CLINICAL REVIEW - 05/03/2026

Neurology | Gastroenterology | Precision Medicine

The Gut–Brain Axis in APOEε4 Carriers:

Microbiome Dysbiosis, Neuroinflammation, and Alzheimer’s Disease Risk

An Evidence-Calibrated Review for Physicians and Advanced Clinicians

Brian Paquette, DO, MPH

Board Certified Neurology, Interventional Pain Medicine

Indiana Polyclinic, JWM Neurology, Indianapolis, IN

Peer-reviewed, evidence-based, AI-enhanced, for educational purposes only

Keywords: APOE4; gut microbiome; neuroinflammation; intestinal permeability; Alzheimer’s disease; lipid metabolism; bile acids; short-chain fatty acids; endotoxemia; blood–brain barrier; evidence calibration

Preface: Evidence Strength by Domain

A central critique of prior reviews in this space — is that mechanistic plausibility and published association data are presented with a degree of certainty that outpaces the available evidence. To address this systematically, the following table summarizes the authors’ assessment of evidence strength by domain, using criteria analogous to GRADE methodology. Readers are encouraged to use this table as a calibration frame when interpreting the sections that follow.

Domain	Evidence Strength	Basis	Key Caveat
APOEε4 → Alzheimer’s disease risk	Strong	Multiple large cohort studies; meta-analyses; mechanistic replication	Does not establish modifiable pathway
APOEε4 → BBB dysfunction	Moderate–Strong	Human CSF/MRI studies (Montagne 2020); mechanistic animal data	Directionality partially uncertain
Gut microbiome → neuroinflammation	Moderate	Germ-free animal models (Erny 2015); SCFA mechanistic data	Causal human evidence very limited
APOEε4 → gut microbiome composition	Weak–Preliminary	Small, heterogeneous human cohorts (Vogt 2017); inconsistent replication	No validated APOE4-specific signature
Endotoxemia → amyloid pathology	Moderate (animal)	LPS infusion models; co-localization data (Zhan 2016)	Human intervention data absent
Bile acid dysbiosis → AD biomarkers	Moderate	ADNI metabolomics (MahmoudianDehkordi 2019)	Associative; confounding not excluded
SCFA depletion → microglial activation	Moderate (animal)	Germ-free mouse reconstitution studies	Human SCFA–microglia data lacking
Mediterranean diet → cognitive protection	Moderate	PREDIMED, MAPT, MIND cohort data	APOE4-stratified trial evidence weak
Probiotics/prebiotics → AD prevention	Preliminary	Small RCTs; no APOE4-stratified biomarker trials	No disease-modifying evidence in humans
Butyrate supplementation → neuroprotection	Preliminary (animal)	Animal model data; limited human trials	Human dose-response and safety data lacking