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Effect of timing of carbohydrate ingestion on endurance exercise performance

McCONELL, GLENN; KLOOT, KATE; HARGREAVES, MARK

Medicine & Science in Sports & Exercise: October 1996 - Volume 28 - Issue 10 - pp 1300-1304
Applied Sciences: Physical Fitness and Performance

This study compared the effects of carbohydrate ingestion throughout exercise with ingestion of an equal amount of carbohydrate late in exercise. Eight well-trained men cycled 2 h at 70 ± 1% ˙VO2 peak, followed immediately by a 15-min performance ride, while ingesting either a 7% carbohydrate-electrolyte solution (CHO-7), an artificially sweetened placebo(CON), or the placebo for the first 90 min then a 21% glucose solution(CHO-0/21). At the start of the performance ride, plasma glucose averaged 4.2± 0.2, 5.2 ± 0.1, and 5.7 ± 0.2 mmol.l-1 in CON, CHO-7, and CHO-0/21, respectively (all different, P < 0.05). Plasma insulin levels were similar just prior to the performance ride in CHO-7 and CHO-0/21, with both higher than CON. A similar pattern was observed with respiratory exchange ratio (RER). Work performed during the performance ride was significantly greater in CHO-7 (268 ± 8 kJ) compared with CON (242± 9 kJ). Performance in CHO-0/21 (253 ± 10 kJ), however, was not improved compared with CON, despite higher plasma glucose levels and plasma insulin levels similar to CHO-7. Seven of the eight subjects performed best in CHO-7. In conclusion, performance was improved, relative to the control trial, only when carbohydrate was ingested throughout exercise. Carbohydrate ingestion late in exercise did not improve performance despite increases in plasma glucose and insulin.

Department of Physiology, The University of Melbourne, Parkville, Victoria, 3052, AUSTRALIA

Submitted for publication June 1995.

Accepted for publication June 1996.

This study was supported by Cadbury Schweppes Pty. Ltd. through the Victorian Institute of Sport.

Address for correspondence: Dr. Glenn McConell, Department of Physiology, Monash University, Clayton, Victoria, 3168 Australia.

©1996The American College of Sports Medicine