Mild depressive symptoms do not influence cognitive functioning in patients with type 2 diabetes

Type 2 diabetes (T2DM) is associated both with cognitive decrements and depressive symptoms. Since depression in itself has been associated with cognitive decrements we aimed to investigate the influence of depressive symptoms on the relation between T2DM and cognitive functioning. Data were derived from three independent studies on cognitive functioning in patients with T2DM (n=366) and controls without diabetes (n=204), two with longitudinal and one with only cross-sectional assessments. Depressive symptoms were measured with self-report inventories (CES-D or BDI-II). The composite z-score of the domains memory, information-processing speed, and attention and executive function was the primary cognitive outcome measure. Mixed linear regression analyses were used in a stepped approach to compare cognitive functioning between (1) patients with T2DM and controls (cross-sectionally and longitudinally), (2) participants with and without depressive symptoms, separately for patients and controls, and (3) patients and controls after adjustment for depressive symptoms. In addition the mediating effect of depressive symptoms was assessed with a bootstrapping technique. Depressive symptoms were present in 11% of the patients with T2DM and in 7% of controls (p=0.15). Cognitive performance in patients with T2DM was worse than in controls (overall difference composite z-score -0.13). However, T2DM was not associated with accelerated cognitive decline over three years of follow-up relative to controls. Controls with depressive symptoms performed worse than those without depressive symptoms, although not statistically significant. Performance in patients with T2DM with and without depressive symptoms was similar. Adjustment for depressive symptoms and estimation of the mediating effect showed that the difference between patients and controls was not mediated by depressive symptoms. In conclusion, the modest cognitive decrements that are associated with T2DM are not due to the presence of mild depressive symptoms.