Net postprandial utilization of [15N]-labeled milk protein nitrogen is influenced by diet composition in humans

The aim of this study was to follow the fate of dietary nitrogen to assess the postprandial utilization of purified milk protein and to determine the acute influence of energy nutrients. For this purpose, a [N-15]-labeling dietary protein approach was used. Twenty-five subjects swallowed an ileal tube and ingested [N-15]-milk protein alone or supplemented with either milk fat or sucrose. The absorption and postprandial deamination of dietary protein was monitored for 8 h, Sucrose delayed the absorption of protein longer than fat, but the ileal digestibility did not differ among groups (94.5-94.8%). Sucrose, but not fat, significantly reduced the postprandial transfer of [N-15]-milk nitrogen to urea. Consequently, the net postprandial protein utilization (NPPU) of milk protein calculated 8 h after meal ingestion was 80% when ingested either alone or supplemented with fat and was significantly greater with sucrose (NPPU = 85%). This study shows that energy nutrients do not affect the nitrogen absorption but modify the metabolic utilization of dietary protein in the phase of nitrogen gain. Our method provides information concerning the deamination kinetics of dietary amino acids and further allows the detection of differences of dietary protein utilization in acute conditions. The diet composition should be carefully considered, and protein quality must be determined under optimal conditions of utilization.