The effect of different quantities of carbohydrate (CHO) intake on CHO metabolism during prolonged exercise was examined in endurance-trained females.
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.
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).
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.
School of Sport and Exercise Sciences, University of Birmingham, Birmingham, UNITED KINGDOM
Address for correspondence: Asker E. Jeukendrup, Ph.D., School of Sport and Exercise Sciences, University of Birmingham, Birmingham B15 2TT, United Kingdom; E-mail: A.E.Jeukendrup@bham.ac.uk.
Submitted for publication April 2006.
Accepted for publication August 2006.