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

Abstract

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 15N from orally administered [15N2]urea or 15NH4Cl 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 occasions. For the last 6 d they ingested [15N2]urea or, in random order, 15NH4Cl (3.45 mg 15N · kg-1 · d-1). On day 10, a 24-h tracer protocol (12 h fasted/12 h fed) was conducted with subjects receiving the 15N tracer hourly. In a similar experiment, 15NH4Cl (3.9 mg 15N · kg-1 · d-1) was given to 7 ileostomates. 15N Enrichments of urinary urea and plasma free and fecal or ileal microbial protein amino acids were analyzed. Results: 15N Retention was significantly higher with 15NH4Cl (47.7%; p < 0.01) than with [15N2]urea (29.6%). Plasma dispensable amino acids after the 15NH4Cl tracer were enriched up to 20 times (0.2-0.6 15N atom% excess) that achieved with [15N2]urea. The 15N-labeling pattern of plasma, ileal, and fecal microbial amino acids (0.05-0.45 15N 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 15N from urea and NH4Cl 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.