This one's good, but vague at the end. It basically says correct doses of sugar and carbs are important, which means you CAN overdose:
http://www.psychologytoday.com/articles/200310/what-is-good-brain-food
I like this one because it simply says too much carbs slows your metabolism and too much sugar speeds up your fat-producing hormones:
http://www.livescience.com/health/090107-brain-food.html
Here's the Wiki on Insulin and I love it because Insulin has the most immediate impact on your brain as you eat. The info is a bit technical, but it basically says:
Sugar intake affects Insulin levels since Insulin is what takes care of sugar in our blood stream. A "sugar rush" is actually an Insulin spike. Too much Insulin will sweep away all the sugar and you will "crash" soon after. Insulin is more controlled when sugar intake is spread over more time in smaller doses. Also, uncontrolled Insulin surges over time can cause Diabetes and Alzheimers Disease:
http://en.wikipedia.org/wiki/Insulin_delivery#Modes_of_administration
I could post more, put I think there's enough there to convert the non-believers. ...Unless someone has some cultish distrust of the mainstream medical community.