There have actually been some good peer-reviewed studies showing that food allergies and sensitivities may truly play a role in ADHD symptoms for some, but not all, children. I think some people upthread mentioned this. However, as others suggested, the way to go about figuring out if this is an issue for your child would indeed be to keep a systematic diary and do a systematic elimination diet, starting with the most common offenders (dairy, soy, corn, wheat, fish, nuts--I think that's right, but I'm no expert). I too am very suspicious of the "converts to sugar" advice.