Effects of pre-exercise carbohydrate feedings on muscle glycogen use during exercise in well-trained runners

Summary

The purpose of this study was to examine the effects of pre-exercise glucose and fructose feedings on muscle glycogen utilization during exercise in six well-trained runners ($\dot V_{{\text{O}}_{{\text{2 max}}} }$=68.2±3.4 ml·kg⁻¹·min⁻¹). On three separate occasions, the runners performed a 30 min treadmill run at 70%$\dot V_{{\text{O}}_{{\text{2 max}}} }$. Thirty minutes prior to exercise each runner ingested 75 g of glucose (trial G), 75 g of fructose (trial F) or 150 ml of a sweetened placebo (trial C). During exercise, no differences were observed between any of the trials for oxygen uptake, heart rate or perceived exertion. Serum glucose levels were elevated as a result of the glucose feeding (P<0.05) reaching peak levels at 30 min post-feeding (7.90±0.24 mmol·l⁻¹). With the onset of exercise, glucose levels dropped to a low of 5.89±0.85 mmol·l⁻¹ at 15 min of exercise in trial G. Serum glucose levels in trials F and C averaged 6.21±0.31 mmol·l⁻¹ and 5.95±0.23 mmol·l⁻¹ respectively, and were not significantly different (P<0.05). There were also no differences in serum glucose levels between any of the trials at 15 and 30 min of exercise. Muscle glycogen utilization in the first 15 min of exercise was similar in trial C (18.8±8.3 mmol·kg⁻¹), trial F (16.3±3.8 mmol·kg⁻¹) and trial G (17.0±1.8 mmol·kg⁻¹), and total glycogen use was also similar in trial C (25.6±7.9 mmol·kg⁻¹), trial F (35.4±5.7 mmol·kg⁻¹) and trial G (24.6±3.2 mmol·kg⁻¹). In contrast to previous research, these results suggest that pre-exercise feedings of fructose or glucose do not affect the rate of muscle glycogen utilization during 30 min of treadmill running in trained runners.