I'm not an ADHD denier, but the 'condition' needs to be approached with caution having worked with children who probably genuinely have the condition whilst others have been labelled with it simply to mask or convientiently explain away extremely bad behaviour without parental criticism or responsibility.
Yes an appropriate sugar free diet is probably more effective than pumping kids with ritalin, but above all, regardless of whether the condition is real or not, firm effective care is essential. All children need to learn appropriate behaviours and boundaries to those behaviours. Unfortunately being diagnosed as ADHD often absolves those children from learning effective behaviours and/or their carers from helping develop and enforce them.
I spent a number of years working within one of Europe's then leading institutions for children with behavioural difficulties. The unit for the youngest children worked on a token system where children were rewarded for non-bad behaviour, and could receive tokens for simply going for a length of time without hitting others. These tokens could be swapped for rewards such as activities, time on the pool table etc.
One 12 year old spent many years in the unit and before his release was weaned off the token economy. On a rare unsupervised trip out he helped an elderly lady across the road, then, when she failed to reward him with a token for his good behaviour, he seriously assaulted her.
This reflects the inherent danger of attempting to manage behaviours through external means such as medication as it relieves the individual from learning internal control mechanisms. Yes, many diagnosed with ADHD find the associated behaviours diminishing as they leave childhood, but for many others as they enter adulthood they find they haven't developed the social skills required to face those challenges and many find themselves within the penal system for violent behaviours.
The best 'cure' is to accept medical advice, have a healthy diet but remaining aware that such medical advice doesn't absolve the recipient from learning the social skills required by all regardless of any prognosis.