Endogenous lysine in ileal digesta in the growing rat determined using different methods

BACKGROUND: Endogenous lysine in ileal digesta has been determined directly using highly digestible proteins and hydrolysates and assuming a complete absorption of dietary amino acids. In this study, the endogenous lysine in the ileal digesta of rats fed casein was determined using the guanidination method (baseline) and directly (assuming complete digestion and absorption) for casein and a highly hydrolysed casein. The peptide alimentation/ultrafiltration technique was also used to determine the endogenous ileal lysine for the casein hydrolysate (Peptopro®).RESULTS: Mean endogenous ileal lysine for rats given the unguanidinated casein and determined directly assuming complete digestion and absorption (644 mg kg⁻¹ dry matter intake (DMI)) or for a highly hydrolysed casein with the same assumption (596 mg kg⁻¹ DMI) was significantly (P < 0.05) higher than the endogenous ileal lysine for casein determined using the guanidination method (274 mg kg⁻¹ DMI). The endogenous lysine in ileal digesta for a casein hydrolysate determined using the alimentation/ultrafiltration method (302 mg kg⁻¹ DMI) was not significantly (P < 0.05) different from that based on the guanidination method but was significantly (P < 0.05) lower than the equivalent endogenous lysine determined directly.CONCLUSION: Dietary amino acids are not completely absorbed from casein or its hydrolysate and determining endogenous amino acids in ileal digesta directly, assuming complete absorption, overestimates the amount of endogenous ileal amino acids.