Purpose: The effect of different quantities of carbohydrate (CHO) intake on CHO metabolism during prolonged exercise was examined in endurance-trained females.
Method: On four occasions, eight females performed 2 h of cycling at approximately 60% V˙O2max with ingestion of beverages containing low (LOW, 0.5 g·min−1), moderate (MOD, 1.0 g·min−1), or high (HIGH, 1.5 g·min−1) amounts of CHO, or water only (WAT). Test solutions contained trace amounts of [U-13C] glucose. Indirect calorimetry combined with measurement of expired 13CO2 and plasma 13C enrichment enabled calculation of exogenous CHO, liver-derived glucose, and muscle glycogen oxidation during the last 30 min of exercise.
Results: The highest rates of exogenous CHO oxidation were observed in MOD, with no further increases in HIGH (peak rates of 0.33 ± 0.02, 0.50 ± 0.03, and 0.48 ± 0.05 g·min−1 for LOW, MOD, and HIGH, respectively; P < 0.05 for LOW vs MOD and HIGH). Endogenous CHO oxidation was lowest in MOD (0.99 ± 0.06, 0.82 ± 0.08, 0.70 ± 0.07, and 0.89 ± 0.09 g·min−1; P < 0.05 for MOD vs all other trials). Compared with WAT, CHO ingestion reduced liver glucose oxidation during exercise by approximately 30% (P < 0.05 for WAT vs all CHO). Differential rates of muscle glycogen oxidation were observed with different CHO doses (0.57 ± 0.07, 0.53 ± 0.08, 0.41 ± 0.07, and 0.60 ± 0.09 g·min−1 for WAT, LOW, MOD, and HIGH respectively; P < 0.05 for MOD vs HIGH).
Conclusion: In endurance-trained women, the highest rates of exogenous CHO oxidation and greatest endogenous CHO sparing was observed when CHO was ingested at moderate rates (1.0 g·min−1, 60 g·h−1) during exercise.