I have known quite a few kids with ADD, ADHD, etc and they all had one thing in commmon - the parenting, lack of consistent disipline. So in answer to your question, it could be the diets as well - an undisiplined child probally does not get the same healthy foods as a well-brought up child.
But personally I believe it is more the lack of proper parenting that causes the problems - I also believe pychologists love to invent new and not-so-wonderful mental issues (not saying all are invented - I am sure some are quite real) which 9 times out of 10 seem to take responsibilty of actions away from the effected -
As was the case (well from how i saw it) on a news artical I saw a few years back, it basically stated that there is a switch in our brain that stops us from doing wrong things and that switch is not fully active until we are 21(i think that was the age - it was 20 something) so from this "finding" psychologist have given anyone under the age of 21 the right to do what ever they want - because at the end of the day they can't help it.
Anyway I think I went a little off topic - and would like to finish with... I think parents need to raise their kids properly and kids need to learn how to be completly responisble for their own actions.