The Effects of Fructose and Glucose on High Intensity Endurance Performance

Abstract

To evaluate the effects of glucose and fructose on endurance performance, six trained women ran at 80% [Vdot]O₂ max until exhaustion, 45 min after the ingestion of a 30% glucose solution (GLU), a 30% fructose solution (FRU), a saccharin placebo solution (PLA), and 300 ml water. A double-blind protocol was used. Heart rates were the same for all four trials, averaging 173 ± 8 beats · min ^-1. The PLA trials were significantly shorter (p < 0.05) than all other trials: 52.2 ± 5 min vs GLU = 63.9 ± 8.5, FRU = 61.9 ± 8.3, or control = 65.0 ± 7.6 min. Glucose treatment yielded an exercise [Vdot]O ₂ of 2.96 ± 0.07 1 · min ^-1, significantly lower than the FRU (3.15 ± 0.14 1 · min ^-1 ) control (3.19 ± 0.14 1 · min ^-1 ) or PLA (3.46 ± 0.11 1 · min ^-1 ) trials. Compared to controls or PLA, R values were significantly higher during the GLU and FRU trials. Venous lactate increased continuously throughout the duration of the control or PLA trials, while lactate was unchanged throughout the FRU and GLU trials. PLA trials resulted in significantly higher blood glucose. GLU treatment caused a significant decline in blood glucose early in exercise. Blood glucose returned to resting levels by the termination of exercise. Compared to controls, free fatty acid levels were significantly reduced during all other trials. It was concluded that prefeeding sugar solutions attenuates high intensity endurance performance.