Plant-derived bioactives, the gut–brain axis, and neurodegenerative diseases: mechanistic roles of diet–microbiota interactions

Diet is increasingly recognized as a potential upstream modulator of the gut-brain axis (GBA) through its effects on the microbiome, microbial metabolites, and host immune and endocrine responses. The GBA is a complex, bidirectional network connecting the gastrointestinal tract and central nervous system, with diet influencing microbial community structure and metabolic output. Plant-based diets, such as Mediterranean and MIND, have been associated with increased production of anti-inflammatory microbial metabolites and improved barrier function, while high calorie/low nutrient diets are often linked to increased immune activation and barrier dysfunction. However, while microbial metabolites, especially short-chain fatty acids, indoles, bile acids, and isothiocyanates, have been proposed as mediators of neuroprotective effects, their role in neurodegenerative diseases remains an area of active investigation, with evidence largely derived from preclinical and associative human studies. Cruciferous vegetables, especially broccoli sprouts, are an emerging focus of research for their bioactive compound sulforaphane, which activates Nrf2-centered cytoprotective pathways. Animal and early human studies suggest sulforaphane can improve cognitive and behavioral outcomes, though larger clinical trials are needed. Personalized, microbiota-targeted dietary interventions may offer scalable strategies for managing neuroinflammatory and neurodegenerative conditions, and we emphasize the need for integrated research across diet, microbiome, and brain health.