Psychobiotic food systems: fermentation-derived microbes and metabolites modulating the microbiota–gut–brain axis

Abstract Psychobiotics are emerging microbiota-targeted interventions with potential relevance for mood, stress resilience, anxiety, sleep, cognition, and neuroimmune regulation. Although initially viewed mainly as probiotic strains acting through the microbiota–gut–brain axis, the concept is now expanding toward food-associated systems integrating live microorganisms, fermented matrices, prebiotic substrates, postbiotic fractions, and neuroactive metabolites. This review examines psychobiotic foods as integrated strain–matrix–metabolite–host systems, emphasizing fermentation as both a source of candidate microorganisms and a biotechnological process generating gut–brain-relevant bioactives. Fermented dairy products, kefir, sourdough, cereals, legumes, vegetables, fruits, tea, cocoa, coffee, and plant-based products may provide strains and metabolites such as γ-aminobutyric acid, short-chain fatty acids, tryptophan derivatives, bioactive peptides, exopolysaccharides, organic acids, vitamins, and phenolic metabolites. These compounds may influence gut–brain communication through microbial ecology, vagal signaling, HPA-axis regulation, immune modulation, intestinal and blood–brain barrier integrity, oxidative-stress control, and enteroendocrine pathways. However, evidence remains heterogeneous, with stronger support for depressive symptoms than for anxiety, sleep, cognition, or other psychiatric outcomes. Future development requires strain authentication, genome-based safety screening, controlled fermentation, metabolomics, simulated digestion, undesirable metabolite monitoring, and well-powered human trials linking food composition to validated mental-health-related endpoints.