Exogenous Glucose Oxidation During Exercise in Relation to the Power Output

In order to study the influence of the power output on the oxidation rate of exogenous glucose and on the contribution of the various substrates to the energy demand, we combined the use of artificially enriched ¹³C-glucose with classical indirect calorimetry during uphill treadmill exercise. Six young male healthy subjects underwent three exercise bouts, in a randomized order and at least two weeks apart, at a low (45 % VO₂max, 1822 ± 194 ml O₂/min for 4 hours), moderate (60 % VO₂max, 2582 ± 226 ml O₂/min for 3 hours), and high intensity (75 % VO₂max, 3036 ± 287 ml O₂/min for 2 hours). After 10 min of exercise, each subject ingested 100 g of artificially ¹³C-labelled glucose dissolved in 400 ml of water. Over the four hours of the exercise at 45 % VO₂max, 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 ingestion and peaked at 120 min. The maximum oxidation rates 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 % VO₂max respectively. 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 total carbohydrates (exogenous + endogenous) progressively increased from 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.

Key words

Exercise - metabolism - oral glucose - stable isotopes