Pre-exercise carbohydrate feedings and endurance performance
Six men were studied to compare the effects of pre-exercise carbohydrate feedings on endurance performance and muscle glycogen utilization during exhaustive exercise. Trials consisted of a cycling ride to exhaustion at 757. of the subjects' maximal oxygen uptake preceded by the ingestion of either fructose (FRU), glucose (GLU), or sweet placebo (CON). No differences were observed between trials for oxygen uptake, respiratory exchange ratio, heart rate, or exercise time to exhaustion. Blood glucose was elevated (P<0.05) as a result of the glucose feeding, but fell rapidly with the onset of exercise, reaching a nadir of 4.02 + O.-'4 mmcl: i (mean + SE) at 15 min of exercise (P<0.05). Serum insulin also increased (P<0.05) following the glucose feeding: by 30 min of exercise, however, insulin had returned to pre-drink levels. No differences in blood glucose and insulin were observed between FRU and CON. Muscle glycogen utilization during the first 30 min of exercise (CON = 46.3 ± 82 mmol/kg w.w., FRU = 56.3 + 3, GLU = 50.0 + 4.9) and total glycogen use (CON = 93.4 + 11.1, FRU = 118.8 + 10.9, GLU = 99.5 + 4.3) was similar between trials (P>O.05). It was concluded that despite more stable blood glucose and insulin levels in FRU and CON, compared with GLU, this provided no advantage to endurance performance or muscle glycogen utilization.