During strenuous exercise (i.e. 70% maximal O2 consumption) there is a progressive shift from muscle glycogen to blood glucose oxidation with increasing duration of exercise. By maintaining blood glucose concentration and the rate of carbohydrate oxidation necessary to exercise strenuously, carbohydrate consumption throughout exercise delays fatigue by 30-60 min in endurance-trained subjects. This requires exogenous glucose supplementation at rates in excess of 1 gram/min (i.e., 16 mg/kg/min) as evidenced by the observation that intravenous glucose infusion at this rate is required to maintain blood glucose at 5 mM. Exogenous glucose must be infused at a rate of 2.6 gram/min (i.e., 37 mg/kg/min), which is similar to the total rate of carbohydrate oxidation, in order to maintain blood glucose at 10 mM after 2 h of exercise. However, carbohydrate supplementation during intense exercise does not spare muscle glycogen utilization in people. This suggests that over the course of 2-4 hours of exercise at 70% VO2max, muscle glycogen and blood glucose contribute equally to total carbohydrate oxidation. Furthermore, during the latter stages of prolonged exercise, exogenous blood glucose supplementation may be capable of supplying almost all of the carbohydrate requirements of exercise at intensities up to 70% VO2max.
