The research says that the sugar/hyperactivity link is a myth.
However, a bad diet will make things worse.
The best diet for ADHD is as unprocessed as possible, lots of protein, lots of healthy fats, and moderate carbs in the form of whole-grains and vegetables.
However, in working with children and families for over 20 years, the only time that I have seen miraculous results due to diet changes have been in infomercials.
Personally, I tend to have a feast or famine cycle. When things are good, my diet, sleep, exercise, and ADHD symptoms are all well-managed.
When one goes, they all seem to go.
Twice, I used carb manipulation to lose weight.
I lost a ton of weight quickly, but had to stop due to the impact on my mood and ADHD symptoms.
I then immediately gained the weight back once I started eating carbs again.