Purpose: To examine the influence of ingesting a carbohydrate-electrolyte (CHO-E) solution on muscle glycogen use and intermittent running capacity after consumption of a carbohydrate (CHO)-rich diet.
Methods: Six male volunteers (mean ± SD: age 22.7 ± 3.4 yr; body mass (BM) 75.0 ± 4.3 kg; V˙O2max 60.2 ± 1.6 mL·kg−1·min−1) performed two trials separated by 14 d in a randomized, crossover design. Subjects consumed either a 6.4% CHO-E solution or a placebo (PLA) in a double-blind fashion immediately before each trial (8 mL·kg−1 BM) and at 15-min intervals (3 mL·kg−1 BM) during intermittent high-intensity running to fatigue performed after CHO loading for 2 d. Muscle biopsy samples were obtained before exercise, after 90 min of exercise, and at fatigue.
Results: Subjects ran longer in the CHO-E trial (158.0 ± 28.4 min) compared with the PLA trial (131.0 ± 19.7 min; P < 0.05). There were no differences in muscle glycogen use for the first 90 min of exercise (~2 mmol of glucosyl units per kilogram of dry matter (DM) per minute). However, there was a trend for a greater use in the PLA trial after 90 min (4.2 ± 2.8 mmol·kg−1 DM·min−1) compared with the CHO-E trial (2.5 ± 0.7 mmol·kg−1 DM·min−1; P = 0.10). Plasma glucose concentrations were higher at fatigue in the CHO-E than in the PLA trial (P < 0.001).
Conclusions: These results suggest that CHO-E ingestion improves endurance capacity during intermittent high-intensity running in subjects with high preexercise muscle glycogen concentrations. The greater endurance capacity cannot be explained solely by differences in muscle glycogen, and it may actually be a consequence of the higher plasma glucose concentration towards the end of exercise that provided a sustained source of CHO for muscle metabolism and for the central nervous system.