A reduced cerebral metabolic ratio in exercise reflects metabolism and not accumulation of lactate within the human brain.
Level 3 - non-randomized controlled study
Non-randomized comparative physiological study with parallel control cohort
PubMed 14608005 · doi:10.1113/jphysiol.2003.055053
What was done
Healthy volunteers participated in two physiological experiments to determine whether lactate taken up by the brain during exhaustive exercise is metabolized or stored. In the first experiment, cerebral arterio-venous (AV) differences for oxygen, glucose, and lactate were measured at rest, during, and after exhaustive exercise in 9 subjects, followed by lumbar puncture for cerebrospinal fluid (CSF) lactate analysis post-exercise (n = 7) compared against resting healthy controls (n = 6). In the second experiment, post-exercise cerebral lactate concentrations were assessed in 5 subjects using proton magnetic resonance spectroscopy (1H-MRS).
What was found
Maximal exercise significantly increased cerebral AV differences for oxygen from 3.2 ± 0.1 mM at rest to 3.5 ± 0.2 mM (P < 0.05), glucose from 0.6 ± 0.0 mM to 0.9 ± 0.1 mM (P < 0.01), and lactate from 0.0 ± 0.0 mM to 1.3 ± 0.2 mM at exhaustion (P < 0.01). Consequently, the cerebral metabolic ratio decreased from 6.0 ± 0.3 to 2.8 ± 0.2 (P < 0.05) and remained depressed during early recovery. Post-exercise CSF lactate (1.2 ± 0.1 mM; n = 7) did not differ from baseline controls (1.4 ± 0.1 mM; n = 6). Post-exercise 1H-MRS brain lactate signals were lower than the estimated detection threshold of approximately 1.5 mM.
Why it matters
This study provides direct human physiological evidence that the brain actively metabolizes circulating lactate during intense exercise rather than passively accumulating it in cerebral fluids or tissues.
Limits
Sample sizes were very small across all experimental groups (n = 5 to 9). Post-exercise CSF lactate was compared against a separate control cohort rather than baseline measurements within the same individuals. Additionally, 1H-MRS had an estimated detection limit of ~1.5 mM, leaving minor increases below that threshold undetectable.
Cited by
- supports During exercise, elevated blood lactate substitutes for glucose as an energy source in the brain.