Evaluation of the 15N-isotope dilution technique for determining the recovery of endogenous protein in ileal digesta of pigs: effect of the pattern of blood sampling, precursor pools, and isotope dilution technique.

The 15N-enrichments (atom percentage excess) were determined in the plasma free amino acids of blood samples taken at the time of feeding and in samples taken hourly and pooled over 12 h, as well as in ileal digesta, crude mucin, and bacteria collected at the distal ileum of pigs fed barley while continuously administered [15N]leucine intravenously. The branched-chain amino acids were the only indispensable amino acids to exhibit incorporation of 15N (P < .05). All dispensable amino acids exhibited some incorporation. Enrichments in free leucine and alanine were higher (P < .02) in blood samples taken at the time of feeding, compared to those in pooled blood samples, resulting in an underestimation of the endogenous ileal recoveries of these amino acids. Enrichments in amino acids in crude mucin were usually similar to those in pooled plasma samples, providing some support for the use of plasma free amino acids to estimate enrichments in endogenous amino acids in ileal digesta. Enrichments in bacteria were not different (P > .05) from those in ileal digesta. The recoveries of endogenous amino acids in ileal digesta determined with the [15N]leucine and 15N-amino acid dilution techniques demonstrate the overestimation of these criteria with the 15N-isotope dilution technique, applied in its current form, and suggest that modifications in the composition of endogenous protein can occur when pigs are fed protein-containing diets. These study supports the use of 15N-isotope dilution techniques, with modifications, for determining the recovery of endogenous protein in ileal digesta of pigs fed protein-containing diets.