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However, autistic children do have a higher rate of constipation and eating issues, such as eating the same foods over and over, according to the study. But any number of factors, including medication, could cause these issues, the researchers said.
The findings appear in the August issue of Pediatrics.
Dr. Patricia Manning-Courtney, medical director of the Kelly O'Leary Center for Autism Spectrum Disorders at Cincinnati Children's Hospital Medical Center, said that the study is an important one because it was the first to scientifically compare the incidence of gastrointestinal (GI) problems in an autistic population with incidence in children who are developing normally.
Even so, it's unlikely to quell a controversy that has been raging for more than a decade.
"A couple of highly publicized cases of autism and loose stools in the late 1990s led to an impression that children with autism had a higher rate of GI dysfunction," she said. "It wasn't well characterized, but that got the story onto the national scene."
This, in turn, led to theories that diets free of gluten and the milk protein casein might help the problems, along with treatment with the hormone secretin. Some even proposed that problems with the gut might be the cause of autistic symptoms, giving the matter added urgency.
But the evidence in favor of these hypotheses was "fuzzy," Manning-Courtney said.
For the new study, researchers followed 124 children with autism and 248 children without autism until they turned 18.
Gastrointestinal diagnoses were classified into five groups: constipation; diarrhea; abdominal bloating, discomfort or irritability; gastroesophageal reflux or vomiting; and feeding issues or selectivity (people with autism often stick with the same food choices and have other "ritualistic tendencies" in eating).
The frequency of GI symptoms was about 77% in the autism group and 72% among the others, not considered a statistically significant difference.
However, almost 34% of the autistic children, compared with nearly 18% of the others, had constipation. Feeding issues were present in about 24% of the autistic children and 16% of those who weren't autistic, the study authors found.
Those differences could stem from what the researchers called neurobehavioral issues connected with autism, such as the ritualistic practices, they said.
"Many patients with autism insist on eating the same thing and might not consume enough fiber," said the study's lead author, Dr. Samar H. Ibrahim, a fellow in gastroenterology and instructor in pediatrics at the Mayo Clinic. "We think this may be contributing to the constipation."
Medications that autistic children take can also interfere with appetite and eating.
"In our study, around 50% of children were on stimulant medications, and those might affect appetite and might have something to do with issues with food," Ibrahim said.
Keith A. Young, vice chairman for research in the psychiatry and behavioral science department at Texas A&M Health Science Center College of Medicine, said he thought "the really interesting thing they found was that there wasn't any indication of celiac disease."
In the group studied, the researchers found just one case. A possible link between celiac disease and autism was proposed as far back as 1961.
"The only difference [between the two groups] was the constipation, which they kind of dismiss," Young said. "And I tend to agree with their conclusion that constipation might be related to the dietary habits of autistic kids."
The constipation and feeding difficulties identified in the study, though, are not the symptoms that people have been buzzing about, Manning-Courtney pointed out. It's mostly been about diarrhea and loose stools.
But the study was a small one and did not rely on rigorous, comprehensive diagnoses of autism for the participants, so it may just stir the pot for people on both sides of the debate.
The researchers, however, are urging parents away from restrictive diets.
"Patients with autism should be investigated the same way as normal patients when they have GI symptoms," Ibrahim said. "And doctors should do a thorough investigation before labeling them with any GI disorder because the overall incidence of any GI disorder did not differ from that of the overall population."
Manning-Courtney concurred. "For me, this study lends support to the recommendation I make to a lot of families that there is no evidence to support restricted diets," she said. "They're dangerous and risky. You have to think long and hard before you, as a parent, make that choice."
SOURCES: Samar H. Ibrahim, M.B.Ch.B., fellow, gastroenterology, and instructor, pediatrics, Mayo Clinic, Rochester, Minn.; Patricia Manning-Courtney, M.D., medical director, Kelly O'Leary Center for Autism Spectrum Disorders, Cincinnati Children's Hospital Medical Center, Cincinnati; Keith A. Young, Ph.D., vice chairman, research, Department of Psychiatry and Behavioral Science, Texas A&M Health Science Center College of Medicine, College Station, Texas, and chairman, Tissue Advisory Board, Autism Speaks; August 2009 Pediatrics
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