I saw this post on a friends Facebook link, a NY Times article entitled "How Exercise Fuels the Brain." In it, researchers in Japan developed a way to see what nutrients are in the brain right after exercise and where they go. For the longest time, it was thought that the only way the brain was supplied was through blood. The researchers found new information that showed that support cells called Astrocytes store small amounts of glycogen. They are able to produce energy for the brain. After exercise, the researchers were able to see that the brain had depleted glycogen in the frontal cortex (thinking, memory and movement) Sounds to me like fatigue.
They studied one single bought of exercise and several boughts of exercise over 4 weeks. They saw that the four week group, had a lot of glycogen stores go to the brain where it had been depleted and can actually increase. Brain carbo load.
What's that mean? If your brain can store more glycogen, essentially, you will be able to perform movement longer, potentially have some cognitive carryover as well. Personally, I've always noticed that your Cross Country teams always seem to have a very high GPA.