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Am J Physiol Endocrinol Metab 283: E1223-E1231, 2002. First published August 20, 2002; doi:10.1152/ajpendo.0311.2001
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Vol. 283, Issue 6, E1223-E1231, December 2002

Phenylalanine kinetics in healthy volunteers and liver cirrhotics: implications for the phenylalanine breath test

I. Tugtekin1, U. Wachter1, E. Barth1, H. Weidenbach2, D. A. Wagner3, G. Adler2, M. Georgieff1, P. Radermacher1, and J. A. Vogt1

1 Sektion Anästhesiologische Pathophysiologie und Verfahrensentwicklung, Universitätsklinik für Anästhesiologie Ulm, und 2 Abteilung Innere Medizin I, Medizinische Universitätsklinik und Poliklinik Ulm, 89070 Ulm, Germany; and 3 Metabolic Solutions, Nashua, New Hampshire 03063

Expired 13CO2 recovery from an oral L-[1-13C]phenylalanine ([13C]Phe) dose has been used to quantify liver function. This parameter, however, does not depend solely on liver function but also on total CO2 production, Phe turnover, and initial tracer distribution. Therefore, we evaluated the impact of these factors on breath test values. Nine ethyl-toxic cirrhotic patients and nine control subjects received intravenously 2 mg/kg of [13C]Phe, and breath and blood samples were collected over 4 h. CO2 production was measured by indirect calorimetry. The exhaled 13CO2 enrichments were analyzed by isotope ratio mass spectrometry and the [13C]Phe and L-[1-13C]tyrosine enrichments by gas chromatography-mass spectrometry. The cumulative 13CO2 recovery was significantly lower in cirrhotic patients (7 vs. 12%; P < 0.01), in part due to lower total CO2 production rates. Phe turnover in cirrhotic patients was significantly lower (33 vs. 44 µmol · kg-1 · h-1; P < 0.05). When these extrahepatic factors were considered in the calculation of the Phe oxidation rate, the intergroup differences were even more pronounced (3 vs. 7 µmol · kg-1 · h-1) than those for 13CO2 recovery data. Also, the Phe-to-Tyr conversion rate, another indicator of Phe oxidation, was significantly reduced (0.7 vs. 3.0 µmol · kg-1 · h-1).

cirrhosis; stable isotopes; 13CO2 recovery; phenylalanine breath test


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