Duration: Longer Exercise Favors Fat Use
Fatigue typically occurs in humans after one to three hours of continuous exercise at intensities of 60-80% VO2max. Carbohydrate depletion is one of the main reasons why fatigue develops. This graph (Coyle, 1986; Romijn, 1993) shows the shift in energy substrate use that occurs during prolonged exercise at 65-75% VO2max. The subjects were endurance trained cyclists who had fasted overnight.
Early in the exercise energy was supplied almost equally by carbohydrate and fat. Gradually, as muscle glycogen levels declined, the contribution of blood glucose to the energy supply increased. Liquid carbohydrate feedings provided at two hours into the cycling session maintained normal blood glucose levels and allowed carbohydrate to continue as a major part of the fuel mix within working muscles. Fatigue occurred after four hours of continuous cycling exercise. Because muscle glycogen levels (in thigh muscles) were very low during this last hour, ongoing carbohydrate oxidation suggests that blood glucose was the major carbohydrate fuel during this period. Additional studies have confirmed these results, and indicated that carbohydrate consumed two to three hours into endurance exercise is oxidized at a rate of one gram per minute.
