The idea that refined sugar causes ADHD or makes symptoms worse is popular, but this notion is not at all supported by sound science.
In fact, far more research debunk rather than support this theory.
In one instance, researchers gave children food containing either sugar or a sugar substitute (like an artificial sweetener) every other day.
The children who received sugar did not demonstrate any different behaviour or learning capabilities compared to those who received the sugar substitute.
Another study in which children were given higher than average amounts of sugar or sugar substitutes showed similar results.
In yet another study, children who were considered "sugar-sensitive" by their mothers were given aspartame, a popular artificial sweetener.
Although all children were consuming aspartame, half of the mothers were told their children were given sugar, while the other half were told the real truth (that their children were given aspartame).
The mothers who thought their children ingested sugar rated their child as "more hyperactive" than the other children, and were more critical of their behaviour, compared to mothers who thought their children received aspartame.
If anything, this experiment demonstrates that the power of suggestion is indeed a strong one.
Source: National Institute of Mental Health