Depression as a Risk Factor for Dementia and Cognitive Decline in Type 2 Diabetes Mellitus: A Systematic Review of Longitudinal Studies

Depression is highly prevalent among individuals with type 2 diabetes mellitus (T2DM) and has been increasingly recognized as a potential contributor to adverse cognitive outcomes. While both depression and diabetes independently increase the risk of cognitive decline and dementia, their combined impact on cognitive health remains an area of growing clinical interest. This systematic review aimed to evaluate the association between depression and cognitive outcomes in individuals with T2DM. A systematic literature search was conducted in PubMed to identify relevant studies published between January 1, 2015, and December 31, 2025. The final search was performed on February 15, 2026. The search strategy combined Medical Subject Headings (MeSH) and free-text terms related to type 2 diabetes, depression, dementia, mild cognitive impairment, and cognitive decline. Studies were eligible if depression was assessed at baseline and cognitive outcomes were evaluated longitudinally through incident dementia diagnosis or repeated cognitive testing, enabling assessment of cognitive decline trajectories. Six studies met the predefined eligibility criteria and were included in the qualitative synthesis. Across the included studies, depression was consistently associated with adverse cognitive outcomes in individuals with T2DM. Longitudinal cohort studies demonstrated increased risk of incident dementia, with reported hazard ratios ranging approximately from 2.17 to 2.59. Other studies reported accelerated cognitive decline or increased risk of mild cognitive impairment among individuals with coexisting depression and diabetes. Depression appears to be an important and potentially modifiable risk factor for cognitive decline and dementia among individuals with type 2 diabetes. Early identification and effective management of depressive symptoms in patients with diabetes may help reduce the risk of adverse cognitive outcomes and support preservation of cognitive health in this high-risk population.