Dementia in patients with type 2 diabetes: a nationwide Danish study

Background The prevalence of dementia is increasing, and type 2 diabetes (T2D) is a recognized risk factor for dementia. Nevertheless, the reported strength of this association varies. Purpose This study aimed to provide a comprehensive assessment of the risk of dementia associated with T2D. Methods Using nationwide Danish registries, we identified and included all patients with T2D between 1998 and 2018, aged >=18 years without dementia. Patients were matched 1:2 by age, sex, and date to individuals without diabetes or dementia. The primary outcome was dementia, follow-up continued until first registered dementia diagnosis, death, or end of study (31.12.2023). Cumulative risks for dementia were estimated using the Aalen-Johansen estimator to account for the competing risk of death. We present analyses using both calendar time and age as the underlying time scale. Results The study included 371,677 patients with T2D and 743,426 controls without T2D. Patients with T2D had a higher prevalence of comorbidities, including hypertension and ischemic heart disease, compared to controls. Over a median follow-up of 11.05 years, 67,069 dementia cases occurred. The absolute risk of dementia was initially higher among patients with T2D. At 1 year, the cumulative incidence of dementia was 0.55% (95% CI 0.53 - 0.58) in the T2D group versus 0.40% (95% CI 0.39 - 0.42) in controls and after 5 years this pattern continued: 2.20% (95% CI 2.16 - 2.25) vs 2.05% (95% CI 2.02 - 2.08). However, at 20 years, there was a shift, and the cumulative incidence of dementia was 8.24% (95% CI 8.12 - 8.36) for T2D and 8.45% (95% CI 8.36 - 8.53) for controls (Figure 1). When considering age as the time scale, this intersection occurred at 80 years (Figure 2). Mortality was consistently higher in patients with T2D compared to controls. Conclusion T2D was associated with a higher risk of dementia in the early years following T2D diagnosis but not in long term follow-up or older age. These findings highlight the need for continued longitudinal research aimed at better understanding differences in cognitive and survival outcomes between patients with T2D and the general population.