The Contribution of Endogenous urea to Faecal Ammonia in man, Determined by 15N Labelling of Plasma Urea

Abstract

To establish the role of endogenous urea as a source of fecal ammonia, the plasma urea of 2 healthy men was labelled with 15N at a constant level of several days and its 15N enrichment was compared with that of fecal ammonia and total N. Feces collected after 1 complete gastrointestinal transit from the onset of plasma labeling had ammonia 15N enrichments which were only 8.5 .+-. 1.2% and total N enrichments which were 6.8 .+-. 0.7% of the plasma urea 15N enrichment. These results show that endogenous urea is not the main precursor of fecal ammonia, which is probably derived by bacterial deamination from the protein of dietary residues, intestinal secretions and shed epithelial cells. The minor contribution of endogenous urea of fecal ammonia suggests that the lumen of the large bowel is not the main site of endogenous urea hydrolysis. The similar labelling of fecal total N and ammonia nitrogen supports other evidence that these fecal N fractions are in a constant state of exchange.