A Meta-Analysis of Cognitive Functioning in Nondemented Adults with Type 2 Diabetes Mellitus

OBJECTIVE: The current investigation sought to determine the pattern and magnitude of cognitive functioning deficits in persons with type 2 diabetes mellitus without dementia using meta-analysis to consolidate findings in the literature. METHODS: MedLine and PsychInfo databases were searched to identify studies of cognitive functioning in persons with type 2 diabetes. Effect sizes (Cohen's d) were calculated for the differences in cognitive functioning between subjects with type 2 diabetes and controls without diabetes on classified cognitive abilities. Average d values were calculated for all cognitive abilities across studies. RESULTS: Twenty-five studies meeting the inclusion criteria were identified. Persons with type 2 diabetes performed significantly lower than controls without diabetes (p<0.05) on all cognitive abilities evaluated, with effect sizes ranging from -0.14 to -0.37. The largest effect sizes were processing speed (with motor task demands), M -0.37 (95% CI, -0.41 to -0.32), and divided attention/shifting, M -0.36 (95% CI, -0.42 to -0.31). CONCLUSIONS: Type 2 diabetes leads to mild to moderate deficits in all measured cognitive abilities. There was a lack of published studies investigating type 2 diabetes-associated variables; therefore, additional meta-analyses investigating the impact of these variables on cognitive functioning in type 2 diabetes could not be performed. As such, data from individual studies must be reported consistently to allow for investigation of variables that may affect the relationship between type 2 diabetes and cognitive functioning. Given the present findings, clinicians working with patients with type 2 diabetes should be alerted to the possibility of cognitive changes that could impact type 2 diabetes treatment management or require referral for neuropsychological assessment.