The study, a review of medical research, found that children with autism are more than four times as likely as their typically developing peers to have digestive difficulties such as abdominal pain, diarrhea or constipation.
The study authors stress that doesn't mean the gastrointestinal troubles are the cause of autism, as one widely discredited theory has suggested, or that something about the biology of autism causes stomach complaints.
No cause-and-effect relationship has been established.
But the researchers say the link between the two problems is strong enough that it deserves more investigation.
"We really searched high and low for good studies, and we only found 15 studies on this over 32 years," said study author William Sharp, a behavioral pediatric psychologist at the Emory University School of Medicine, in Atlanta.
"This is something that is a prominent concern for kids with autism, and that we should refocus our scientific endeavors to get an evidence-based approach for assessment and treatment of these kids," said Sharp, who specializes in feeding problems in children with autism.
For the review, published online April 28 and in the May print issue of Pediatrics, Sharp and his colleagues combed the medical literature looking for studies of digestive problems in autism. Then they narrowed their search to studies that included a comparison ("control") group. These control groups were usually typically developing peers or siblings of children with autism.
Out of a pool of 961 possible studies, only 15 included a control group. But even those studies had some problems, Sharp conceded. They used different definitions for frequent complaints such as diarrhea and constipation. Many relied on reports from parents to document a problem while others only counted stomach issues if they were noted in a child's medical chart.
After smoothing out those differences as well as they could, researchers found high rates of gastric problems in children with autism. Diarrhea and constipation were the most frequently reported problems. Kids with autism were almost four times as likely to have diarrhea or constipation compared to typically developing children, the investigators found. And they were more than twice as likely to experience abdominal pain.
Doctors don't yet know why kids with autism are more prone to digestive problems, but they believe a lot of it may be behavioral.
Some studies have suggested that as babies, children with autism don't breast-feed well. Breast milk helps to develop and protect an infant's intestinal tract. That may be something children with autism miss out on.
They can also be picky eaters.
"If a parent says their child has occasional constipation, is that enough to say a kid has a GI issue, if they're very selective and only eat white rice?" asked Cynthia Johnson. She is the director of the autism center at Children's Hospital of Pittsburgh. She was not involved in the research.
Until more is known, Johnson said, parents should bring digestive issues to a doctor's attention.
"They need to know to raise their concerns with their pediatrician to make sure their child doesn't need to be seen by a gastroenterologist," she said. "But if it seems diet-related, they probably need to talk to a dietitian for a recommendation."
For more about autism, visit the U.S. National Institute of Mental Health.