Lactate accumulates in the cerebrospinal fluid after prolonged exercise.
Level 4 - case-series / case-control
Prospective single-arm pre-post physiological study without a separate control group.
PubMed 41355164 · doi:10.1177/0271678X251399006
What was done
Thirteen healthy human participants underwent 2 hours of mixed-intensity cycling exercise to induce sustained increases in brain carbohydrate uptake. Researchers measured cerebral net lactate exchange and cerebrospinal fluid (CSF) concentrations before and after exercise to test whether CSF lactate accumulation accounts for excess carbohydrate uptake relative to oxygen consumption.
What was found
Exercise shifted the brain from net lactate release to extraction (p = 0.034), producing an excess brain uptake of 14.3 ± 3.7 mmol of carbohydrate over 2 hours. CSF glucose concentration remained unchanged (3.0 ± 0.2 vs 3.0 ± 0.1 mmol/L; p = 1.0), whereas CSF lactate concentration doubled (1.1 ± 0.05 vs 2.2 ± 0.3 mmol/L; p < 0.0001) and correlated with cerebral lactate uptake (r = 0.68, p = 0.015). This CSF accumulation represents a 15% increase in carbohydrate-based ATP availability, but accounts for only 6% of the unexplained carbohydrate extracted during exercise.
Why it matters
This study provides direct evidence in humans that prolonged exercise leads to lactate accumulation in the CSF. However, because CSF lactate stores represent only a small fraction of the surplus carbohydrate extracted by the brain, the primary metabolic fate of exercise-induced cerebral carbohydrate uptake remains largely unknown.
Limits
The study had a small sample size (n = 13) and lacked a non-exercise time-control group. It evaluated only healthy participants and could not directly measure intracellular parenchymal storage, brain glycogen synthesis, or other non-oxidative metabolic fates that might account for the remaining 94% of excess carbohydrate.
Cited by
- supports Human studies show that lactate generated from vigorous exercise crosses into the brain.