I recall a study showing that eating lower-GI breakfast cereals helps schoolchildren focus. Perhaps this is related to blood glucose's relation to willpower?

Up until recently my diet was around 50% fruit and fruit-juice, but lately I've tried cutting fruit out and replacing it with carbs and fat and protein. I'm not sure whether this has strongly affected my willpower. My willpower /has/ improved, but I started exercising more and went onto cortisone around the same time, so I'm not sure what's doing it. However, the first few days without sugary food, especially in the mornings, I actually became depressed and tired-feeling, almost the same feeling you get during caffeine withdrawal. That feeling has disappeared now.

Thoughts?