CONTEXT: Persons with severe mental illness (SMI) are at increased risk to develop type 2 diabetes.

OBJECTIVE: It is unclear whether persons with diabetes and SMI are also at increased risk of diabetes complications and the potential age-specific differences in development of these.

DESIGN, SETTING, AND PARTICIPANTS: Utilizing nationwide register data we followed the entire Danish population with type 2 diabetes from 01.01.1996 to 31.12.2018. Exposure was SMI (schizophrenia, bipolar or depression disorders). Outcome was diabetes complications (nephropathy, retinopathy, lower limp amputations, and cardiovascular disease). We applied Poisson regression models to estimate overall incidence rate ratios (IRRs) and age-specific incidence rates (IRs) and IRRs of first event of each complication in persons with SMI compared to persons without SMI. The models were adjusted for sex, age, diabetes duration, calendar year, education, and migration status.

RESULTS: We followed 371,625 persons with type 2 diabetes, of which 30,102 had coexisting diagnosed SMI. Persons with SMI had a higher IR of nephropathy (IRR: 1.15; 95% CI: 1.12-1.18), amputations (IRR: 1.15; 95% CI: 1.04-1.28), and cardiovascular disease (men: IRR: 1.10; 95% CI: 1.06-1.15, women: IRR: 1.18; 95% CI: 1.13-1.22) but a lower IR of retinopathy (IRR: 0.75; 95% CI: 0.70-0.81) when compared to persons without SMI, after adjustment for confounders. For all complications except amputations, the difference in IR was highest in the younger age groups.

CONCLUSIONS: Persons with type 2 diabetes and SMI had a higher risk and an earlier onset of several diabetes complications diagnosis, emphasizing focusing on improving diabetes management in younger age groups with SMI.