Gut Microbial Metabolites at the Host Redox Interface: Mechanistic Evidence and Implications for Precision Animal Nutrition

Gut microbial metabolites can influence host redox regulation, but their relevance to precision animal nutrition depends on evidence linking microbial origin, tissue exposure, and a necessary host mechanism. This narrative review evaluates short-chain fatty acids, microbially modified bile acids, tryptophan derivatives, and sulfur metabolites across cellular, rodent, and livestock studies. Mechanisms are distinguished as direct chemical, metabolism-mediated, receptor-mediated, or associative, with receptor dependence assessed separately from redox dependence. Butyrate provides a strong metabolic example linking epithelial oxidation, oxygen consumption, hypoxia-inducible factor, and barrier function. Specific bile acid metabolites affect immune differentiation through mitochondrial reactive oxygen species, whereas necessary redox nodes remain less clearly established for indole derivatives. Sulfide can support mitochondrial electron transfer or inhibit respiration, depending on exposure and disposal capacity. Mechanistic evidence is strongest in experimental models; livestock studies commonly lack source attribution, tissue measurements, or targeted perturbation. Fermentable carbohydrate selection, balanced protein reduction, and defined bile acid or microbial preparations illustrate candidate nutritional applications rather than validated redox prescriptions. Translation requires species-specific exposure measurements, causal testing, processing stability, and independent evaluation of health, feed efficiency, and economic return. The framework prioritizes testable redox mechanisms while limiting claims unsupported by animal-level outcomes.