I find this subject interesting. There's some pretty scary stuff in food, including petroleum-based dyes and preservatives. Carcinogen candy, anyone?
Apparently the artificial stuff in food can actually cause mental and behavioral problems (the latter being similar to ADHD). A friend of mine can't eat anything modified or artificial because he gets emotional, aggressive, etc. I know from experience that it can really mess with your head.
As for obesity and the like...I'm not really sure. I think people need to stop eating junk and actually exercise. But I'm not exactly a doctor. So that's not really valid. :T