I don't seem to have a problem either with wheat or dairy (but I know that many people with ASDs do). Still, diet is very important. If I don't eat enough, I get cranky - teary, touchy, irritable, tense. I'm much more likely to have that "hazed" feeling and not be able to think straight. Finally, I may simply get physically weak. That can be a problem when my appetite disappears.
I tried to become a vegetarian when I was around 15, and didn't stick to it too long. It just isn't for me. Not eating meat also makes me crazy and slow like this and it isn't nice.