Assessing adipokines as potential biomarkers of dementia, Alzheimer's disease, and mild cognitive impairment: A systematic review and meta-analysis

Midlife obesity and late-life weight loss confer a greater risk for developing dementia and Alzheimer's disease (AD), but the exact mechanisms behind this phenomenon are currently unknown. The answer could lie on the involvement of gastrointestinal factors, such as adipokines (e.g., leptin, adiponectin, and resistin) and ghrelin. In this context, we conducted a pre-registered systematic review and meta-analysis of 42 cross-sectional and 13 longitudinal studies targeting the associations between leptin, adiponectin, resistin, and ghrelin and the prevalence of general dementia, AD, and mild cognitive impairment (MCI). We also examined the relationship between the four gastrointestinal factors and neurocognitive outcomes and AD-related cerebrospinal fluid biomarkers. Patients with AD had lower blood leptin and higher resistin levels than cognitively normal participants. Lower leptin and higher resistin were associated with higher degree of cognitive impairment. Additionally, lower late-life leptin levels might be associated with higher prospective risk of dementia and AD, although more studies are needed to corroborate this. Results in ghrelin and adiponectin were not conclusive, with age, sex distribution, obesity, and severity of dementia seemingly acting as moderators across several analyses. Our work might contribute to the identification of new preclinical blood markers of MCI and AD.