Duodenal vs. gastric administration of labeled leucine for the study of splanchnic metabolism in humans

Low-rate (6 ml/h) intragastric infusion of stable, isotope-labeled amino ac ids is commonly used to assess the splanchnic handling of amino acids in hu mans. However, when used in the postabsorptive state, this method yields un reliable plasma isotopic enrichments, with a coefficient of variation >10%. In this metabolic condition, we confirmed in six subjects that an intragas tric infusion of L-[H-2(3)]leucine at 6 ml/h yields an unreliable isotopic steady state in plasma amino acids with a coefficient of variation of 43 +/ - 12% (mean +/- SD). In five additional subjects, we assessed the effects o f 1) increasing the rate of delivery of a leucine tracer in an isotonic pla smalike solution at 240 ml/h into the gastric site, and 2) changing the sit e of infusion from gastric to duodenal with this same high rate of delivery . In contrast to the gastric route, and regardless of the rate of delivery, only the intraduodenal route allowed 1) isotopic plasma steady state (i.e. , coefficients of variation were <10%: 5 +/- 3%), and 2) reproducible leuci ne extraction coefficients (22 +/- 5%). We conclude that an infusion site t hat bypasses the gastric emptying process, i.e., the duodenal route, along with delivery of a plasmalike solution, is necessary to reach isotopic stea dy state in plasma when labeled leucine is infused into the gastrointestina l tract in the postabsorptive state.