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The aim of this study was to examine the effect of carbohydrate (CHO) ingestion on glycogen degradation in type I and type II muscle fibers during prolonged running by using a quantitative biochemical method. To this end, eight male subjects ran at 70% maximal oxygen uptake to exhaustion on a motorized treadmill on two occasions, 1 wk apart. On each occasion, the subjects ingested 8 ml/kg body wt of either placebo (Pl) or a 5.5% CHO-electrolyte solution (CHO-E) immediately before the start of the run and 2 ml/kg body wt every 20 min thereafter. Needle biopsy samples were obtained from the vastus lateralis muscle before and after each trial and also at the time coinciding with Pl exhaustion in the CHO-E trial. Running time to exhaustion was longer (P < 0.01) in the CHO-E trial compared with the Pl trial (132.4 +/- 12.3 and 104.3 +/- 8.6 min, respectively). A 25% reduction in glycogen utilization in type I fibers only was observed in the CHO-E trial compared with the Pl trial (215.2 +/- 27.5 vs. 285.4 +/- 30.1 mmol/kg dry wt; P < 0.01). Furthermore, in the CHO-E trial, in contrast to the Pl trial, both muscle ATP and phosphocreatine concentrations were well maintained throughout exercise. Therefore, because in both the Pl and CHO-E trials the type I fibers were glycogen depleted at the point of exhaustion (31.6 +/- 10.3 and 28.1 +/- 7.1 mmol/kg dry wt, respectively), it is proposed that CHO ingestion improved endurance capacity by contributing to oxidative ATP production specifically in type I fibers and by doing so delayed the development of glycogen depletion in this fiber type.
Tsintzas et al. (Thu,) studied this question.