An isotopic study of the effect of dietary carbohydrate on the metabolic fate of dietary leucine and phenylalanine

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Abstract
The fate of dietary leucine and phenylalanine was studied in five healthy, young adult men, by using a dual, stable isotope-tracer infusion approach to estimate amino acid fluxes, splanchnic (Sp) uptake, and delivery of absorbed amino acid to the peripheral circulation. Subject received two, 4-h tracer infusions of [l-13C]leucine and [15N]phenylalanine infused through a feeding tube placed in the duodenum, and [5,5,5-2H3]leucine, [ring-2H5]phenylalanine, and [6,6-2H2]glucose infused simultaneously by vein. In one experiment subjects received an amino acid mixture (83 mg amino acid · kg−1 · h−1) via the feeding tube and in the other experiment amino acids were supplied with carbohydrate (CHO) (167 mg · kg−1 · h−1). Sp uptake of dietary leucine decreased with added dietary CHO (29% of ingested leucine for amino acids alone vs 20% with CHO; P < 0.05) but was not different for phenylalanine (P > 0.05). Addition of CHO decreased both release of leucine via protein breakdown and leucine oxidation and increased body leucine balance (P < 0.05).

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