Postbiotics in broiler nutrition: a critical review of mechanisms, efficacy, and applications in antibiotic-free production systems

The global phase-out of antibiotic growth promoters (AGP) in broiler production has intensified the need for safe, effective alternatives that maintain bird health and performance without driving antimicrobial resistance. Postbiotics, defined as preparations of inanimate microorganisms and their components that confer a health benefit on the host, have emerged as a promising class of feed additive. Postbiotics are processing-stable, free of viable-cell biosafety concerns, and amenable to compositional standardization. This review synthesizes evidence from 58 peer-reviewed studies (2010 to 2025) identified through a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-guided search of Scopus, Web of Science, PubMed, and Google Scholar, covering postbiotics derived from lactic acid bacteria (LAB), Bacillus spp., and Saccharomyces cerevisiae. Postbiotic supplementation consistently improved intestinal morphology, increasing jejunal villus height to crypt depth ratios by 9 to 36%, enhancing tight junction protein expression (occludin, claudin-1, ZO-1), and promoting beneficial shifts in cecal microbiota. These effects translated into improved growth performance, with average daily gain (ADG) increasing by 2 to 12% and feed conversion ratio (FCR) reduced by 0.05 to 0.18, alongside carcass yield improvements of 1.2 to 3.5% and enhanced meat quality. Under stress conditions, including heat stress, necrotic enteritis (NE), Salmonella infection, and mycotoxin exposure, postbiotics reduced pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) and oxidative damage, as indicated by lower malondialdehyde concentrations and increased antioxidant enzyme activity. Mechanistically, these effects involve microbial-associated molecular pattern signaling, short-chain fatty acid-mediated enterocyte proliferation, and pathogen exclusion via β-glucan and mannan. However, substantial heterogeneity in postbiotic characterization, dosage, and experimental design limits cross-study comparability. Future research should prioritize compositional standardization, dose-response evaluation, and large-scale commercial validation to support consistent field application.