THE USE OF N-15-LABELED DIETARY PROTEINS FOR DETERMINING TRUE ILEAL AMINO-ACID DIGESTIBILITIES IS LIMITED BY THEIR RAPID RECYCLING IN THE ENDOGENOUS SECRETIONS OF PIGS

We assessed the use of N-15-labeled dietary proteins as a possible too l for the determination of the true ileal amino acid (AA) digestibilit y in pigs. The first experiment was designed to study the dietary N ex cretion pattern at the ileum subsequent to the ingestion of a single N -15-labeled meal. In a second experiment, we compared ileal endogenous AA outputs and true AA digestibility estimates obtained in pigs inges ting N-15-labeled dietary proteins in a single meal vs. intravenous in fusion of I [N-15]leucine for 10 d during the ingestion of a pea-based diet and a protein-free starch diet. The proportion of endogenous N f ound in the ileal digesta differed when the label was delivered orally (50%) vs. intravenously (72%) and changed with time. As a consequence , the true ileal AA digestibilities measured with labeled diets were l ower. A third experiment demonstrated that this was due to the rapid r ecycling of labeled dietary N in endogenous moieties, because N-15 was found in blood within 10 min of consuming the labeled meal, within 50 min of consumption in pancreatic enzymes, 90 min in bile and 4 h in i leal mucins. We conclude that the use of N-15-labeled meals for determ ination of true ileal AA digestibilities is limited by the fast recycl ing of dietary N in endogenous secretions following a single N-15-labe led meal. The accuracy of results will depend on meaningful estimates of AA flow during a limited period and accurate estimates of N-15 in A A.