Effect of the route of nutrient administration on whole-body protein kinetics in man

The kinetic mechanism resulting in nitrogen (N) retention during oral or continuous intravenous feeding (IVF) was investigated in normal subjects and hospitalized patients. The whole-body protein turnover rate (Q) in these adult subjects was estimated using a primed-constant infusion of 15N-glycine and measuring the isotope enrichments in urinary urea and ammonia. Based on a stochastic model, the total protein synthesis (S) and breakdown (C) rates were calculated from N excretion and intake. These protein kinetic responses were studied either after brief (two or three days) or extended (ten days) fasting or during the fourth day of defined formula oral feeding or the tenth to 14th day of parenteral repletion by IVF. The mean daily intake during feeding was 270 +/- 7 mg N/kg and 28 +/- 2 nonprotein kcal/kg. The results from a total of 120 studies are summarized. In the absence of any N intake, depleted patients with cancer demonstrated a higher Q (+16%), with increased S (+25%) than depleted patients without cancer or acutely depleted normals. In all groups, protein kinetics were increased to a greater extent by oral feeding, compared to IVF. All subjects are in positive N balance during feeding and the mean N retention is 32 +/- 3%, which is not different among groups. Protein accretion during oral feeding is due to an increase in S and a similar decrease in C of body protein. During IVF this is achieved by a larger decrease in C with relatively little or no change in S.