Respective oxidation of exogenous glucose and fructose given in the same drink during exercise

We computed the respective amounts of exogenous glucose (G) and fructose (F), which are oxidized during exercise when ingested simultaneously, with the use of 13C labeling. Six subjects exercised for 2 h at 60.7 +/- 2.9% of maximal O2 uptake on a cycle ergometer while ingesting 50 or 100 g of G or F or a mixture of 50 g each of G and F in 500 ml of water. The amount of exogenous G oxidized increased from 37.8 +/- 2.2 to 58.3 +/- 8.1 g when the total amount ingested increased from 50 to 100 g. The amount of F oxidized was significantly lower (32.2 +/- 1.2 and 45.8 +/- 2.6 g for the 50 and 100 g ingested, respectively). When 50 g each of G and F were simultaneously ingested in the same drink, the amounts oxidized (39.5 +/- 4.8 and 34.1 +/- 1.5 g, respectively) were similar to those observed when 50 g of G or F were ingested separately. The cumulative amount of exogenous hexoses oxidized (73.6 +/- 6.6 g) was 21% larger than when 100 g of G were ingested. This finding could be due to the fact that the routes for absorption and metabolism of exogenous G and F are at least partly different, resulting in less competition for oxidation when a mixture of these two hexoses is ingested than when an isocaloric amount of G is ingested. From a practical point of view, these data may provide experimental support for using mixtures of carbohydrates in the energy supplements for endurance athletes.