The Importance of the Gut–Muscle Axis: From Mechanistic Insights in Cell Culture and Rodent Models to Descriptive and Associative Evidence in Livestock

The gut microbiota is a metabolically active ecosystem that influences host physiology through bioactive metabolites and interactions with host signaling pathways. Recent research has established a bidirectional gut–muscle axis in which microbial metabolites and muscle-derived factors (myokines) regulate muscle protein synthesis, degradation, regeneration, fiber-type specification, and overall muscle performance. Studies using germ-free, antibiotic-treated, probiotic-supplemented, and fecal microbiota transplantation models demonstrate that the gut microbiota is a critical determinant of skeletal muscle mass and function. Key mediators include short-chain fatty acids, bile acids, aromatic amino acid metabolites, microbial-associated molecular patterns, and methylamine metabolites. This review summarizes current mechanistic knowledge of gut–muscle communication and its relevance to livestock production. In monogastric livestock, particularly pigs and poultry, microbiota transplantation experiments and targeted probiotic interventions provide causal evidence that gut microbial communities influence muscle growth, muscle fiber composition, intramuscular fat deposition, carcass traits, and meat quality, including tenderness, marbling, water-holding capacity, and flavor. Several studies have also identified specific microbial taxa and metabolites capable of transferring desirable production phenotypes. In contrast, evidence in ruminants remains largely associative and originates mainly from multi-omics and dietary intervention studies. Future research should validate causal mechanisms, identify robust microbial biomarkers, and develop species-specific microbiome-based strategies for precision livestock production.