Abstract
The resting brain is fuelled by glucose with a small release of lactate. During exercise, the brain switches to extracting lactate from blood and this increases brain carbohydrate uptake in great excess to that of oxygen. The fate of this excess carbohydrate uptake is unknown. Studies investigating the fate of brain carbohydrate uptake use brief periods of brain activation and so it is possible that the dissociation between brain glucose and oxygen metabolism is temporal and not material. In 13 healthy humans, we induced sustained increases in brain carbohydrate uptake via 2 h of mixed-intensity cycling exercise and hypothesized that lactate accumulation in the cerebrospinal fluid would account for some of this excess carbohydrate uptake. Exercise shifted the brain from releasing to extracting lactate (p = 0.034), causing an excess uptake of 14.3 ± 3.7 mmol of carbohydrate over 2 h of exercise. Although CSF glucose remained perfectly stable (3.0 ± 0.2 vs 3.0 ± 0.1 mmol/L; p = 1.0), CSF lactate concentration doubled (1.1 ± 0.05 vs 2.2 ± 0.3 mmol/L; p < 0.0001) and was correlated to cerebral lactate uptake (r = 0.68, p = 0.015). This accumulation of lactate in CSF represents a 15% increase in carbohydrate-based ATP availability, but accounts for only 6% of the unexplained carbohydrate extracted by the brain during exercise.