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

Abstract

Simple Summary The gut microbiota, the community of microorganisms inhabiting the gastrointestinal tract, is increasingly recognized as a key regulator of skeletal muscle growth, metabolism, and function through a bidirectional gut–muscle axis. Microbial metabolites, including short-chain fatty acids and bile acids, influence muscle protein synthesis, fiber-type composition, and energy metabolism, while muscle-derived signaling molecules can affect intestinal function and microbial composition. Experimental studies show that disruption of the gut microbiota impairs muscle development and performance, whereas microbiota transplantation and probiotic supplementation can improve muscle mass and function. In livestock, however, the strength of evidence differs among species. In monogastric animals such as pigs and poultry, fecal or cecal microbiota transplantation and targeted probiotic interventions provide causal evidence that gut microbial communities regulate intramuscular fat deposition, muscle fiber characteristics, carcass traits, and meat quality attributes including marbling, tenderness, water-holding capacity, and flavor. In contrast, evidence in ruminants is derived largely from microbiome–metabolome association studies and dietary interventions, which reveal links between microbial metabolites, muscle gene expression, and meat quality but have not yet established causality. This review summarizes current knowledge of the gut–muscle axis, highlights key metabolites and signaling pathways involved in muscle regulation, and discusses opportunities for microbiome-based strategies to improve livestock performance and meat quality.