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Manuscript received 4 March 1998. Initial reviews completed 27 April 1998. Revision accepted 3 July 1998.
, and
, **, ,
* Departments of Nutritional Sciences and Paediatrics, University of Toronto, Toronto, ON, Canada M5S 3E2; ** The Research Institute, The Hospital for Sick Children, Toronto, ON, Canada M5G 1X8; and Department of Agricultural, Food & Nutritional Sciences, University of Alberta, Edmonton, AB, Canada T6G 2P5
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 
0.2%). Steady fed state conditions were established by feeding subjects eight hourly meals beginning 4 h before the start of the oral infusion protocol. Isotopic plateau in CO2, plasma and urine was achieved within 120 min of phenylalanine or lysine infusion. At isotopic plateau, the mean ratio of plasma to urine enrichment was 1.0 ± 0.04 (SEM), 0.39 ± 0.03 and 0.97 ± 0.02 for L-[1-13C]phenylalanine, L-[1-13C]lysine (D-lysine = 1.6%) and L-[1-13C]lysine (D-lysine 0.2%), respectively. There was good agreement between isotopic enrichment in plasma and urine for L-[1-13C]phenylalanine and L-[1-13C]lysine (D-lysine 0.2%). However, use of L-[1-13C]lysine (D-lysine = 1.6%) resulted in significantly higher enrichment in urine, probably due to renal tubular discrimination of D-lysine. Mean flux rates for phenylalanine and lysine were consistent with the literature. Thus, the oral, primed, equal dose infusion produces the isotopic steady-state condition required for amino acid flux and oxidation determination within an 8-h period.
The Journal of Nutrition Vol. 128 No. 11 November 1998,
pp. 1913-1919
Copyright ©1998 by the American Society for Nutritional Sciences
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