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 15 N from orally administered [ 15 N 2 ]urea or 15 NH 4 Cl to plasma free and intestinal microbial 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 occa-sions. For the last 6 d they ingested [ 15 N 2 ]urea or, in random order, 15 NH 4 Cl (3.45 mg 15 N.kg -1 .d -1 ). On day 10, a 24-h tracer protocol (12 h fasted/12 h fed) was conducted with subjects receiving the 15 N tracer hourly. In a similar experiment, 15 NH 4 Cl (3.9 mg 15 N.kg -1 .d -1 ) was given to 7 ileostomates. 15 N Enrichments of urinary urea and plasma free and fecal or ileal microbial protein amino acids were analyzed. Results: 15 N Retention was significantly higher with 15 NH 4 Cl (47.7%; P < 0.01) than with [ 15 N 2 ]urea (29.6%). Plasma dispensable amino acids after the 15 NH 4 Cl tracer were enriched up to 20 times (0.2-0.6 15 N atom% excess) that achieved with [ 15 N 2 ]urea. The 15 N-labeling pattern of plasma, ileal, and fecal microbial amino acids (0.05-0.45 15 N atom% excess) was similar. Appearance of microbial threonine in plasma was similar for normal subjects (0.14) and ileostomates (0.17). Conclusion: The fate of 15 N from urea and NH 4 Cl differs in terms of endogenous amino acid metabolism, but is similar in relation to microbial protein metabolism. Microbial threonine of normal and ileostomy subjects appears in the blood plasma but the net contribution to the body threonine economy cannot be estimated reliably from the present data.