Exogenous and endogenous contributions to nitrogen fluxes in the digestive tract of pigs fed a casein diet. III. Recycling of endogenous nitrogen.

The aim of the present study was to measure the incorporation of infused 15N in blood fractions, urine, digesta, faeces and in the exocrine pancreatic and biliary secretions, in order to estimate the endogenous part of nitrogen in the ileal digesta and in the faeces of pigs fed a casein diet and to calculate the total endogenous nitrogen secretion as well as its recycling in the digestive tract. For 8 d 11 Large White female pigs (50.1 +/- 1.8 kg) received a continuous infusion of L-[15N]leucine via a catheter in the jugular vein. The 15N-enrichment was measured in several fractions. The 15N-level of the pancreatic juice was higher than that in the biliary secretion, TCA-blood fractions, and urine during the whole experimental period. Using the 15N-isotope dilution method it was found that casein was completely digested up to the terminal ileum and that all the nitrogen in the ileal digesta was of endogenous origin. The total endogenous secretion was estimated at approximately 11 g N/d. The reabsorption of endogenous nitrogen amounted to 79% up to the end of the small intestine and 88% over the whole digestive tract.