When muscles are under maximum exertion, they produce lactate as a result of incomplete glucose breakdown in the presence of insufficient oxygen. This can build up in the form of lactic acid, causing pain and a reduction in performance.

But researchers in Denmark and The Netherlands have now discovered that the human brain can use this lactate by-product as an alternative source of energy during periods of strenuous muscle activity.

The researchers compared samples of blood running to and from the heads of volunteers engaged in strenuous exercise. The blood on its way to the brain was found to contain considerably more lactate than blood flowing away from the brain.

And the brain was not storing the lactate that had come from the muscles during exercise but using it as fuel. In fact, not only was the brain clearing lactate from the circulation but it was leaving more glucose free to be used as fuel for the muscles.

The findings help to explain why the brain is able to function properly at times when the body’s demands for fuel and oxygen are highest. They also go a step further to show that the brain actually moves into a higher gear in terms of activity.

From an evolutionary perspective, it is not difficult to deduce how individuals with the capacity to think clearly while running, perhaps from predators, or catching prey, would have an advantage over those without.

This opens the door to new areas of brain research related to understanding the specific neurological effects of lactic acid.

Other research, in California, has indicated that lactic acid can be used as an emergency fuel in the immediate hours following traumatic brain injury, instead of glucose, the current standard. It was found that in the first 12 to 48 hours following traumatic injury, the brain takes up and apparently consumes more lactate than at any other time.

Also, glucose requires 10 enzymatic steps before it generates energy, while lactate requires many fewer steps, making it a faster and more efficient source of fuel in such circumstances.