Abstract
Twenty‐four crossbred pigs of 15 kg initial bodyweight were fed four semi‐synthetic diets for 10 days according to a completely randomised design. The study aimed to determine the effects of state of body nitrogen balance and the presence of dietary peptides and protein in the digestive tract on the excretion of endogenous amino acids from the ileum of the pig. Endogenous lysine excretion was determined for pigs given a protein‐free (PF) diet, an enzymically hydrolysed casein‐ (EHC), a zein‐ (ZN) or a synthetic amino acid‐ (SAA) based diet. Digesta from the EHC‐fed animals were centrifuged and ultrafiltered after collection and the precipitate plus retentate fraction was used to determine the endogenous flows. Such processing excludes unabsorbed dietary amino acids from the measure of endogenous loss. ZN is naturally deficient in lysine and tryptophan and these two amino acids were omitted from the synthetic amino acid‐based diet to allow direct measurement of endogenous lysine flow. Pigs given the ZN and SAA diets received free lysine and tryptophan orally throughout the study except for the final 2 days of the study, when these amino acids were infused intravenously. Endogenous flows for amino acids other than lysine were determined for pigs given the PF and EHC diets. On the final day of the study the pigs were given their daily dietary allowance hourly and killed 10 h after the start of feeding. Digesta were collected from the terminal ileum (20 cm anterior to the ileo‐caecal junction) and endogenous flows were determined by reference to the marker chromic oxide. The mean endogenous ileal lysine flows for the ZN‐ and EHC‐fed pigs were not significantly different (overall mean, 419 mg kg^−1^ dry matter intake), but were higher (P < 0.05) than those for the PF‐ and SAA‐fed pigs (overall mean, 268 mg kg^−1^ dry matter intake) whose mean flows were not significantly different from each other. The mean endogenous ileal flows for amino acids other than lysine were higher (P < 0.05) for the EHC‐fed pigs compared to the animals on the PF diet, except for proline, glycine and arginine. The similar endogenous ileal lysine excretion for pigs receiving a SAA‐based diet and in positive body nitrogen balance, and PF‐fed pigs in negative body nitrogen balance, indicates that negative body nitrogen balance per se does not lead to a lowered endogenous ileal excretion. It would appear, however, that the presence of dietary peptides or protein in the gut increases amino acid excretion at the terminal ileum above that found with PF or SAA alimentation. Consequently, endogenous ileal amino acid flow in the pig may be underestimated when determined by the traditional PF method.