Suitability of a recently proposed noninvasive L-[13C]leucine breath test for assessment of whole body leucine oxidation in maple syrup urine disease (MSUD) was examined. Oral L-[1-13C]leucine loads (38 μmol/kg body weight) were performed in overnight fasted MSUD patients (n= 6, classical form), obligate heterozygote parents (n= 6), and control subjects (n= 10). Three-hour13CO2exhalation kinetics were evaluated using curve fitting procedures. Venous blood was obtained in most cases and analyzed for13C-labeled plasma metabolites. In control subjects, maximal13CO2exhalation was reached attmax= 55 ± 18 min. Cumulative13CO2output at 3 h amounted to 4.7 ± 0.7 μmol × (kg body weight)-1. Estimated total13CO2exhalation was 7.2± 1.4 μmol × (kg body weight)-1(19.0 ± 3.6% of the dose). Half of this amount was expired att1/2= 130± 18 min. The data show a considerable degree of intersubject variability. Intraindividual variability was comparable, however, when checked in two volunteers. In obligate heterozygotes,13CO2kinetics were similar to controls (tmax= 35 ± 8 min,t1/2= 95 ± 16 min). Total13CO2output[5.7 ± 1.4 μmol × (kg body weight)-1] tended to be in the lower control range. None of the MSUD patients under study exhibited a significant increase in13CO2output after load. Maximal increase of label in plasma 4-methyl-2-oxopentanoate, the physiologic precursor of13CO2, was 16.1 ± 3.5 MPE in control subjects. In MSUD, label dilution was increased and correlated with the patients' leucine/4-methyl-2-oxopentanoate plasma levels. Considering the generally high variability of13CO2output and the unstable substrate pools in MSUD, we discuss the limitations of whole body leucine oxidation measurements by noninvasive approaches.Abbreviations: APE,atom percent13C-label enrichment;BCAA,branched chain L-amino acids;BCOA,branched chain 2-oxo acids;BCOA-DH,branched-chain 2-oxo acid dehydrogenase complex;KIC,4-methyl-2-oxopentanoate;MPE,mole percent13C-label enrichment;MSUD,maple syrup urine disease;WBLO,whole body leucine oxidation