When the human body kicks into high gear, the brain can run on recycled, alternative energy to make the whole system more efficient, scientists have discovered.

The body breaks carbohydrates from food into glucose, the primary fuel of muscles and the mind. When muscles work hard, they produce lactate as a byproduct. Though recent studies have shown that muscles can re-use the lactate as fuel, conventional wisdom holds that it can also build up in the form of lactic acid, reducing performance and causing pain.

The new research shows that the brain can switch to lactate during strenuous exercise, using the muscles' byproduct as backup fuel.

The researchers, in Denmark and The Netherlands, looked at existing studies that had compared the 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 from the brain. Further investigation showed that the brain was not storing the lactate which had come from the muscles during exercise, but rather using it as fuel. In fact, the brain helped to clear lactate from the circulation, thereby leaving glucose to the muscles that need it for the hard work they were performing.

The finding helps explain why the brain is able to work properly when the body's demands for fuel and oxygen are highest, and it shows that the brain actually shifts into a higher gear in terms of activity. This opens doors to entirely new areas of brain research related to understanding lactate's specific neurological effects that could lead to new treatments for common brain ailments, the researchers said in a statement today.

The findings are detailed in the October issue of the FASEB Journal.

"From an evolutionary perspective, the result of this study is a no-brainer," said Dr. Gerald Weissmann, editor-in-chief of the journal. "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."