"Sometimes you have parents who pursue a treatment that has not been shown to work, to the detriment of treatments that have been shown to work," he said.
But parents who want hard scientific evidence before trying the casein-free, gluten-free diets may have to wait a while.
"For many researchers this would be a lower priority. The problem is there's limited federal dollars, it's an amazing amount of work to get these [treatment] studies done," said Volkmar.
Complicating the problem is the vast spectrum of behaviors and medical issues within autism. Gastrointestinal issues may be more common in some parts of the spectrum but not in others. Doctors say lumping children with autism into one diet study may confuse the results.
"Studying them [patients] in large groups of people who are heterogeneous -- very different from one another -- is not the best way to learn what is going on with these diets," said Dr. Martha Herbert of Massachusetts General Hospital in Boston. "You will wash out any effect in a subgroup by blurring them in with others who have different problems."
Dr. Arthur Beaudet, who contributed to the published statements, said he believes genetic studies may one day lead to better research of gastrointestinal problems in children with autism.
For instance, children with autism symptoms caused by one genetic mutation may be more likely to develop gastrointestinal issues than children who have a different genetic mutation -- or none at all.
Beaudet said current research has already pinpointed subgroups of people with autism who have a specific mutation and distinct behavior, such as the estimated 1 percent of people with autism who have the mutation known as "16p11.2"
"There's a very broad spectrum of autism from very severe handicaps to very mild handicaps… we have to break these down to the smallest extent possible," said Beaudet.