Not a doctor, so I might be stating the obvious to anyone who is, but couldn't this explain why exercise seems to make people feel more energetic? If a buildup of pollutants in the brain can make you feel fatigued, then better cardiovascular fitness and healthy kidneys would help clear these pollutants out more effectively, right? It isn't that you have more energy, it's just that being healthier is helping clear the things that make you mentally fatigued.

I guess you could also turn this on its head for people with chronic fatigue by looking at their cardiovascular and kidney function.