The Effects of Differing Glycemic Index Meals on Substrate Utilization and Endurance Performance.

Few studies have investigated the effects of pre-exercise meals with differing glycemic responses on substrate utilization and endurance performance in subsequent exercise. Those few which have been done have looked at foods of limited application to a pre-exercise meal consumed prior to a morning event. This study investigated the effects of commonly eaten breakfast cereals on exercise performance. Ten physically active male subjects participated in this study. The subjects reported to the performance laboratory in the morning following an overnight fast (10-12 hours). Upon arrival, the subjects were weighed and then fed either a high glycemic index (GI) meal (corn flakes, banana, 1% low-fat milk) or moderate glycemic index meal (oatmeal, banana, 1% low-fat milk) containing 100 grams of carbohydrate (77% carbohydrate). A third trial using a continued fast served as a control. Trials were performed in random order. After consumption of the meal, the subjects remained seated for one hour before beginning a one-hour submaximal cycling trial at 70% VO2 max followed immediately by a maximal performance test to exhaustion. Blood was sampled pre-meal, pre-exercise, every 15 minutes of exercise and at completion of the performance test and analyzed for plasma glucose, free fatty acids, and lactate. Metabolic measurements collected every 15 minutes of exercise and during the maximal performance test included heart rate (Ilk), pulmonary ventilation (VE), oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory exchange ratio (RER).