Cerebral carbohydrate cost of physical exertion in humans

Above a certain level of cerebral activation the brain increases its uptake of glucose more than that of O-2, i.e., the cerebral metabolic ratio of O-2/( glucose + 1/2 lactate) decreases. This study quantified such surplus brain uptake of carbohydrate relative to O-2 in eight healthy males who performed exhaustive exercise. The arterial-venous differences over the brain for O-2, glucose, and lactate were integrated to calculate the surplus cerebral uptake of glucose equivalents. To evaluate whether the amount of glucose equivalents depends on the time to exhaustion, exercise was also performed with beta(1)-adrenergic blockade by metoprolol. Exhaustive exercise (24.8 +/- 6.1 min; mean +/- SE) decreased the cerebral metabolic ratio from a resting value of 5.6 +/- 0.2 to 3.0 +/- 0.4 ( P < 0.05) and led to a surplus uptake of glucose equivalents of 9 +/- 2 mmol. beta(1)-blockade reduced the time to exhaustion ( 15.8 +/- 1.7 min; P < 0.05), whereas the cerebral metabolic ratio decreased to an equally low level (3.2 +/- 0.3) and the surplus uptake of glucose equivalents was not significantly different ( 7 +/- 1 mmol; P = 0.08). A time-dependent cerebral surplus uptake of carbohydrate was not substantiated and, consequently, exhaustive exercise involves a brain surplus carbohydrate uptake of a magnitude comparable with its glycogen content.