Postprandial modulation of dietary and whole-body nitrogen utilization by carbohydrates in humans

Background: Sucrose exerts a sparing effect on whole-body protein metabolis m, mainly during the absorptive phase. Objective: We aimed to characterize the acute postprandial effect of additi on of sucrose on deamination of dietary and endogenous nitrogen, with parti cular consideration being given to the effects of bioavailability. Design: Twenty-one subjects equipped with ileal tubes ingested N-15-labeled soy protein combined with [C-13]glycine, with (n = 10) or without (n = 11) sucrose. Dietary and endogenous ileal flow of nitrogen were determined fro m the ileal effluents. The kinetics of dietary amino acid transfer to the b lood were characterized by (CO2)-C-13 enrichment in breath and N-15 enrichm ent in plasma amino acids. Deamination of dietary and endogenous amino acid was determined from body urea, urinary nitrogen, and N-15 enrichment. Results: (CO2)-C-13 recovery in breath and N-15 plasma amino acid enrichmen ts were highly correlated (R-2 greater than or equal to 0.95, P < 0.001, fo r both meals) and markedly delayed by sucrose (half-(CO2)-C-13 recovery: 27 4 min compared with 167 min), whereas exogenous and endogenous ileal nitrog en kinetics and balances remained unchanged. Addition of sucrose halved the early (0-2 h) deamination peak of dietary nitrogen and reduced endogenous nitrogen oxidation over the first 4 h. Both were reduced by 18-24% over the 8-h period after the meal. Conclusions: Without changing the nitrogen absorptive balance, sucrose mark edly affected the bioavailability profile, which is governed by gastric emp tying. Endogenous and dietary nitrogen were not spared in the same way and over the same periods, showing that the metabolism of endogenous and dietar y nitrogen may be affected differently by nutritional modulation, even if t he effects are of a similar magnitude over the entire postprandial period.