LEUCINE AND PHENYLALANINE KINETICS DURING MIXED MEAL INGESTION - A MULTIPLE TRACER APPROACH

To estimate whole body and splanchnic metabolism of dietary amino acids, phenylalanine and leucine kinetics were determined simultaneously in six normal volunteers before and during the constant administration of a complete mixed meal, employing multiple tracers of these amino acids. L-[5,5,5-H-2]leucine and L-[2,6-H-3]phenylalanine were infused intravenously; L-[1-C-13]leucine and L-[1-C-14]phenylalanine were administered orally with the meal. During the meal, steady-state leucine concentration rose from 136 +/- 6 to 190 +/- 14-mu-mol/l (P < 0.01), phenylalanine from 44 +/- 4 to 61 +/- 6-mu-mol/l (P < 0.01), total leucine rate of appearance (R(a)) from 1.29 +/- 0.03 to 1.77 +/- 0.07 (P < 0.01, +37 +/- 3%), and phenylalanine R(a) from 0.73 +/- 0.05 to 0.80 +/- 0.07-mu-mol.kg-1. min-1 (P < 0.05, +8 +/- 3%). Splanchnic uptake of dietary phenylalanine was greater (P < 0.001) than that of leucine (58 +/- 4 vs. 25 +/- 4%, respectively), 44 +/- 3% of circulating leucine derived from the diet vs. 20 +/- 2% of circulating phenylalanine (P < 0.01). Endogenous leucine and phenylalanine R(a) were significantly suppressed (P < 0.05). In summary: 1) splanchnic uptake of dietary phenylalanine is onefold greater than that of leucine; 2) dietary contribution to systemic phenylalanine R(a) is about half of that to leucine R(a); and 3) endogenous appearance of both leucine and phenylalanine after the meal is suppressed. In conclusion, splanchnic metabolism of dietary leucine and phenylalanine differs markedly and can be quantitated in vivo without catheterization.