Fuelling cerebral activity in exercising man.
Level 5 - mechanism / opinion, no new human data
Narrative review synthesizing physiological mechanisms without primary trial data or systematic search.
PubMed 16395281 · doi:10.1038/sj.jcbfm.9600256
What was done
This narrative review describes cerebral substrate uptake and energy utilization in exercising humans, focusing on changes in the cerebral metabolic ratio (MR, defined as O2 uptake divided by glucose + 1/2 lactate uptake).
What was found
At rest, cerebral MR is close to 6, reflecting balanced glucose oxidation. Light exercise maintains a stable MR, but demanding exercise reduces MR by 30% to 40% due to surplus glucose uptake relative to oxygen. During vigorous exercise with elevated blood lactate, brain lactate uptake matches glucose uptake and is oxidised without intra-cerebral accumulation. The surplus uptake of glucose equivalents reaches approximately 10 mmol, matching estimated total brain glycogen levels. The review hypothesizes that low MR marks an energy shortage that limits motor activation in central fatigue.
Why it matters
It outlines how high-intensity exercise shifts cerebral energy substrate dynamics from pure glucose oxidation to lactate utilization, providing a metabolic hypothesis for central fatigue.
Limits
The abstract reports a narrative review with no systematic search criteria, quality appraisal, or study count (n = ?). Numerical findings are presented as broad physiological estimates without variance measures or confidence intervals, and the link between low MR and central fatigue remains hypothetical.
Cited by
- supports During exercise, elevated blood lactate substitutes for glucose as an energy source in the brain.