CARBOHYDRATE INGESTION AND SINGLE MUSCLE-FIBER GLYCOGEN-METABOLISM DURING PROLONGED RUNNING IN MEN

The aim of this study was to examine the effect of carbohydrate (CHO-I 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 e xhaustion on a motorized treadmill on two occasions, 1 wk apart. On ea ch occasion, the subjects ingested 8 ml/kg body wt of either placebo ( Pi) 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 b iopsy samples were obtained from the vastus lateralis muscle before an d after each trial and also at the time coinciding with Fl exhaustion in the CHO-E trial. Running time to exhaustion was longer (P < 0.01) i n the CHO-E trial compared with the P1 trial (132.4 +/- 12.3 and 104.3 +/- 8.6 min, respectively). A 25% reduction in glycogen utilization i n type I fibers only was observed in the CHO-E trial compared with the P1 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 Fl trial, both m uscle ATP and phosphocreatine concentrations were well maintained thro ughout exercise. Therefore, because in both the P1 and CHO-E trials th e 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 propo sed 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.