111 - Validation of the Indicator Amino Acid Oxidation Technique in the Domestic Cat

There is a lack of data on the minimum Met or total sulfur AA (SAA) requirements for adult cats. Non-invasive and precise techniques, such as indicator AA oxidation (IAAO) can be used for these purposes. This pilot study aimed to validate the IAAO technique in cats and investigate the effect of dietary Met concentrations and sources on fasted plasma SAA concentrations. Six cats were fed three experimental diets in random order: Met-deficient diet (BASAL; 0.23% Met+Cys-DM) and two Met-sufficient diets in which either DL-Met (DLM) or Met-hydroxy-analogue (MHA) were supplemented, respectively, on an equimolar basis to meet the total SAA requirement (0.34%-DM). After 2d diet adaptation, IAAO studies were performed. Cats were offered 13 ½-hourly small meals. The 6^th meal contained a priming dose (4.8 mg/kg-BW) of L-[1-¹³C]-Phe and the remaining meals the constant dose (1.04 mg/kg-BW). Breath samples were collected ½-hourly to measure ¹³CO₂ enrichment. The following morning after an 16hr fast, blood samples were collected. Isotopic steady-state was evaluated through linear regression models. Plasma AA data were analyzed using PROC GLIMMIX procedure in SAS (Version 9.4). Met concentrations tended to be higher in cats fed DLM compared to BAS (P = 0.0877), but similar to those fed MHA (P > 0.05). Total cysteine plasma concentrations were similar between treatments (P > 0.05). Cats fed BAS had greater concentrations of plasma HCys compared to DLM and MHA (P = 0.0093). Cats did not reach isotopic steady-state in atom percent excess (P >0.05) regardless of the dietary treatment provided. Non-steady-state kinetics models are under development to predict ¹³CO₂ flux. This study indicates that the short-term feeding of a Met-deficient diet in cats results in greater plasma Hcys and a tendency for lower plasma Met concentrations. A higher prime dose of L-[1-¹³C]-Phe is warranted to achieve isotopic steady-state and successfully apply the IAAO to estimate AA requirements.