Cerebral metabolic response to submaximal exercise

We studied cerebral oxygenation and metabolism during submaximal cycling in 12 subjects. At two work rates, middle cerebral artery blood velocity increased from 62 +/- 3 to 63 +/- 3 and 70 +/- 5 cm/s as did cerebral oxygenation determined by near-infrared spectroscopy. Oxyhemoglobin increased by 10 +/- 3 and 25 +/- 3 mu mol/l (P < 0.01), and there was no significant change in brain norepinephrine spillover. The arterial-to-internal-jugular-venous (a-v) difference for O-2 decreased at low-intensity exercise (from 3.1 +/- 0.1 to 2.9 +/- 0.1 mmol/l; P < 0.05) and recovered at moderate exercise (to 3.3 +/- 0.1 mmol/l). The profile for glucose was similar: its a-v difference tended to decrease at low-intensity exercise (from 0.55 +/- 0.05 to 0.50 +/- 0.02 mmol/l) and increased during moderate exercise (to 0.64 +/- 0.04 mmol/l; P < 0.05). Thus the molar ratio (a-v difference, O-2 to glucose) did not change significantly. However, when the a-v difference for lactate (0.02 +/- 0.03 to 0.18 +/- 0.04 mmol/l) was taken into account, the O-2-to-carbohydrate ratio decreased (from 6.1 +/- 0.4 to 4.7 +/- 0.3; P < 0.05). The enhanced cerebral oxygenation suggests that, during exercise, cerebral blood flow increases in excess of the O-2 demand. Yet it seems that during exercise not all carbohydrate taken up by the brain is oxidized, as brain lactate metabolism appears to lower the balance of O-2-to-carbohydrate uptake.