To study the fate of enterally delivered essential amino acids, leucine and phenylalanine, 14 healthy adults were infused in the postabsorptive state with [1-13C]leucine, [5,5,5-2H3]leucine, and [phenyl-2H5]phenylalanine for 7 h in a crossover design by intravenous and nasogastric tube routes. The amount of enterally delivered tracer that was sequestered by the splanchnic bed on the first pass was 21 +/- 1, 17 +/- 3, and 29 +/- 2 for the [13C]leucine, [2H]leucine, and [2H]phenylalanine tracers, respectively. Less than 2% of the nasogastric [1-13C]leucine tracer was oxidized on the first pass. We estimate that 40% of the nasogastric leucine tracer that was sequestered on the first pass was converted to alpha-ketoisocaproate and released, and 50% was incorporated into newly synthesized proteins. Assuming that less phenylalanine is incorporated into protein than leucine because of the lower abundance of phenylalanine in protein compared with leucine, we estimate that 80% of the extracted nasogastric phenylalanine tracer was converted to tyrosine. The study design also indicated a significant effect of duration of tracer infusion on the results, presumably due to recycling of tracer from rapidly turning over protein.