BREATH-BY-BREATH MEASUREMENTS FOR THE ANALYSIS OF EXOGENOUS GLUCOSE-OXIDATION DURING INTENSE ENDURANCE EXERCISE USING [C-13]-ISOTOPES

Up to now only the analysis of (CO2)-C-13 in separate breath-gas sampl es after administration of [C-13]-labelled carbohydrates with interval s of several minutes in between has been available for analyses of the exogenous glucose oxidation during exercise. Our studies show the use of rapid respiratory mass spectrometry for breath-by-breath analysis to determine exogenous glucose utilization. Six male triathletes perfo rmed two exercise tests of 100-min duration each on a cycle ergometer. In both tests 5 min after the beginning of cycling glucose was admini stered. in test 1 30 g natural glucose was given, in test 2 a mixture of 1.5 g [C-13(1)]-glucose and 28.5 g natural glucose. The work rate w as regulated to keep constant 90% of the VO2 at the VO2 levelling off (VO(2)max) measured in a short ramp test. The resulting work rate was at 260.2 +/- 9.5 watts in test 1 and at 276.3 +/- 12.1 watts in test 2 . Respiration gases and the end-tidal concentrations of (CO2)-C-12 (12 C) and (CO2)-C-13 (13C) were calculated breath-by-breath online. The c ourse of the ratio [13C/12C] reflects the course of exogenous glucose utilization. Onset of utilization was 10.5 +/- 5.5 min following oral administration. Maximum utilization was attained at 53.1 +/- 9.8 min w ith a maximal rate of oxidation of 0.36 +/- 0.05 g/min. The beginning of the energy supply of exogenous glucose could be determined quite so on after oral administration.