Whereas prolonged intense exercise when one is dependent upon glucose/glycogen progressively depletes fuel reserves available to sustain the brain, the keto-adapted athlete may benefit from the opposite effect. As endurance exercise progresses, unlike the blood glucose concentration, the blood ketone concentration does not decline but tends to rise somewhat, ensuring a stable supply of fuels to the brain. Consistent with this improved cerebral fuel delivery, ultra-endurance athletes frequently report that mental clarity is maintained better during prolonged exercise in the keto-adapted state. This is in stark contrast to the problems of central fatigue and “hitting the wall” that commonly occur in athletes who follow a high-carbohydrate fuelling strategy and to which a progressive hypoglycemia that develops during prolonged exercise may contribute to fatigue. While ingesting carbohydrate during exercise may delay these fuel flow problems in athletes consuming high-carbohydrate diets, it does not always prevent them.