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

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 [[1-[sup.13]C]leucine and [sup.15]N]phenylalanine infused through a feeding tube placed in the duodenum, and [5,5,5-[sup.2][H.sub.3]]leucine, [ring-[sup.2][H.sub.5]phenylalanine, and [[6,6[sup.2][H.sub.2]]glucose infused simultaneously by vein. In one experiment subjects received an amino acid mixture (83 mg amino acid . [kg.sup.-1] . [h.sup.-1]) via the feeding tube and in the other experiment amino acids were supplied with carbohydrate (CHO) (167 mg . [kg.sup.-1] . [h.sup.-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). Am J Clin Nutr 1993;57:161-9