The 15N amino acid dilution method allows the determination of the real digestibility and of the ileal endogenous losses of the respective amino acid in pigs.

We assessed the influence of sampling site when using the isotope dilution method to determine ileal endogenous N losses. Three growing pigs were prepared with ileorectal anastomoses and fitted with three catheters (portal, jugular and carotid). A 15N-leucine solution was infused for 24 d, alternating between the carotid artery and the jugular vein. Blood was sampled from the portal catheter and from the systemic catheter not used for the infusion. The pigs were fed successively a protein-free diet, an isolated pea protein diet and a hydrolyzed pea protein diet according to a Latin-square design. The 15N was transferred from leucine to isoleucine, valine, alanine, glycine and proline. Free 15N alanine, glycine and valine enrichments were closer to the respective amino acid enrichments in secretory tissues in the portal vein than in the systemic blood. The enrichment of total nitrogen was higher in the trichloroacetic acid-soluble fraction of the plasma than in the ileal digesta of pigs fed the protein-free diet. Lysine enrichment was significantly different from zero in all tissues except muscle, suggesting that essential amino acids can be synthesized by microflora and used for protein synthesis in the host. We conclude that the total nitrogen isotope dilution method is inappropriate to determine the endogenous loss of amino acids. Moreover, the amino acid dilution method should be performed with portal blood sampling. The main limitation of this method is that a number of essential amino acid losses cannot be determined.