Dietary Protein Structure Affects Endogenous Ileal Amino Acids But Not True Ileal Amino Acid Digestibility in Growing Male Rats1–3

Abstract

Background: The amount of endogenous, as opposed to undigested dietary, protein in digesta is a measure of fundamental interest related to gut physiology and function.Objective: The objective of this study was to determine whether alimentation with proteins having differing amino acid compositions influenced endogenous ileal amino acids (EIAAs) and true ileal amino acid digestibility (TIAAD) values.Methods: Male rats (n = 8) were fed a purified diet containing 100 g/kg of 1 of 5 protein hydrolysates, each derived from a different semipurified intact protein source [gelatin, beef muscle (BM), casein, soy protein isolate (SPI), and lactalbumin] devoid of antinutritional factors or fiber. The rats were fed their respective hydrolysate-based diet for 1 d after receiving the same diet but containing the corresponding intact protein source for 7 d. Titanium dioxide was used as an indigestible marker. Ileal digesta were collected after the rats were killed, and EIAAs were determined (precipitate + retentate) after centrifugation and ultrafiltration of the digesta. The TIAAD values of the intact protein sources were determined using EIAA flows based on each protein hydrolysate.Results: Mean EIAA flows differed (P < 0.05) across protein hydrolysates for most amino acids, with the mean ± SEM EIAA flows across amino acids being 262 ± 17, 253 ± 12, 248 ± 18, 226 ± 14, and 191 ± 20 mg/kg dry matter intake for the gelatin, BM, casein, SPI, and lactalbumin hydrolysates, respectively. The only difference (P < 0.05) for the mean EIAA flows across amino acids within each protein hydrolysate was observed between gelatin (262 ± 17 mg/kg) and lactalbumin (191 ± 20 mg/kg) hydrolysates. Except for Trp (P < 0.001) in gelatin and lactalbumin hydrolysates, EIAA flows determined using the casein hydrolysate were not different (P ≥ 0.05) from EIAA flows determined using the other protein hydrolysates. TIAAD values were not generally different (P ≥ 0.05) regardless of the hydrolysate used to determine the EIAA flows.Conclusions: Protein source affected EIAA flows, although the differences had little effect on TIAAD. Enzyme hydrolyzed casein is a suitable model hydrolysate for determining TIAAD with the enzyme-hydrolyzed protein-ultrafiltration technique.