One of the things the AoAers and many biomed parents like to trot out is the idea that intestinal issues are to blame for autism. As someone with lifelong issues in that area, I absolutely and from a personal perspective get how these issues impact quality of life and certainly mood. When you're in pain, it's really difficult to deal with things. However, there is no good science linking autism to intestinal distress.
Are intestinal issues common in autistic individuals? Yes, they are. Science Daily reports on a new study: "A new study conducted by Autism Speaks' Autism Treatment Network (ATN) shows that GI symptoms occur in nearly half of children with ASD, and the prevalence increases as children get older."
Are they common in the general public? Yes, they are: "Results: In the 10 selected studies, the reported prevalence of upper abdominal symptoms (mostly upper abdominal pain or discomfort) ranged from approximately 8% to 54%, while the prevalence of heartburn and/or regurgitation ranged from 10% to 48% for heartburn, from 9% to 45% for regurgitation and 21% to 59% for both/either" (Heading, 1999). And that's just the upper GI problems.
Why might intestinal and digestive issues be so common to both the general population and those with autism? The reasons are myriad. One factor that must be looked at is diet. It's a fair assumption that individuals with autism with restrictive diets that don't meet the nutritional requirements are going to have some problems, especially if fiber isn't being consumed in sufficient quantity. If you have a low fiber diet, you're going to have pain associated with the formation of the fecal matter. How many people make sure that they get enough fiber? That they don't consume more fat than they can break down? That they don't consume too many foods that cause bloating and gas? If the average joe isn't expending a whole lot of time making sure to consume foods so as to minimize gastrointestinal distress, then, it isn't any surprise that individuals with even more limited diets are going to have even more issues.
Yes, intestinal issues are common in individuals with autism, but that has nothing to do with causing autism. Ibrahim et al. (2009) conclude: "As constipation and feeding issues/food selectivity often have a behavioral etiology, data suggest that a neurobehavioral rather than a primary organic gastrointestinal etiology may account for the higher incidence of these gastrointestinal symptoms in children with autism."
I know, expecting parents who have felt the need to blame everything on the autism and everything for the autism are going to be really poor at sifting through all the data and coming to a sound, scientifically-based conclusion. And they're not likely going to get any better at figuring out what behaviors are caused by the differences in neurology and what are not, which symptoms are autistic symptoms and which are symptoms caused by something else, like, for example, the quack treatments they put their children through.