Sugar does not cause hyperactivity in children.[230][231] Double-blind trials have shown no difference in behavior between children given sugar-full or sugar-free diets, even in studies specifically looking at children with attention-deficit/hyperactivity disorder or those considered sensitive to sugar.[232]
The results indicate that children’s performance declines throughout the morning and that this decline can be significantly reduced following the intake of a low GI cereal as compared with a high GI cereal on measures of accuracy of attention (M=-6.742 and −13.510, respectively, p<0.05) and secondary memory (M=-30.675 and −47.183, respectively, p<0.05).
I mean, if you are oscillating between sugar highs and crashes, it is difficult to concentrate, plus it causes diabetes etc..
Is this what you have in mind?
wikipedia
No, I have this in mind:
http://www.ncbi.nlm.nih.gov/pubmed/17224202
I don’t have time to evaluate which view is less wrong.
Still, I was somewhat surprised when I saw your first comment.
Upvoted for not wasting time!