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 · kg¹ · h¹; 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 · kg¹ · h¹) 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 · kg¹ · h¹).