FIGURE 2.

Typical study day protocol for each DAAO study in which graded intakes of phenylalanine were provided in random order. The experimental diet was given as 8 isocaloric, isonitrogenous meals representing 1/10th of each subject's daily requirement. Priming doses of the isotopes NaH¹³CO₃ and l-[1–¹³C]phenylalanine were given at the fifth meal; an hourly dose of l-[1–¹³C]phenylalanine was given simultaneously and continued throughout the remaining 4 h. Three baseline breath samples and 3 baseline urine samples were collected 45, 30, and 15 min before the tracer protocol began. Four plateau urines and 6 plateau breath samples were collected at isotopic steady state beginning 2.5 h after the tracer protocol began. The carbon dioxide production rate (VCO₂) was measured by indirect calorimetry after the fifth hourly meal. DAAO; direct amino acid oxidation.