Dietary arginine uptake by the splanchnic region in adult humans

To determine the uptake of dietary arginine and leucine by the splanchnic region, two experiments were carried out, each involving four healthy young adult men who received a diet supplying 1 g protein · kg^-1 · day^-1 for 7 and 10 days before conducting a primed constant tracer infusion protocol. In study 1, subjects received for 8 h (3-h fast; 5-h fed state, achieved by a constant intragastric infusion of the diet formula) L-[5,5-²H₂; guanidino- ¹⁵N₂]arginine ([M₄]Arg), L-[guanidino-¹³C]arginine ([¹³C]Arg), and L- [5,5,5-²H₃]leucine ([²H₃]Leu) simultaneously by an intragastric infusion on day 7 and a repeat of this protocol on day 10 except with tracer administration given by vein. Plasma arginine fluxes were essentially the same for the two arginine tracers but differed significantly with route of administration. In study 2 the subjects received on day 7 a constant intravenous infusion of [¹³C]Arg and [²H₃]Leu and a simultaneous intragastric infusion of [M₄]Arg and [1-¹³C]leucine. On day 10 the routes of administration of these tracer pairs were reversed. During the fed state in study 1, splanchnic uptake of dietary arginine was 31 ± 10 and 34 ± 8%, based on the [¹³C]Arg and [M₄]Arg tracers, respectively, and it was significantly higher (P < 0.01) than for leucine, which was 10 ± 6%. In study 2, splanchnic uptake of dietary arginine, estimated from a series of tracer-protocol combinations for the fed state, was ~38% compared with a lower (P < 0.01) value of ~15% for leucine.