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Incorporation of urea and ammonia nitrogen into ileal and fecal microbial proteins and plasma free amino acids in normal men and ileostomates^1,2,3

¹ From the Massachusetts Institute of Technology, Laboratory of Human Nutrition and Clinical Research Center, Cambridge, MA; the German Institute of Human Nutrition, Bergholz-Rehbrücke, Germany; and the Rowett Research Institute, Aberdeen, Scotland.

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.

Key Words: Ammonia • urea • nitrogen metabolism • dispensable amino acids • indispensable amino acids • gut • microbial protein • stable isotopes • transamination • ileostomy • men

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