Prospective Study Reveals Associations Between Colorectal Cancer and Type 2 Diabetes Mellitus or Insulin Use in Men

BACKGROUND & AIMS: Type 2 diabetes mellitus (DM) is associated with an increased risk of colorectal cancer (CRC); it is not clear if this association varies by sex or other factors. Insulin use might also be associated with CRC risk. We investigated associations of type 2 DM and insulin use with CRC risk. METHODS: The Cancer Prevention Study II Nutrition Cohort is a prospective study of cancer incidence. In 1992 or 1993, adult participants (n = 184,194) completed a detailed, self-administered questionnaire. Follow-up questionnaires were sent in 1997 and every 2 years thereafter. Cox proportional hazards regression analysis was used to calculate relative risks (RR) and 95% confidence intervals (CI), adjusting for covariates. RESULTS: After exclusions, 73,312 men and 81,663 women remained in the final analytic cohort; 1567 men (227 with type 2 DM) and 1242 women (108 with type 2 DM) were diagnosed with colon or rectal cancer by 2007. Among men, type 2 DM was associated with increased risk of incident CRC compared to not having type 2 DM (RR: 1.24; 95% CI: 1.08-1.44); risk was higher for participants with type 2 DM using insulin (RR: 1.36; 95% CI: 1.05-1.78), and participants with type 2 DM not using insulin (RR: 1.22, 95% CI: 1.04-1.45). Among women, type 2 DM and insulin use were not associated with risk of incident CRC (RR: 1.01; 95% CI: 0.82-1.23 and RR: 0.95; 95% CI: 0.64-1.41, respectively). CONCLUSIONS: There is a modest association between type 2 DM and CRC among men, but not women. Insulin use is not associated with a substantially increased risk of CRC.