The rate at which digested protein enters the small intestine modulates the rate of amino acid digestibility throughout the small intestine of the growing pig

Background: Actinidin, a cysteine protease in kiwifruit (KF), increases both the gastric digestion and gastric emptying rate of beef muscle protein. Objective: To determine the relationship between the rate of digested nitrogen (N) entering the small intestine, SI (a function of the extent of gastric digestion and gastric emptying rate), and the disappearance of amino acids (AAs) in different parts of the SI, at set times post-feeding. Methods: Entire male pigs (n=90; mean±SD: 28±2.9 kg bodyweight) were given a beef meat-based diet for three days. The beef-based diet was supplemented with green KF pulp (containing actinidin), gold KF pulp supplemented with actinidin, or gold KF pulp alone (no actinidin). On day 3, pigs were euthanized at 0.5, 1, 3, 5 and 7 h postprandially. Stomach chyme was analyzed to determine the rate of digested N entering the SI. Apparent AA digestibility at set times, was determined in the proximal, medial and distal SI. Results: The rate of digested N entering the SI was higher (P<0.001) with actinidin. Actinidin also increased the apparent AA digestibility at the proximal and medial SI (P≤0.05) at set times, but not in general for the distal SI (P>0.05). At the proximal SI, the apparent AA digestibility was greater correlated with the digested N entering the SI (r=0.73, P<0.001; n=57) than it was correlated with gastric emptying (r=0.64, P<0.001; n=57) or gastric protein digestion (r=0.57, P<0.001; n=57). Similar trends were observed for the medial SI. Conclusions: The rate of digested N entering the SI is an accurate predictor of the rate of AA digestibility in the SI. Proteins entering the SI that are already highly digested are mainly digested and absorbed in the first half of the SI, while proteins entering the SI poorly digested are largely absorbed in the second half of the SI.