ADHD and effects of diet on severity of symptoms is a well debated topic in the attention research!
There is a good evidence base for omega 3's for improving focus and concentration. Sugar, on the other hand, whilst it does not cause ADHD, can contribute to the severity of children's behaviour difficulties.
We all know someone whose children run around madly after a sugar hit, quickly followed by tears and sometimes unwanted behaviours. Current research shows that eliminating added sugar from our children's diet has many health benefits. For the child with ADHD, anecdotal evidence suggests that reducing sugar intake can have a positive outcome in terms of behaviour, focus and concentration.
We have an interest at our clinic in whether a ketogenic diet (extremely low sugar) may be beneficial in kids with ADHD. Current research is inconclusive. Generally, a good quality low sugar diet does has conclusive evidence for improved health and well-being and we all need to strive or this as a minimum standard for our kids.