Comparison of fructose and glucose ingestion before and during endurance cycling to exhaustion

Abstract

The aim of this study was to compare the effect of pre-exercise and exercise ingestion of fructose and glucose during prolonged cycling exercise. The primary purpose was to determine if ingesting fructose before and during exercise was as beneficial or more beneficial than glucose ingestion. Seventeen trained subjects performed a control cycle to exhaustion. At least one week later, each subject performed a second ride to exhaustion ingesting either fructose or glucose before and during exercise. Blood was drawn before and at timed intervals during exercise to determine blood glucose, lactate and free fatty acid (FFA) levels for all three conditions (control, fructose, glucose). Gas measurements (Beckman Metabolic Measurement Cart) were taken at approximately 10 minute intervals, to ensure each subject was cycling at 75% VO2 max. and to determine respiratory exchange ratio (RER). Exercise time to exhaustion for the control group was significantly less (see document) than either the fructose (p < .02) or glucose (p < .001) group, but the fructose and glucose groups were not significantly different from each other. Blood glucose levels in the fructose group remained more stable than the glucose group and actually increased throughout the exercise test to exhaustion. Prior to the onset of exercise, the blood lactate level of the control group was significantly (see document) lower than either the fructose (p < .002) or the glucose (p < .01) group. The fructose and glucose groups did not show any significant differences in blood lactate over time. There were no significant differences in blood FFA between the treatment groups during the exercise test to exhaustion, but the control group FFA level was significantly (see document) higher than that of the fructose group (p <.02) prior to the onset of exercise. All three groups demonstrated gradual declines in RER throughout the exercise test to exhaustion. In this study it was established that fructose and glucose are of equal value in prolonging exercise time to exhaustion in endurance cycling performance. Ingesting fructose before and during exercise allowed for a more constant supply of glucose to be available to the working muscles than glucose ingestion. The more stable blood glucose levels with fructose ingestion may be beneficial in reducing perceived exhaustion, increasing mental alertness and postponing the athletes' perception of 'hitting the wall’, thereby allowing for an enhancement in exercise performance.