artificial flavors and
colors, some preservatives, sugar, caffeine
some fats. I know many ADHDers, and many of them have been on diets that cut out foods with those ingredients. The diets made no impact. However, we should all (ADHD or not) cut those ingredients down to a minimum in our diets, as it's beneficial for our health.
In all honesty, I have found that caffeine and sugar can offer some level of self-medication for someone with ADHD - but I don't encourage it!