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Published on: September 19, 2019

Gut microbiota alterations in Alzheimer's disease and mild cognitive impairment: A systematic review and

Yujia Liu1,2, Xiuru Wang1, Weilin Huang1

  • 1School of Public Health, Key Laboratory of Tropical Translational Medicine of Ministry of Education, Hainan Medical University, Haikou, Hainan Province, China.

Journal of Alzheimer'S Disease : JAD
|July 2, 2026
PubMed

Insights

Gut microbiome diversity does not reliably distinguish Alzheimer's disease (AD) or mild cognitive impairment (MCI) from normal aging. Community structure and specific microbes may offer insights, but more standardized research is needed.

Area of Science:

  • Microbiome Research
  • Neurodegenerative Diseases
  • Aging

Background:

  • Altered gut microbiota is linked to Alzheimer's disease (AD) and mild cognitive impairment (MCI), but human study findings are inconsistent.
  • Existing research lacks consensus on specific microbiome alterations across AD and MCI stages.
  • The role of gut microbiome in cognitive decline requires further clarification.

Purpose of the Study:

  • To synthesize observational evidence on gut microbiota differences in older adults with AD or MCI compared to cognitively normal (CN) controls.
  • To assess the interpretive value of reported microbiome measures across different stages of cognitive decline.
  • To provide a meta-analytic overview of gut microbiome alterations in the AD continuum.

Main Methods:

  • Systematic literature search of major databases (PubMed, Embase, Scopus, Web of Science, Cochrane Library) for observational studies.
  • Inclusion of studies published from January 2012 to December 2025, focusing on adults aged 60+ with AD, MCI, or CN status.
  • Random-effects meta-analysis for alpha-diversity; narrative synthesis for beta-diversity and taxonomic changes.

Main Results:

  • Twenty-three studies involving 698 AD, 485 MCI, and 1060 CN participants were analyzed.
  • No statistically significant differences in alpha-diversity (Shannon, Chao1, ACE indices) were found between AD and CN groups, or MCI and CN groups.
  • Beta-diversity and taxonomic analyses suggested disrupted community structure and altered microbial composition in AD, but findings for MCI were less consistent.

Conclusions:

  • Alpha-diversity is not a reliable biomarker for differentiating AD or MCI from normal cognitive aging.
  • Community structure and specific microbial taxa patterns show potential for informing about cognitive status but are limited by study heterogeneity.
  • Future research necessitates standardized, function-oriented, and biomarker-informed approaches to elucidate microbiome changes in the AD continuum.
Keywords:
Alzheimer's diseasegut microbiotamicrobiomemild cognitive impairmentα-diversityβ-diversity

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