Pre-exercise carbohydrate ingestion : effect of the glycaemic index on metabolism and endurance performance
Carbohydrates are a major substrate contributing to energy transduction during medium to high intensity exercise, and the body's levels of this substrate can be manipulated by dietary and exercise behaviours. Nutritional strategies employed before and during exercise affect endurance exercise performance by altering the metabolism of carbohydrate within the body. Carbohydrate feeding during endurance exercise has repeatedly been demonstrated to be beneficial to the athlete. Studies investigating preexercise carbohydrate feeding, particularly in the hour before exercise, have produced conflicting results and justify further investigation. The study reported in this dissertation aimed to further investigate the role of the meal, in particular, examine the effect of differing glycaemic indices of carbohydrate foods on metabolism and exercise performance. A total of eight, endurance trained subjects participated in this study which involved the ingestion of carbohydrate food with differing glycaemic indices 45 min before cycling at a submaximal workload corresponding to 70% VO2max for 50 min, followed by a self-paced 15 min performance ride. In all trials blood samples were taken from a forearm vein and analysed for metabolites and hormones. The results from this study demonstrate that the pre- exercise ingestion of carbohydrate foods with different glycaemic indices alter metabolism during rest and subsequent submaximal exercise. The data from this study demonstrated that pre-exercise ingestion of a high glycaemic index (HGI) food resulted in a hyperglycaemic response followed by an insulin-mediated hypoglycaemia at the onset of exercise. In addition, the elevated insulin during the HGI trial resulted in an attenuation in circulating FFA and higher rate of carbohydrate oxidation compared with the ingestion of a low glycaemic index food (LGI) or placebo (CON). Despite the changes in metabolism associated with pre-exercise CHO feeding, exercise performance following 50 min of submaximal exercise was not affected.
1106 Human Movement and Sports Science, 1111 Nutrition and Dietetics, School of Sport and Exercise Science
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