The research shows diet has little to no impact on ADD ... although caffeine acts like Ritalin only a shorter-term effect. Caffeine will temporarily remove the hyperactivity and helps them focus.
Considering diet directly affects your insulin levels and therefore your hormone regulation I’d say that research is lacking. Changing your diet can drastically alter your energy levels and general mood. Hyper active neural activity may not be directly impacted but an erratic emotional state can not possibly help.