have previously demonstrated that feeding carbohydrate throughout exercise at 70%-74% of maximal O₂ uptake (i.e., ) can delay fatigue by 30 to 60 min (e.g., from 3 h to 4 h) (Coyle et al., 1983, 1986). A major finding was that carbohydrate feedings did not spare muscle glycogen utilization and that trained cyclists were able to exercise for the additional hour when fed carbohydrate without relying upon muscle glycogen for a fuel (Coyle et al., 1986). Instead, it appears that when the blood glucose concentration is maintained at 5 mM by carbohydrate feeding, highly trained cyclists are capable of relying upon blood glucose for almost all of their carbohydrate energy during the later stages of prolonged strenuous exercise. When exercising without feedings, the blood glucose concentration declines progressively after the first hour and reaches hypoglycemic levels (i.e., <2.5 mM) after 3 h of exercise (Coyle et al., 1983, 1986). Figure 8-1 describes our theory that the source of carbohydrate energy shifts from muscle glycogen to blood glucose as the duration of exercise progresses. Thus, blood glucose appears to be the most important source of energy after 3 h of strenuous cycling. It is therefore important that people have adequate glucose in their blood during the later stages of exercise in order to delay fatigue.

FIGURE 8-1 Theoretical representation of the sources of energy during prolonged cycling at 70% of maximal oxygen uptake. Source: Redrawn from Coyle et al. (1986).