Postbiotics in functional foods: Emerging bioactive metabolites, mechanisms of action, gut microbiome modulation, and health benefits: A review

Postbiotics are preparations of inanimate microorganisms and/or their components that confer a health benefit on the host, as defined by the International Scientific Association for Probiotics and Prebiotics (ISAPP). They have emerged as a distinct category of functional food ingredients because they do not require microbial viability and they may therefore reduce some technological constraints associated with live probiotic cultures. This narrative review synthesizes evidence on the bioactive components and metabolites associated with postbiotic preparations, including short-chain fatty acids (SCFAs), exopolysaccharides (EPS), bacteriocins, bioactive proteins and peptides, cell-wall fragments, organic acids, tryptophan metabolites, enzymes, and vitamins. Particular attention is given to mechanisms involving intestinal barrier reinforcement, immunomodulation, antimicrobial activity, gut–microbiome interactions, and the gut–brain axis, while distinguishing postbiotic preparations from isolated or purified metabolites, which are not themselves postbiotics under the ISAPP definition. Clinical and preclinical evidence covering metabolic health, gastrointestinal disorders, immune function, and emerging psychobiotic applications is considered with emphasis on effect sizes, study limitations, and the distinction between experimental and human evidence. The review also examines functional-food applications, delivery technologies, safety considerations, regulatory challenges, and priorities for standardization and clinical validation.