I have an 8 yr old with extreme aggression issues. He's diagnosed with ADHD (innatentive type) and apraxia of speech. Nothing wromg with his intelligence. We also have a family history of mental illness including the ones you've mentioned.
At the suggestion of friends I tried removing gluten from his diet. He was already dairy free. (That had helped up to a point.) With the removal of gluten I have not seen any physical changes (he looked healthy to begin with). By the end of the second week the improvement in behavior was dramatic. We've tried a few challenges in the past few weeks; a donut (may not be fair because it also had milk protein), a dairy free granola bar (oat), a sandwhich with wheat bread(no dairy). Within half an hour the aggression and impulsive behavior was back and lasted for the remainder of the day. It has not been a cure all, when he is stressed (hungry, tired) the maladaptive behaviors kick in. But, it is sooo much better.
I'm trying to get my family on board to see if it will help any of them.
I actually came to this site looking for why the diet change might be helping.
Jodie
There have been research studies of diets with OCD, however, nothing has been scientifically proven that they really has any affect on the behavior of OCD. A dietary recommendation for anxiety disorders is to limit caffeine containing foods which may help decrease the anxiety. The best management for OCD is behavioral modification and medication, if necessary, followed by a Doctor. The Child Behavior Forum may have additional information for you.