Human studies often report gut microbiome differences in mild cognitive impairment and Alzheimer's disease, but intervention effects on cognition and microbiota remain inconsistent.
Evidence
This scoping review searched PubMed, Scopus, and Cochrane Library through February 2023 and included 58 original studies and systematic reviews or meta-analyses from 4751 screened records.
Caveat
Methodological heterogeneity, limited follow-up, and the review's scoping design prevent causal conclusions about dysbiosis, cognition, or treatment effects.
Simplified
Alzheimer's disease (AD) is projected to become the highest-burden neurological disorder globally. Mounting evidence implicates the gut microbiome in AD pathogenesis. This scoping review of gut microbiomes in mild cognitive impairment (MCI) and AD included dietary and probiotic interventions. We included original research and systematic reviews/meta-analyses. Animal and non-English studies were excluded. We searched PubMed, Scopus, and Cochrane Library through February 2023. Using Arksey and O'Malley's framework and the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA)-Extension for Scoping Reviews (ScR) checklist, we screened 4751 articles, with 58 meeting predefined inclusion criteria. Our results demonstrated that was frequently reported in MCI and AD, including increased Pseudomonadota and Actinomycetota in AD and reduced diversity in some cases. Probiotic and dietary interventions showed promise in modulating cognition and microbiota, inconsistently. Emerging evidence links dysbiosis to cognitive decline; however, methodological heterogeneity and limited follow-up impede causal inference. Research should prioritize standardized protocols, functional microbiome analysis, and longitudinal human studies to clarify therapeutic potential. HIGHLIGHTS: Gut dysbiosis is a common feature of MCI and AD, with phylum-level microbial shifts frequently observed. Pseudomonadota and Actinomycetota are enriched in AD across multiple human studies. Beneficial genera like Faecalibacterium and Roseburia are consistently reduced in MCI and AD in a small number of studies. Probiotic and dietary interventions are promising to modulate the microbiota-cognition axis. More longitudinal human studies are needed to assess causal microbiome relationships.
Key numbers
58 studies
Study Inclusion
Final count of studies included in the review
1529 studies
Removed
Number of removed during the study selection process
4751 studies
Initial Studies Identified
Total studies identified during the initial search
Full Text
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Gut-Brain Axis