Reformulation of a crystalline amino acid reference diet improves oxidation response but remains insufficient for assessing methionine bioavailability in extruded dog diets

Previously, a semi-synthetic crystalline amino acid (AA) reference (BAS) diet, limited in methionine (Met) but providing indispensable AAs (IDAA) at 120% of Association of American Feed Control Officials (AAFCO) recommendations for adult dogs had no oxidation response, indicating another limiting AA. Thus, we could not determine the metabolic availability (MA) of dietary Met in peas and chicken meal (ChM), the latter having the lowest oxidation level. Therefore, we sought to determine if reformulating the BAS diet AA profile to match the IDAA content of the ChM diet would allow for determination of the MA of Met in peas and ChM over two experiments. In the reformulated BAS63+ diet, we increased arginine, histidine, isoleucine, leucine, lysine, threonine, and valine relative to the original BAS63- diet, while maintaining Met at 63% of requirement (0.33% DM). In Exp 1, one neutered male dog (3 years old; 30.3 kg BW) was used in a two-period switchback design to receive BAS63- and BAS63+. Following a 2-d adaptation, the indicator AA oxidation (IAAO) technique was used. The dog received 13 small meals, with meal 6 containing a priming dose (9.4 mg/kg BW) of L-[1-13C]-phenylalanine (Phe, 99%), followed by a constant dose (2.4 mg/kg BW) in meals 6-13. Breath samples were collected to measure 13CO2 enrichment, and oxidation was calculated (F13CO2/kg BW/h). The BAS63+ diet improved oxidation by 84% and was used in Exp 2. In Exp 2, seven neutered male mixed-breed dogs (3 years old; 26.3 ± 2.0 kg BW) were used in a partially replicated 5 × 5 Latin square design to receive isonitrogenous and isoenergetic dietary treatments: BAS63+, ChM63 (ChM and lamb-based diet), and PEA63 (green pea and lamb-based diet), all providing Met at 63% (0.33% DM) of requirement. Two additional BAS diets were created with Met at 19% (BAS19+; 0.10% DM) and 40% (BAS40+; 0.21% DM) of requirement. This created two and three graded levels of Met for ChM and peas, and the BAS diet, respectively, allowing for a slope ratio approach to quantify MA using the BAS19+ diet as the common first point. The IAAO technique was performed in Exp 2 using the same protocol. Data were analyzed using Proc GLIMMIX with dog and period as random effects and diet, %Met, and their interaction as fixed effects. Overall, ChM had the lowest oxidation level, and the MA of Met in peas was 58% relative to ChM. The higher oxidation in the BAS63+ compared to ChM suggests another AA may still be limiting in the BAS diet.