Objective:

To summarize current human evidence linking changes in the gut microbiome to mild cognitive impairment (MCI) and Alzheimer’s disease (AD), highlighting the significance of these links.

Key Findings:

• Individuals with MCI or AD frequently showed differences in gut microbiota compared with cognitively healthy controls, with beta diversity analyses indicating distinct microbial community profiles.
• In MCI, higher abundance of bacteria such as Faecalibacterium and Roseburia was reported.
• AD was associated with increases in bacteria from groups such as Pseudomonadota and Actinomycetota.
• Alpha diversity was often similar between individuals with MCI and controls but lower in AD.
• Escherichia coli was the only organism consistently reported as increased in AD.
• Altered microbial profiles were linked to changes in cerebrospinal fluid amyloid-β and phosphorylated tau levels.

Interpretation:

While certain microbial patterns appear repeatedly, differences in study design and methods complicate comparisons and the definition of reliable biomarkers, highlighting the need for standardized approaches.

Limitations:

• Variability in study design, populations, sequencing methods, and reporting.
• Inconsistent results from intervention studies due to small sample sizes and short follow-up periods, impacting reliability.

Conclusion:

Standardized sampling, longitudinal studies, and integration of microbiome data with existing biomarkers are needed for clinical application, with specific areas for future research suggested.