Pooling data from 23 observational studies (698 AD patients, 485 with mild cognitive impairment, 1,060 cognitively normal controls), this meta-analysis found no statistically significant differences in alpha-diversity metrics between groups. Shannon diversity (SMD = -0.23, 95% CI: -0.57 to 0.11), Chao1, and ACE indices all failed to distinguish AD or MCI from normal aging. Beta-diversity and taxonomic composition, however, showed more consistent disruption in AD, suggesting microbial community structure rather than species richness is where the signal resides.
The gut-brain axis has attracted intense research interest, with dozens of animal studies demonstrating that microbiome manipulation can alter amyloid burden and neuroinflammation. This meta-analysis delivers a necessary corrective to overenthusiastic biomarker claims: the commonly reported alpha-diversity measure—essentially a count of microbial richness—adds no diagnostic value for AD or MCI at the population level. This matters because numerous clinical papers have cited reduced diversity as a hallmark of neurodegeneration without adequately powering their studies.
The more actionable signal lies in specific taxonomic shifts—changes in Bacteroidetes, Firmicutes, and short-chain fatty acid producers—though heterogeneity across sequencing platforms, dietary confounders, and cohort demographics currently prevents consensus. The aggregate sample size (under 2,300) remains modest for a condition this complex. Until standardized metagenomic protocols and longitudinal biomarker-confirmed designs become routine, microbiome diagnostics for AD remain scientifically premature. This is a confirmatory, field-clarifying finding that should redirect research investment toward functional and compositional markers.
