Gastrointestinal symptoms in a sample of children with pervasive developmental disorders.
I have not read the full text of the study as of yet, but from the abstract it talks about GI symptoms in a population of 172 children with autism. In this sample there were "thirty nine (22.7%) were positive for GI problems, primarily constipation and diarrhea". There were no differences on some measures of adaptive functioning between the groups but the group with GI problems "showed greater symptom severity on measures of irritability, anxiety, and social withdrawal. Those with GI problems were also less likely to respond to treatment."
The problem that is immediately apparent is that there isn't any information about how common GI problems are in a sample of normal children. However I would imagine that if 22% of "normal" children had persistent GI issues that it would have been reported by now.
I find this study interesting because it documents that there is an identifiable subset of children with autism that have GI issues. If there is a GI issue then it is possible that some of the symptoms of autism can be corrected by addressing the dietary issues, especially if the group with GI issues show greater symptom severity as eluded to by the abstract.
But there are those that do not accept that this is the case or say that it is unrelated to the autism. For example, the self-styled "Autism Diva" frequently argued that there was no relation as in this sequence of comments:
The GFCF diet should NOT be tried on all autistic kids and all kids with behavior issues. This is just bizarre and has not basis in fact.
If a child is getting sick after eating wheat or dairy, then fine, change the diet, but that does nothing to the autism.
If you read through the comments you will see a lot of talk against the relation between autism and diet/GI issues. Now I did partially instigate this sequence of comments but this is fairly representative of a normal exchange.
I do know that my daughters both had GI issues and that their stomaches were bloated consistently for the first two years of their lives. Once we changed their diet the issues diminished and then faded (although there are still some issues occasionally). As a side benefit their attention, ability to focus, and eye contact went up as we restricted their diet.
But that is my experience, your millage will vary.