Proteins turn over continuously. Protein breakdown generates toxic ammonia. Glutamine and urea synthesis are the main pathways for ammonia detoxification, but their mutual dependence is still not sorted out. A ~5% reduction in urea synthesis causes lethal hyperammonemia, but a 35% reduction in glutamine synthesis produces no phenotype, even though glutamine synthetase is, thus far, held responsible for preventing hyperammonemia. Using tissue-specific knockout mice and stable isotope-based fluxomics, we investigate the interorgan dependence and (patho )physiological determinants of glutamine and urea synthesis, and the requirement of glutamine synthesis for urea synthesis. Environmental conditions that are studied include high-protein diets (high-protein diets are satiating and facilitate weight reduction), fasting (a proxy for cachexia), and intestinal bacterial overgrowth and acidosis (the encephalopathy-provoking condition in cirrhotic patients).