CHANGES IN BREATH (CO2)-C-13 (CO2)-C-12 DURING EXERCISE OF DIFFERENT INTENSITIES/

The measurement of breath (CO2)-C-13/(CO2)-C-12 is commonly used durin g exercise to evaluate the oxidation rate of exogenous carbohydrates e nriched in C-13. The aim of this study was to investigate whether exer cise itself affects the C-13/C-12 ratio in expired air CO2 in relation to exercise intensity. The relative abundance of C-13 and C-12 in exp ired air CO2 was determined by isotope-ratio mass spectrometry and exp ressed as delta(13)C (in parts per thousand) by using Craig's formula and calibrated standards. Five healthy young men exercised on a treadm ill after an overnight fast during greater than or equal to 105 min on four occasions and in a randomized order Work rates were performed at similar to 30, 45, GO, and 75% of their maximal O-2 uptake (VO2max). delta(13)C in expired air CO2 and respiratory exchange ratio (RER) wer e determined every 15 or 30 min during exercise. At 30 and 45% VO2max, a slight and not statistically significant increase in delta(13)C lev els was observed at 30 min. In contrast, at 60 and 75% VO2max, the ris e mas statistically significant and averaged 0.83 and 0.99 parts per t housand, respectively. Average delta(13)C (between 0 and 105 min) prog ressively increased with the intensity of exercise. Individual values of delta(13)C and RER were positively correlated (r = 0.653, P = 0.002 ) as were values of delta(13)C and endogenous carbohydrates utilized ( r = 0.752, P < 0.001). Factitious or ''pseudooxidation'' of a C-13-enr iched exogenous glucose load (indeed noningested) was calculated from the changes in expired air delta(13)C, Over the whole period of exerci se it was not statistically significant at 30 and 45% VO2max. However, over the first 60 min of exercise, such pseudooxidation of exogenous glucose was significant at 30 and 45% VO2max. In conclusion, by modify ing the mix of endogenous substrates oxidized, exercise at 60% VO2max and above significantly increases the C-13/C-12 ratio in expired air C O2. At these intensities, this could lead to overestimation of the oxi dation of C-13-labeled substrates given orally. At lower intensities o f exercise, such overestimation is much smaller and affects mainly the values recorded during the initial part of the exercise bout.