Oxidation of 13C-glucose and 13C-fructose ingested as a preexercise meal: effect of carbohydrate ingestion during exercise.

The oxidation of 13C-labeled glucose and fructose ingested as a preexercise meal between 180 and 90 min before exercise was measured on 6 subjects when either a placebo or sucrose was ingested during the exercise period. Labeled hexose oxidation, which occurred mainly during the first hour of exercise, was not significantly modified when sucrose was ingested, but exogenous glucose oxidation was significantly higher than exogenous fructose oxidation in both situations. The results suggest that the absorption rate of exogenous hexoses was high when exercise was initiated but diminished thereafter, and that glucose and fructose released from sucrose ingested during exercise did not compete with glucose or fructose ingested before exercise for intestinal absorption, for conversion into glucose in the liver (for fructose), or for uptake and oxidation of glucose in peripheral tissues. However, as already shown, in terms of availability for oxidation of carbohydrates provided by the preexercise meal, glucose should be favored over fructose.