Pre-exercise carbohydrate ingestion: effect of the glycemic index on endurance exercise performance. Med. Sci. Sports Exerc., Vol. 30, No. 6, pp. 844-849, 1998.
Purpose: This study aimed to examine the effect of glycemic index of pre-exercise carbohydrate (CHO) ingestion on exercise metabolism and performance.
Methods: Eight endurance trained men ingested a high glycemic index (HGI), low glycemic index (LGI), or a placebo (CON) meal 45 min before exercise and then cycled for 50 min at 67% V˙O2max. Subjects subsequently performed a 15-min self-paced performance ride in which total work (kJ) was recorded.
Results: Plasma glucose concentrations were higher (P < 0.01) after ingestion in HGI compared with LGI and CON (7.53 ± 0.64 vs 5.55 ± 0.21 and 4.65 ± 0.14 mmol·L−1 for HGI, LGI, and CON, respectively, 30 min postprandial; mean ± SE) but declined at the onset of exercise and were lower (P < 0.01) compared with LGI and CON (4.03 ± 0.31 vs 4.64 ± 0.24 and 5.09 ± 0.16 mmol·L−1 for HGI, LGI, and CON respectively; mean ± SE) at 10 min of exercise. Plasma glucose remained depressed (P < 0.01) until 30 min into exercise in HGI compared with other trials. Plasma insulin concentrations were higher (P < 0.01) following ingestion during rest and exercise in HGI compared with LGI and CON. Plasma FFA concentrations were lower (P < 0.05) following ingestion in HGI and LGI compared with CON and higher (P < 0.05) in LGI compared with HGI at the start and end of exercise. RER and CHO oxidation was higher (P < 0.01) in HGI compared with LGI and CON during submaximal exercise. There were no differences in work output during the performance cycle.
Conclusions: These data indicate that pre-exercise CHO feedings with varying glycemic indexes do not affect exercise performance following short term submaximal exercise despite alterations in metabolism.
Department of Physical Education and Recreation, Victoria University of Technology, Footscray, 3001; Exercise Physiology and Metabolism Laboratory, Department of Physiology, The University of Melbourne, Parkville, 3052, AUSTRALIA
Submitted for publication March 1996.
Accepted for publication September 1997.
The authors acknowledge the contribution of Associate Professor Michael Carey for the generous use of the Exercise Metabolism Laboratory. The authors also acknowledge the technical assistance of Ian Fairweather, Danny Rutar, Sally Clark, Jo Parkin, and Steve Fraser.
Address for correspondence: Dr. Mark. A. Febbraio, Department of Physiology, The University of Melbourne, Parkville, 3052, Australia. E-mail: firstname.lastname@example.org.