Children with chronic or recurring stomach pain without a clear medical explanation were also more likely to have an anxiety disorder than those without stomach problems, in a new study.
By the time kids with stomach pain reached age 20, just over half had had symptoms of an anxiety disorder at some point, most often social anxiety, researchers found.
Anxiety tended to start in early childhood, around the same time as the chronic stomach problems.
Past studies suggest between eight and 25 percent of all youth have chronic stomach pain, researchers noted. When there's no clear medical cause for the pain - such as inflammatory bowel disease or celiac disease - it's known as functional abdominal pain.
"It's very prevalent, and it's one of the most common reasons that children and adolescents end up in their pediatrician's office. It's one of the most common reasons kids are missing school," said Dr. Eva Szigethy, head of the Medical Coping Clinic at the Children's Hospital of Pittsburgh Inflammatory Bowel Disease Center.
One small study of children with that type of pain found they were at a higher than average risk of anxiety disorders as young adults.
To build on those findings, Lynn Walker from the Vanderbilt University School of Medicine in Nashville, Tennessee, and her colleagues followed 332 children who visited a doctor for unexplained stomach pain between age eight and 17.
For comparison, they also tracked 147 youth from the same area schools without stomach problems.
When participants were 20 years old, on average, the researchers interviewed them in person or over the phone about symptoms of anxiety and depression. At that point, four in 10 of those with a history of stomach pain still had a gastrointestinal disorder.
Based on the interviews, Walker's team found 51 percent of people with stomach pain as children had ever had an anxiety disorder and 30 percent currently met the criteria for a diagnosis.
In comparison, 20 percent of people in the no-stomach pain group had ever had an anxiety disorder and 12 percent currently had one.
"What was striking was the extent to which anxiety disorders were still present at follow-up," Walker told Reuters Health.
Anxiety was more common among people who continued having stomach pain compared to those whose childhood symptoms went away, she and her colleagues wrote Monday in Pediatrics.
Although the researchers couldn't tell from their analysis which came first - the pain or the anxiety - most anxiety disorders traced back to early childhood.
A 'vicious cycle'
Szigethy, who wasn't involved in the new research, said that in her experience it's "extremely common" to find functional abdominal pain and anxiety occurring together.
"We've noticed clinically that often the anxiety does predate the onset of pain," she told Reuters Health.
She said children with anxiety may be more sensitive to pain, and may constantly worry about any pain they do feel.
"People who are anxious tend to be very vigilant to threat, scanning their environment or their body for something that might be wrong," she said. Those children are more likely to get into a "vicious cycle" of staying home from school due to a stomachache, getting behind on schoolwork and becoming more anxious, Walker added.
The researchers both said doctors treating kids with unexplained stomach pain should be asking about anxiety as well.
The interviews also showed 40 percent of participants with childhood stomach pain had been depressed at some point, versus 16 percent of those in the comparison group.
Szigethy noted that the study didn't track how kids were treated for anxiety or stomach pain and whether that affected their symptoms as young adults. That, she said, is "a next step to be looking at in this type of work."