N-15 LEUCINE AND N-15-ISOLEUCINE ISOTOPE-DILUTION TECHNIQUES VERSUS THE N-15-ISOTOPE DILUTION TECHNIQUE FOR DETERMINING THE RECOVERY OF ENDOGENOUS PROTEIN AND AMINO-ACIDS IN DIGESTA COLLECTED FROM THE DISTAL ILEUM IN PIGS

Twelve gilts, with an average initial BW of 38 kg, were fitted with simple T-cannulas at the distal ileum and two catheters in the external jugular veins. The animals were fed twice daily (0800 and 2000) 700 g of one of four diets containing either soybean meal, canola meal, wheat, or barley as the sole protein source. N-15-leucine was infused continuously for 9 d at a rate of 40 mg/kg of BW daily via one of the jugular catheters. Blood samples were taken during feeding. Ileal digesta samples were collected continuously for 24 h on d 7 and 9 of the infusion period. N-15-enrichment excess in leucine and isoleucine in the trichloroacetic acid (TCA)-soluble fraction of blood and in digesta were measured using single-ion monitoring gas chromatography-mass spectrometry. Assuming that the N-15-enrichment excess in leucine and isoleucine in the TCA-soluble fraction of the blood is similar to that in endogenous protein secreted into the digestive tract and that the amino acid composition of endogenous protein is constant, the contribution of endogenous to total protein in ileal digesta was calculated using the N-15-isotope dilution technique for leucine and isoleucine, respectively. These contributions were much smaller for the N-15-leucine than for the N-15-isoleucine isotope dilution technique: 7.1 to 11.0 vs 21.8 to 24.9 g of protein/kg of DM intake. The values obtained with the N-15-isoleucine isotope dilution technique were close to those obtained with the N-15-isotope dilution technique as described in a previous publication.