KEY POINTS·Fatigue during prolonged heavy exercise is often associated with low or depleted muscle glycogen stores and with lowered blood glucose.·Eating a high-carbohydrate meal 2-4 h before constant-pace exercise can improve endurance capacity (time to exhaustion) but may not be advantageous in time-trials,especially if sports drinks are consumed during the trials.·Contrary to earlier reports,there appear to be no adverse effects on endurance capacity of consuming well-formulated carbohydrate solutions within the hour before exercise. KEY POINTS · Fatigue during prolonged exercise is often associated with low or depleted muscle glycogen stores and with lowered blood glucose. · Eating a high-carbohydrate meal for 2-4 h before constant-pace exercise can improve endurance capacity (time to exhaustion) but may not be advantageous in time-trials, especially if sports drinks are consumed during the trials.