Part 3: Carbohydrate feedings prior to exercise

High intensity aerobic exercise for 1 hour decreases liver glycogen by about 55%, whereas a 2-hour strenuous workout almost depletes the glycogen of the liver and active muscle fibers.

Prior to exercise:

Confusion exists regarding the potential endurance benefits of preexercise ingestion of simple sugars. Some researchers argue that consuming rapidly absorbed high glycemic carbohydrates within 1 hour before exercising accelerates glycogen depletion. This negatively affects endurance performance by the following mechanism.

• A rapid rise in blood sugar triggers an overshoot in insulin release. An excess of insulin cause a relative hypoglcemia (called rebound hypoglycemia). Significant blood sugar reduction impairs central nervous system function during exercise.

• A large insulin release facilitates the influx of glucose into muscle, which disproportionately increases glycogen catabolism in exercise. At the same time, high insulin levels inhibit lipolysis, which reduces fatty acid mobilization from adipose tissue. Augmented carbohydrate breakdown and depressed fat mobilization contribute to premature glycogen and early fatigue.

Research in late 70s indicated that drinking a highly concentrated sugar solution before exercise precipitated early fatigue in endurance activities. However, subsequent research has not corroborated the negative effects of concentrated preexercise sugar feeds on endurance. This discrepancy in research findings has no clear explanations.

One way to eliminate any potential for negative effects of prexercise simple sugars is to ingest them at least 60 mins before exercising. This provides sufficient time to reestablish hormonal balance before exercise begins.