Comparison of the enzyme-hydrolyzed casein, guanidination, and isotope dilution methods for determining ileal endogenous protein flow in the growing rat and pig

The objectives of the two studies were to determine whether the guanidination and isotope dilution methods applied both by labeling the animal (15N-infusion method) and by diet (15N-dilution method) give similar estimates of ileal endogenous lysine (EL) and endogenous nitrogen (EN) flows, respectively, to that of the enzyme-hydrolyzed casein (EHC) method in the growing pig and to determine whether the guandination and 15N-dilution methods give similar estimates of EL and EN flows, respectively, to that of the EHC method in the rat. For the first study, the test diet contained guanidinated and enzymatically hydrolyzed casein (molecular weight < 5,000 Da), which was also labeled with 15N. Rats (n = 30; mean BW 178 g) and pigs (n = 6; mean BW 19.2 kg) received a preliminary EHC-based diet for 7 d. The test diet was then given to the rats and pigs on d 8. Digesta were sampled from the terminal 20 cm of ileum of killed animals. The EL flows determined using the guanidination method were lower than those determined using the EHC method (means of 298 vs. 382, and 214 vs. 287 microg/g of DMI, in the rat and pig, respectively; P < 0.05 for the rat and P < 0.01 for the pig). The EN flows determined with the 15N-dilution method were lower than those determined using the EHC method (means of 1,034 vs. 1,942 and 1,011 vs. 1,543 microg/g of DMI, in the rat and pig, respectively, P < 0.001 for the rat and P < 0.05 for the pig). In the second study, pigs (n = 6; mean BW 27 kg) were continuously infused via the jugular vein with 15N-leucine for 11 d. The pigs received an EHC-based diet (molecular weight < 5,000 Da) during this 11-d period, after which digesta were sampled at the terminal ileum under anesthesia. The EN flow determined using the 15N-infusion method (mean of 1,971 microg/g DMI) was higher (P < 0.01) than that determined using the EHC method (mean of 1,233 microg/g DMI). The guanidination method gave a lower estimate of EL flow than did the EHC method in both the pig and rat. The 15N-dilution method also gave a lower estimate of EN flow than the EHC method in the pig and rat, and the 15N-infusion method gave a higher estimate of EN flow than the EHC method in the pig.