopies. Cases were patients with any adenoma on index colonoscopy. Patients without any adenoma composed the control group. Multivariable logistic regression was used to calculate odds ratios (OR) and associated confidence intervals (CI). Results: Compared with the controls, case patients (n = 196) did not have a significantly increased odds of insulin exposure, when exposure was defined as 12 months or more of insulin use compared with no insulin. However, the odds of insulin exposure among the cases was significantly increased when exposure was defined as 18 months or more (OR 1.6, 95% CI 1.1-2.5), 24 months or more (OR 1.7, CI 1.1-2.6), and 36 months or more (OR 2.0, 95% CI 1.2-3.4) of insulin use (test for trend P = 0.05). A similar trend in insulin exposure was seen among type 2 diabetics with advanced adenomas. Adenoma location was not significantly affected by insulin therapy. Conclusions: Chronic insulin therapy is associated with increased risk of colorectal adenomas in patients with type 2 diabetes. Impact: Diabetes patients receiving insulin may need more stringent colon cancer screening.
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