Bioavailability of rumen-protected histidine, lysine, and methionine assessed using different in vivo methods.

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The objective of this experiment was to estimate the bioavailability (BA) of rumen-protected (RP) His, RPLys, and 2 RPMet products using 3 in vivo methods: area under the curve (AUC), plasma dose-response (PDR), and fecal free AA (FFAA) methods. We used 8 rumen-cannulated cows in a replicated 4 × 4 Latin square experiment with 16-d periods. Treatments were (1) abomasal infusion of water (control), (2) abomasal infusion of free His, Lys, and Met (FAA), (3) administration of RPHis + RPLys + RPMet1 (rumen-protected methionine protected with ethyl cellulose; RPAA1), and (4) administration of RPHis + RPLys + RPMet2 (rumen-protected methionine protected with a pH-sensitive polymer; RPAA2). On d 7 of each experimental period, a pulse-dose of water (control) or FAA were infused into the abomasum of the cows, or RPAA were placed directly in the rumen, and blood samples were taken from the jugular vein through a catheter 11 times over a 24-h period for the AUC method. Following the AA pulse-dose, infusion lines were installed into the abomasum for continuous infusion of FAA for the PDR method, or RPAA were fed from d 12 to d 16 and cows were fitted with urinary catheters for total collection of feces for the FFAA method. Fecal collection and blood sampling were conducted from d 14 to 16. Due to technical issues likely leading to unrealistic BA estimates, data for the PDR method are reported in the supplemental material. Relative BA based on the AUC method (computed as AUC of RPAA treatment plasma AA concentration divided by AUC of FAA treatment plasma AA concentration) was lower for RPMet1 compared with RPMet2 (43% vs. 61%) and was 45% (SEM = 3.35) and 72% (SEM = 5.99), for RPHis and RPLys, respectively. Rumen escape fractions of RPAA, estimated in a previous study using an in situ method, and digestibility data from the current study were used for calculations of BA for the FFAA method. Bioavailability based on the FFAA method was lower for RPMet1 (67%) compared with RPMet2 (91%) and was 87% (SEM = 0.71) and 75% (SEM = 2.75) for RPHis and RPLys, respectively. The relative differences in estimated BA based on both the AUC and FFAA methods between the RPMet products were as expected, based on literature, and data for all 4 RPAA products corresponded well with previously estimated BA using the FFAA method. The unrealistic data for the PDR method were likely caused by technical deviations from the original method (e.g., once-daily dosing of RPAA and inability to capture representative plasma concentration data with the sampling time points). Therefore, comparison of the PDR method with the AUC and FFAA methods were not possible in this study. Further comparisons are needed without deviations from the original PDR method. Variability in BA data and differences in estimated BA between the in vivo methods highlight the current challenges for accurate measurements of relative in vivo BA of RPAA products. Different protection technologies may call for different methodology to be used for BA estimations. Further research and standardization of in vivo BA methods are warranted.