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Ide, Kojiro, Allan Horn, and Niels H. Secher. Cerebral metabolic response to submaximal exercise. J. Appl. Physiol. 87(5): 1604-1608.-We studied cerebral oxygenation and metabolism during submaximal cycling in 12 subjects. At two work rates, middle cerebral artery blood velocity increased from 62 Ϯ 3 to 63 Ϯ 3 and 70 Ϯ 5 cm/s as did cerebral oxygenation determined by near-infrared spectroscopy. Oxyhemoglobin increased by 10 Ϯ 3 and 25 Ϯ 3 µmol/l (P Ͻ 0.01), and there was no significant change in brain norepinephrine spillover. The arterial-to-internal-jugularvenous (a-v) difference for O 2 decreased at low-intensity exercise (from 3.1 Ϯ 0.1 to 2.9 Ϯ 0.1 mmol/l; P Ͻ 0.05) and recovered at moderate exercise (to 3.3 Ϯ 0.1 mmol/l). The profile for glucose was similar: its a-v difference tended to decrease at low-intensity exercise (from 0.55 Ϯ 0.05 to 0.50 Ϯ 0.02 mmol/l) and increased during moderate exercise (to 0.64 Ϯ 0.04 mmol/l; P Ͻ 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 Ϯ 0.03 to 0.18 Ϯ 0.04 mmol/l) was taken into account, the O 2 -to-carbohydrate ratio decreased (from 6.1 Ϯ 0.4 to 4.7 Ϯ 0.3; P Ͻ 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. blood pressure; epinephrine; glucose; heart rate; lactate; near-infrared spectroscopy; norepinephrine; norepinephrine spillover CONTROVERSY EXISTS as to whether the metabolic activity of the brain as a whole increases during physical exercise. For example, there appears to be no change in brain O 2 uptake (V O 2 ) during cycling (18, 31), whereas, during vigorous exercise on the treadmill, there is reported to be an increase in brain V O 2 (28). Although the cerebral metabolic rate for O 2 has been taken as the variable most closely coupled to metabolic activity of the brain (18), V O 2 may not be the most sensitive index for an evaluation of the metabolic activity of the brain. In response to neural activation, regional V O 2 increases much less than the increase in cerebral blood flow (7). Also, cerebral oxygenation determined by near-infrared spectroscopy (NIRS) exceeds the increase in O 2 demand in response to motor stimulation (22). Moreover, the increase in regional uptake of glucose surpasses that of O 2 (8), and similar observations have been made for the global value (15). Thus the molar cerebral V O 2 -to-glucose uptake ratio becomes reduced (15, 16).During exercise, blood lactate increases progressively with work rate, and lactate may be of importance for brain metabolism (14). Yet, when there is no increase in blood lactate, as during mental activation, no net brain uptake of lactate has been demonstrated (17). Rather, the increase in lactate uptake by the brain becomes apparent during hyperlactemia ...