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

Am J Clin Nutr. 1999 Dec;70(6):1046-58. doi: 10.1093/ajcn/70.6.1046.


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 occasions. 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.

Publication types

  • Clinical Trial
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Amino Acids / biosynthesis
  • Amino Acids / metabolism*
  • Ammonium Chloride / pharmacokinetics*
  • Bacterial Proteins / metabolism*
  • Diet
  • Feces / chemistry
  • Feces / microbiology
  • Female
  • Humans
  • Ileostomy
  • Ileum / metabolism*
  • Male
  • Middle Aged
  • Nitrogen / metabolism*
  • Urea / pharmacokinetics*


  • Amino Acids
  • Bacterial Proteins
  • Ammonium Chloride
  • Urea
  • Nitrogen