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Unformatted text preview: Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance MARK HARGREAVES, 1 * JOHN A. HAWLEY 2 and ASKER JEUKENDRUP 3 1 Centre for Physical Activity and Nutrition Research, School of Health Sciences, Deakin University, Burwood, VIC 3125, Australia, 2 Exercise Metabolism Group, School of Medical Sciences, RMIT University, Bundoora, VIC 3083, Australia and 3 School of Sport and Exercise Sciences, University of Birmingham, Birmingham B15 2TT, UK Accepted 7 August 2003 A key goal of pre-exercise nutritional strategies is to maximize carbohydrate stores, thereby minimizing the ergolytic effects of carbohydrate depletion. Increased dietary carbohydrate intake in the days before competition increases muscle glycogen levels and enhances exercise performance in endurance events lasting 90 min or more. Ingestion of carbohydrate 34 h before exercise increases liver and muscle glycogen and enhances subsequent endurance exercise performance. The effects of carbohydrate ingestion on blood glucose and free fatty acid concentrations and carbohydrate oxidation during exercise persist for at least 6 h. Although an increase in plasma insulin following carbohydrate ingestion in the hour before exercise inhibits lipolysis and liver glucose output, and can lead to transient hypoglycaemia during subsequent exercise in susceptible individuals, there is no convincing evidence that this is always associated with impaired exercise performance. However, individual experience should inform individual practice. Interventions to increase fat availability before exercise have been shown to reduce carbohydrate utilization during exercise, but do not appear to have ergogenic benefits. Keywords : fatigue, free fatty acids, glucose uptake, insulin, muscle glycogen. Introduction The importance of carbohydrate for exercise perfor- mance has been recognized since the classic respira- tory exchange studies of Christensen and Hansen in the late 1930s and the biopsy studies of Bergstrom and colleagues (Bergstrom et al ., 1967), who measured muscle glycogen during various dietary and exercise interventions. Since then, considerable attention has focused on nutritional strategies to maximize endogenous carbohydrate stores (liver and muscle glycogen), thereby minimizing the potential ergolytic effects of carbohydrate depletion (Coyle et al ., 1986). In this review, attention will focus on dietary carbohydrate during training in the days (1 7) leading up to competition and on carbohydrate and fat ingestion in the hours immediately before exercise and their effects on exercise metabolism and performance. Carbohydrate loading in the days before exercise In the classic carbohydrate loading study of Bergstrom et al . (1967), the ingestion of a high carbohydrate diet, following a period of relative carbohydrate deprivation, resulted in a marked increase (supercompensation) in muscle glycogen (to as high as 200 mmol kg 7 1 wet mass) and enhanced subsequent endurance exercise...
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