Except actully, there's some evidence that caffeinne, for example, is helpful for people with ADHD. Prior to getting DS properly diagnosed and on medication. As with many others with ADHD children, the occasional use of caffeine is something we found helpful prior to DS being medicated. I can't really talk about the chemicals in many drinks etc, and certainly I'm not wild about them, but there's a lot of evidence that it's all very low in the bigger scheme of things.
I think there's more than enough truth to the fact that diets can make existing conditions worse/ more manageable. But to blame them for causing ND is ridicuolous. And in our house, DS is the one who has always been such a good eater that he's been eating all cooked from scratch, mostly the same food we eat, pretty much since he was weaned. While DD who is a fussy pain in the ass eats much more processed food as a result and doesn't have so much as a hint of any ND whatsoever.