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Wong, S H. FACSM1; Lok, A1; Morris, J1
1Department of Sports Science and PE, The Chinese University of Hong Kong, Shatin, Hong Kong
It has been shown that pre-exercise ingestion of low glycemic index (GI) foods produced a smaller fluctuation in the blood glucose level and lesser disturbance of the hormonal homeostasis than did the consumption of high GI foods. However, the research findings on exercise performance are inconsistent.
This study examined the effect of pre-exercise low and high GI carbohydrate (CHO) meals on running performance.
Eight endurance-trained male runners (mean+S.E.M.; age 33+1.7 years; VO2 max 63+1.8 ml/kg/min) completed two trials that were separated by at least seven days in a counterbalanced design. Two hours before the run and after an overnight fast, each subject consumed an isocaloric meal containing either low (GI = 37, LGI) or high (GI = 77, HGI) GI CHO foods (2.4 MJ; 65% CHO; 15% protein; 20% fat) that provided 1.5 g CHO/kg body mass in random order. Each trial consisted of a 21-km performance run on a level treadmill. The subjects were required to run at 70% VO2 max during the first 5-km of the run. They then completed the remaining 16-km as fast as possible.
All of the subjects achieved a faster performance time after the consumption of the LGI meal (LGI vs HGI: 98.7+2 vs 101.5+2 min, p <0.01). Blood glucose and serum free fatty acids concentrations were higher throughout the performance run in the LGI trial. Serum insulin concentrations were higher in the HGI trial during the postpandial resting period. However, there were no differences in serum insulin, serum cortisol, and blood lactate concentrations between the trials throughout the exercise. Compared with the HGI trial, CHO oxidation was 9.5% lower and fat oxidation was 17.9% higher during exercise in the LGI trial.
Consumption of a LGI meal 2 hours before a 21-km performance run is more effective in improving the run time when compared to an isocaloric HGI meal.
©2003The American College of Sports Medicine
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