A good diet and lots of exercise does seem to relieve many "symptoms" of disorders which are probably misdiagnosed in the first place.
However, knowing a couple of kids who REALLY DO have Asperger's, they're totally different from "regular" kids. No one with an actual Asperger's kid is going "gosh, we thought he was a little different but we didn't pay it much mind" because these kids are fucking INTENSELY different from "normal" kids. That's one reason I don't buy these social retards who are dicks and then say "but we have Asperger's! It just wasn't diagnosed until I was an adult because my parents didn't notice!"
BULL SHIT. I wasn't diagnosed with ADHD until I was an adult because when I was a kid, only boys could have it. I was, however, subjected to all sorts of testing trying to figure out what WAS wrong with me, and I was such a terror in school that my mom pulled me out and "home-schooled" me (which mostly meant ignoring me while she partied with her little circle of hippie activist actor friends). There is no "my parents didn't notice". If your kid has a really real disorder, you fucking notice because the very first thing they do is make YOUR life insanely difficult.
I had a friend, Mike, when I was little, and he had Aspergers, and it was really, really obvious something was up with him (to everybody but geeky me, who couldn't fathom why a kid so smart was in special ed.) Had just about zero ability to control his weird crap and made his mom's life hell (or so I hear. Again, I wasn't paying attention, but I vaguely remember him doing weird aspie things whenever his mom wanted him to do something.) He's a far cry from some antisocial nerd making up excuses for being weird, and in his case, meds & a long stay in a private school for autism spectrum kids was the only way to keep him under control.