MONDAY, Sept. 16 (HealthDay News) -- Kids who are the victims of bullies are often reluctant to report the abuse. But a new study shows that frequent and unexplained physical symptoms are common in bullied children, and experts say they can be signs that should alert parents and teachers to a problem.

The research is a combined look at 30 studies representing almost 220,000 school-aged children from 14 countries.

Taken together, the studies show that kids who are bullied are more than twice as likely as kids who aren't to report feeling bad or sick, even when there's no obvious explanation for their symptoms.

Headaches, stomach aches, back aches, neck or shoulder pain, dizziness, trouble breathing, tense muscles, nausea, diarrhea and bedwetting were among the most common complaints.

"The results of this study suggest that any recurrent and unexplained physical symptom can be a warning sign of bullying," said study author Gianluca Gini, an assistant professor of developmental psychology at the University of Padua, in Italy.

The new research was published online Sept. 16 in Pediatrics and will appear in the October print issue of the journal.

So when is a headache just a headache, and when is it a cry for help? Experts say paying attention and asking the right questions can usually help parents discover the truth.

"Children do not easily talk about their bullying experiences," Gini said, but asking if they feel safe at school is sometimes a good way to get them to open up.

Another expert says paying attention to the timing of these complaints can help, too.

"A lot of kids refuse to go to school. These headaches and stomachaches are there before a school bell or a school bus and then magically disappear and then they're fine," said Marlene Snyder, a faculty member in the Institute on Family and Neighborhood Life at Clemson University, in South Carolina.

Bullied kids may also seem sad or depressed or just not themselves, Snyder said.

"I think the first thing is really finding time to really observe, watch, and talk with kids so you know when something is off," said Snyder, who was not involved in the research. "Ask them questions: 'How was your day? Who did you sit with at lunch?'"

That may be easy enough to do with younger children, but high school-aged kids are sometimes tougher nuts to crack.

"You have to start taking a look at what's happening with the friend routine. Has the phone stopped ringing? Are they afraid to check their text messages? There are so many things that can tip off an aware parent," she said.

More information

For more about bullying, visit Stopbullying.gov.