Timing matters: a life-course review of midlife type 2 diabetes, exposure duration, and late-life dementia risk.

Purpose: This review synthesizes the evidence regarding Type 2 diabetes mellitus (T2DM) as a critical modifiable risk factor for dementia through a life-course lens. The primary purpose is to move beyond the general association and determine how the timing and severity of T2DM, specifically age at onset, duration of disease, and glycemic instability, influence cognitive decline. Consequently, this paper seeks to answer the central research question: How do midlife versus late-life T2DM onset and cumulative glycemic exposure independently contribute to the long-term risk and biological pathways of late-life dementia? Methods: A structured narrative review was conducted using predefined search strategies and study selection criteria, focusing on peer-reviewed studies published 2020-2025 and key longitudinal cohorts. Studies included large prospective cohorts, population-based analyses, or meta-analyses reporting hazard ratios (HRs), relative risks (RRs), or odds ratios (ORs) for incident dementia, Alzheimer's disease (AD), or vascular dementia (VaD). Key exposures included midlife onset, duration, cumulative glycemic measures, and severe hypoglycemia. Results: T2DM was associated with a 1.3-1.8-fold increased risk of incident dementia. Earlier onset and longer duration showed graded risk increases, while higher cumulative glycemic burden and severe hypoglycemia were linked to greater dementia incidence. Associations were stronger for VaD than AD. Conclusions: Midlife T2DM appears to be a strong exposure-dependent risk factor for late-life dementia. Preventing T2DM onset, maintaining stable glycemic control, and minimizing hypoglycemia may be associated with lower future dementia risk, pending confirmation in interventional studies.