Nutrition and gut-brain axis: opposing effects of dietary fiber and Western-style diets on Alzheimer's disease.

PURPOSE OF REVIEW: This review summarizes how diet shapes the gut-brain axis and contributes to Alzheimer's disease (AD) pathology, emphasizing the contrasting effects of Western-style diets and dietary fiber.

RECENT FINDINGS: Western diets rich in sugar and saturated fat disrupt gut microbial balance, increase intestinal permeability, and promote systemic inflammation, oxidative stress, and lipid metabolic imbalance, all of which accelerate neurodegeneration. In contrast, dietary fiber supports microbial diversity, improves lipid and glucose metabolism, and reduces neuroinflammation through both short-chain fatty acid (SCFA)-dependent and independent pathways involving bile acids, microbial lipids, and immune modulation. Recent animal and clinical data show that mixed-fiber supplementation can restore metabolic stability and cognitive function. In the 5xFAD mouse model (a transgenic AD model overexpressing five familial AD mutations), adding low-dose fiber to a high-sugar diet reshaped gut microbiota and improved AD-like pathology, identifying a reproducible set of fiber-sensitive bacterial taxa.

SUMMARY: Dietary patterns exert opposing effects on gut-brain communication. Nutrient excess drives dysbiosis and neuroinflammation, while dietary fiber promotes metabolic balance and neuronal resilience. Understanding these context-dependent microbial and metabolic interactions may guide precision dietary strategies for AD prevention and therapy.