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 samples after administration of [C-13]-labelled carbohydrates with intervals 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 performed two exercise tests of 100-min duration each on a cycle ergometer. In both tests 5 min after the beginning of cycling glucose was administered. 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 was 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 (12C) and (CO2)-C-13 (13C) were calculated breath-by-breath online. The course 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 with 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 soon after oral administration.