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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 r.-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, 15NH4C1 (3.45 mg 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 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 deal 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, deal, 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. Copyright (c) 1999 American Society for Clinical Nutrition [ABSTRACT FROM AUTHOR]

This review examines the involvement of the gastrointestinal tract in the utilization of nitrogen, the identities of the nitrogenous substances entering and leaving the gut, and the significance of this recycling in the overall nitrogen economy of the body. It is concerned with nonruminant mammals, including man. [ABSTRACT FROM AUTHOR]

Bioactive peptides either present in foods or released from food proteins during digestion have a wide range of physiological effects, including on gut function. Many of the bioactive peptides characterized to date that influence gut motility, secretion, and absorption are opioid agonists or antagonists. The authors review a body of experimental evidence that demonstrates an effect of peptides from food proteins on endogenous (nondietary) protein flow at the terminal ileum of simple-stomached mammals, including adult humans. At least some dietary peptides (1000-5000 Da) significantly enhance the loss of protein from the small intestine, causing an increased amount of protein to enter the colon. Food-derived peptides appear to either stimulate protein secretion into the gut lumen or inhibit amino acid reabsorption or influence both processes simultaneously. The effect of dietary peptides on small-intestine secretory-protein dynamics is discussed in the context of the major components of gut endogenous protein, sloughed cells, enzymatic secretions, mucin, and bacterial protein. [ABSTRACT FROM AUTHOR]