Incorporation of urea and ammonia nitrogen into ileal and fecal microbial proteins and plasma free amino acids in normal men and ileostomates

Background: The importance of urea nitrogen reutilization in the amino acid economy of the host remains to be clarified. Objective: The objective was to explore the transfer of N-15 from orally ad ministered [N-15(2)]urea or (NH4Cl)-N-15 to plasma free and intestinal micr obial amino acids. Design: Six men received an L-amino acid diet (167 mg N . kg(-1).d(-1); 186 kJ . kg(-1) . d(-1)) for 11 d each on 2 different occasions. For the last 6 d they ingested [N-15(2)]urea or, in random order,(NH4Cl)-N-15 (3.45 mg N -15 . kg(-1) . d(-1)). On day 10, a 24-h tracer protocol (12 h fasted/12 h fed) was conducted with subjects receiving the N-15 tracer hourly. In a sim ilar experiment, (NH4Cl)-N-15 (3.9 mg N-15 . kg(-1) . d(-1)) was given to 7 ileostomates. N-15 Enrichments of urinary urea and plasma free and fecal o r ileal microbial protein amino acids were analyzed. Results:N-15 Retention was significantly higher with (NH4Cl)-N-15 (47.7%; P < 0.01) than with [N-15(2)]urea (29.6%). Plasma dispensable amino acids af ter the (NH4Cl)-N-15 tracer were enriched up to 20 times (0.2-0.6 N-15 atom % excess) that achieved with [N-15(2)]urea. The N-15-labeling pattern of pl asma, ileal, and fecal microbial amino acids (0.05-0.45 N-15 atom% excess) was similar. Appearance of microbial threonine in plasma was similar for no rmal subjects (0.14) and ileostomates (0.17). Conclusion: The fate of N-15 from urea and NH4Cl differs in terms of endoge nous amino acid metabolism, but is similar in relation to microbial protein metabolism. Microbial threonine of normal and ileostomy subjects appears i n the blood plasma but the net contribution to the body threonine economy c annot be estimated reliably from the present data.