According to a study by researchers from Denmark and The Netherlands published in the October 2008 print issue of The FASEB Journal, the brain, just like muscles, works harder during strenuous exercise and is fueled by lactate, rather than glucose.
One thing that this might indicate is that even when you are running low on glucose, your brain can be fuelled in an alternative way. I'm aware that the brain can run on ketones. Here is another energy source and another reason why carbs may not be so necessary......
This press release suggests an evolutionary perspective:
"From an evolutionary perspective, the result of this study is a no-brainer (a pun perhaps?!). Imagine what could have or did happen to all of the organisms that lost their wits along with their glucose when running from predators. They were obviously a light snack for the animals able to use lactate."
You need a fuel for when the glucose runs out. Of course you do! This is beautiful stuff
Not only does this finding help explain why the brain is able to work properly when the body's demands for fuel and oxygen are highest, but it goes a step further to show that the brain actually shifts into a higher gear in terms of activity.
Lactate fuels the human brain during exercise
The human brain releases a small amount of lactate at rest, and even an increase in arterial blood lactate during anesthesia does not provoke a net cerebral lactate uptake. However, during cerebral activation associated with exercise involving a marked increase in plasma lactate, the brain takes up lactate in proportion to the arterial concentration. Cerebral lactate uptake, together with glucose uptake, is larger than the uptake accounted for by the concomitant O2 uptake, as reflected by the decrease in cerebral metabolic ratio (CMR) [the cerebral molar uptake ratio O2/(glucose+ lactate)] from a resting value of 6 to <2.> CMR also decreases when plasma lactate is not increased, as during prolonged exercise, cerebral activation associated with mental activity, or exposure to a stressful situation. The CMR decrease is prevented with combined β1- and β2-adrenergic receptor blockade but not with β1-adrenergic blockade alone. Also, CMR decreases in response to epinephrine, suggesting that a β2-adrenergic receptor mechanism enhances glucose and perhaps lactate transport across the blood-brain barrier. The pattern of CMR decrease under various forms of brain activation suggests that lactate may partially replace glucose as a substrate for oxidation. Thus, the notion of the human brain as an obligatory glucose consumer is not without exceptions.—Quistorff, B., Secher, N. H., and Van Lieshout, J. J. Lactate fuels the human brain during exercise.