The primed, continuous intravenous infusion of amino acids labeled with 13C together with measurement of isotopic enrichment in plasma is commonly used to study amino acid metabolism. However, a less invasive, oral infusion that also produces an isotopic steady state in CO2 and urine would be useful, particularly for pediatric studies. We measured the 13C enrichments of expired CO2, plasma and urine free phenylalanine and lysine and estimated flux and oxidation rates in adult humans (n = 12) who received a 4-h oral, primed, equal dose infusion of either L-[1-13C]phenylalanine, L-[1-13C]lysine (D-lysine = 1.6%) or L-[1-13C]lysine (D-lysine

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