Revisiting gut microbiota-driven ammonia metabolism: from disease burden to physiological adaptation.

Ammonia homeostasis is governed by interconnected host and microbial pathways, including hepatic ureagenesis, glutamine synthesis, and gut microbiota-mediated urea hydrolysis, proteolysis, and amino acid deamination. Ammonia has historically been viewed primarily as a nitrogenous waste product and neurotoxic molecule, a concept strongly supported by studies of hyperammonemia and hepatic encephalopathy. Impaired hepatic clearance, portosystemic shunting, and enhanced gut-derived ammonia input increase systemic ammonia burden and are linked to neurological dysfunction, muscle wasting, immune dysregulation, and progression of liver disease. In the gut lumen and mucosal environment, however, gut microbes not only generate ammonia but can also reuse ammonia-derived nitrogen for amino acid synthesis, microbial biomass formation, and community nitrogen exchange. Evidence from selected physiological and preclinical models indicates that microbiota-derived ammonia may contribute to nitrogen recycling, microbial community maintenance, enteric neural regulation, or metabolic adaptation under defined conditions. These roles are more context-specific and less broadly established than the pathological effects of systemic hyperammonemia. When epithelial barrier integrity is compromised, hepatic clearance declines, or host buffering reserves are depleted, elevated ammonia can increase epithelial exposure, portal ammonia input, or peripheral blood ammonia. Conversely, evidence that reduced microbiota-derived ammonia contributes to disease remains limited and currently comes mainly from selected animal models. This review re-examines ammonia metabolism from a gut microbiota-centered perspective. We discuss the ecological origins, spatial distribution, exposure characteristics, host-interface effects, and context-specific outcomes of microbiota-derived ammonia in physiology and disease, and discuss how ammonia should be measured, stratified, and targeted in microbial nitrogen metabolism.