Brain substrate utilization during prolonged exercise

Substrate utilization by the brain was studied in 7 subjects at rest and during moderately heavy bicycle exercise for one hour. Blood samples from the internal jugular vein and a peripheral artery were obtained at rest and at timed intervals during exercise. At rest the a-v difference for glucose across the brain was 0.50±0.09 mmoles/l which, if oxidized, could account for l00±13 % of the oxygen a-v difference. During exercise, when the concentrations of lactate, pyruvate, and glycerol rose very considerably, no change was detected in the brain's utilization of glucose, nor was there a consistent uptake of any other substrate. It is concluded that although the brain has the enzymatic capacity to alter its substrate utilization, no such adaptation takes place during exercise of this type and duration.