Table 2.

Feeding, isotope dosing, and sampling schedule on the day of each DAAO study

	Fasting state			Fed state
Time, min	−75	−50	−25	0	10	20	45	70	95	120	145	170	195	220	245	270	295
Food1				♦	♦	♦	♦	♦	♦	♦	♦	♦	♦	♦	♦	♦
		Background									Isotope steady state phase
L-[1-13C]-Phe2									♣*	♣	♣	♣	♣	♣	♣	♣
Samples
Indirect calorimetry3		♠	♠	♠			♠	♠	♠
Breath4							♥	♥	♥	♥	♥	♥	♥	♥	♥	♥	♥
Blood																	✔

Symbols denote dogs receiving the respective procedure during that time period. DAAO = direct amino acid oxidation; Phe = phenylalanine.

¹The food offered to each dog consisted of a Phe-deficient test diet top-dressed with 1 of 7 isonitrogenous Phe-alanine solutions to deliver total dietary Phe concentrations of 0.24, 0.29, 0.34, 0.44, 0.54, 0.64, and 0.74% (as-fed basis). Each meal represented one-thirteenth of the daily feed intake for each dog.

²Priming dose of L-[1-¹³C]-Phe was started with the sixth meal (time: 95 min) followed by continuous L-[1-¹³C]-Phe supply through the remaining 200 min of the study. Both priming and continuous isotope dosing solutions were top-dressed on the dry kibble meal.

³Three 25-min measures of respiratory gases were obtained prior to feeding to calculate resting volume of CO₂ produced (VCO₂). Starting at 45 min, VCO₂ was measured in 25-min intervals for the duration of the study.

⁴Three 25-min breath collections for ¹³CO₂ background breath samples were obtained at 45, 70, 95 min (fed state) before the isotope dosing. Breath samples were then collected every 25 min for the duration of the study.