EXOGENOUS GLUCOSE-OXIDATION DURING EXERCISE IN RELATION TO THE POWER OUTPUT

In order to study the influence of the power output on the oxidation r ate of exogenous glucose and on the contribution of the various substr ates to the energy demand, we combined the use of artificially enriche d C-13-glucose with classical indirect calorimetry during uphill tread mill exercise. Six young male healthy subjects underwent three exercis e bouts, in a randomized order and at least two weeks apart, at a low (45 % VO(2)max, 1822 +/- 194 ml O-2/min for 4 hours), moderate (60 % V O(2)max 2582 +/- 226 ml O-2/min for 3 hours), and high intensity (75 % VO(2)max, 3036 +/- 287 ml O-2/min for 2 hours). After 10 min of exerc ise, each subject ingested 100 g of artificially C-13-labelled glucose dissolved in 400 mi of water. Over the four hours of the exercise at 45 % VO(2)max, the amount of exogenous glucose oxidized was 89.5 +/- 5 .9 g from the 100 g ingested. In all exercise bouts, the oxidation of exogenous glucose already began during the first 30 min after ingestio n and peaked at 120 min. The maximum oxidation Fates averaged 0.64 +/- 0.07, 0.75 +/- 0.04, and 0.63 +/- 0.08 g/min, and the mean amounts of exogenous glucose oxidized over the first two hours averaged 51.7 +/- 8.0, 61.5 +/- 6.6 and 50.9 +/- 8.45 g, at 45, 60 and 75 % VO(2)max re spectively. The contribution of the oxidation of exogenous glucose to the total energy supply progressively decreased when the power output increased, from 19.6 to 12.2 %. In the meantime, the contribution of t otal carbohydrates (exogenous + endogenous) progressively increased fr om 55.1 to 77.8 % while the contribution of lipids decreased from 35.5 to 16.6 %. In conclusion, exogenous glucose ingested during exercise is largely oxidized and strongly contributes to the energy supply. The oxidation rate first increases with the power output, but levels off or even decreases at high intensity exercise.