A Prospective Study of the Associations Between Treated Diabetes and Cancer Outcomes

OBJECTIVE: To quantify the association of treated diabetes with cancer incidence and cancer mortality as well as cancer case fatality and all-cause mortality in adults who subsequently develop cancer and to calculate attributable fractions due to diabetes on various cancer outcomes. RESEARCH DESIGN AND METHODS: Prospective data on 599 diabetic and 17,681 nondiabetic adults from the CLUE II (Give Us a Clue to Cancer and Heart Disease) cohort in Washington County, Maryland, were analyzed. Diabetes was defined by self-reported use of diabetes medications at baseline. Cancer incidence was ascertained using county and state cancer registries. Mortality data were obtained from death certificates. RESULTS: From 1989 to 2006, 116 diabetic and 2,365 nondiabetic adults developed cancer, corresponding to age-adjusted incidence of 13.25 and 10.58 per 1,000 person-years, respectively. Adjusting for age, sex, education, BMI, smoking, hypertension treatment, and high cholesterol treatment using Cox proportional hazards regression, diabetes was associated with a higher risk of incident cancer (hazard ratio 1.22 [95% CI 0.98–1.53]) and cancer mortality (1.36 [1.02–1.81]). In individuals who developed cancer, adults with diabetes had a higher risk of cancer case fatality (1.34 [1.002–1.79]) and all-cause mortality (1.61 [1.29–2.01]). For colorectal, breast, and prostate cancers, the attributable fractions resulting from diabetes were larger for cancer fatality and mortality than cancer incidence. CONCLUSIONS: In this prospective cohort, diabetes appears to exert a greater influence downstream on the risk of mortality in people with cancer than on upstream risk of incident cancer.