Preexercise sugar feeding does not alter prolonged exercise muscle glycogen or protein catabolism.

The purpose of this study was to determine the effects of preexercise sugar feedings (glucose [GLU] or fructose [FRU]) on muscle glycogen and protein catabolism during prolonged exercise in fed men. Seven men cycled to exhaustion on three different occasions at 70% VO2max, 45 min after ingestion (700 ml) of either a 0.476 mol.L-1 carbohydrate (CHO) solution or a sweetened placebo (PLA). With GLU, serum insulin was significantly increased prior to exercise. As a result, serum glucose was significantly lower at 15 and 30 min of exercise with GLU, but was similar to the other treatments thereafter. Time to fatigue was absolutely longer with the GLU feeding, and exercise muscle glycogen catabolism was absolutely lower during the FRU trial, but the observed differences did not attain statistical significance due to intersubject variability. Protein catabolism was similar for all treatments. These data indicate that a 60-g preexercise glucose or fructose feeding following CHO loading procedures has minimal effects on muscle glycogen or protein catabolism during prolonged exercise.