Carbohydrate loading and its effect on ECG responses

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Virginia Polytechnic Institute and State University

Six white volunteer males less than 35 years old, who ran less than 35 miles per week completed a loading regimen. This consisted of a succession of mixed, high-fat and highCHO diets for at least 72 hours each. Subjects ran to exhaustion after each diet stage. Mean times to exhaustion were 61, 63 and 95 minutes for the mixed, high-fat, and high-CHO diets, respectively. Since questions have been raised concerning a detrimental effect of CHO-loading on heart function, subjects were closely examined for evidence of negative effects associated with this procedure. An incomplete right bundle branch block (RBBB), sinus arrhythmia, and early repolarization after the mixed diet was noted in three of the six subjects. These changes were not observed during the high-fat nor high-CHO diets. Prior to the high-CHO exhaustive run, it was also found that the width of the QRS complex was significantly more narrow than the mixed and high-fat diets. The above electrocardiographic (ECG) changes were noted as not being clinically significant by an internal medicine physician. No changes were noted in blood pressures, serum free fatty acids (FFA) and post exhaustive run body weights for diets not pre/post exercise bouts. Serum glucose was significantly higher for the pre-run high-CHO diet when compared to the mixed and high-fat diets pre-run values, yet it remained within normal limits. Body weight following the high-CHO diet was significantly greater than during the high-fat and mixed periods. This may be due to water retention occurring with increased glycogen storage. This probably explains the longer time to exhaustion for the high-CHO diet as compared to the mixed and high-fat trials which both yielded similar times to exhaustion. Although research indicates that a mixed diet prolongs the onset of exhaustion more than a high-fat diet, the similar endurance capacity for the high-fat and mixed diets could be related to a learning effect on the treadmill and/or psychological considerations of consuming a high-fat diet. Also since heart rate and blood pressure were not significantly different for the first 30 minutes of exercise while rate of perceived exertion (RPE) indicated the mixed diet trial most demanding and the high-CHO trial least, one might suspect that some other factor besides physiological values, may cause fatigue. In summary, CHO-loading appeared to enhance endurance of the novice runner with no apparent detrimental effects on cardiac function in these six subjects.