Our aim was to characterize the postprandial total and dietary N fluxes in the portal drained viscera (PDV) and whole body after administration of a single meal in young pigs. Seven 4-wk-old piglets, implanted with a portal flow probe and portal, arterial and venous catheters, received a primed constant [18 O]urea intravenous infusion and were studied for 8 h after a bolus mixed meal ingestion (46 mmol N/kg body wt) intrinsically labeled with 15 N to trace dietary N fluxes. The real cecal digestibility of the formula was 94.3% (SD 1.8). PDV output of dietary N was found principally in the pool of circulating protein (51% of the measured dietary N PDV output), in the free ␣-amino N pool (44%), and to a lesser extent in ammonia (5%). Dietary N release in ␣-amino N and ammonia mainly occurred during the first 3 h. Total and exogenous postprandial urea productions were 5.8 and 2.0 mmol N/kg body wt, respectively. At the end of the postprandial period, losses of dietary N amounted to 10.3% of the dose: 5.7% through ileal losses and 4.6% by deamination and transfer to urea. Net postprandial retention of dietary N was 90.4% (SD 1.3), of which 20% was found in splanchnic zone (small intestine 10%, liver 5%, and plasma protein 3%) and 42% in peripheral zone (muscle 31%, skin 6%). In conclusion, our results show a high efficiency of dietary N utilization for muscular uptake and anabolic utilization. However, the results obtained point out the necessity to further explore the form of dietary N released into the portal blood. dietary nitrogen; postprandial metabolism; portal drained viscera; nonsteady state; pigs DURING THE POSTPRANDIAL PHASE, the amino acids absorbed by the gut are subjected to extensive metabolism during passage through the intestinal mucosa and liver before being available to peripheral tissues, particularly muscle. Tissue protein synthesis is enhanced in most protein pools in this absorptive state, and both dietary and endogenous amino acids are used to replete protein stores, which are depleted during the postabsorptive state (maintenance), and to allow the net accretion of tissue protein, especially in very young growing animals (11). However, uncertainties remain regarding the form in which dietary N is absorbed, metabolized within the enterocyte, released through the portal circulation, and made available for peripheral tissues after hepatic metabolism (45). The temporal fluxes of both dietary and proteolysis-derived amino acids through catabolic or synthetic pathways in different organs after a meal are also unclear and have received little attention despite evidence of a specific time frame during which stimulation of protein synthesis occurs (8,36).The lack of information about the kinetics and metabolic fate of dietary nitrogen is due to technical and ethical constraints for accessing different organs and tissues, particularly in nonsteady-state conditions, where studies are rare. Studies in humans are indirect, because it proves difficult to access compartments other than blood, giving insight ...
