Childhood obesity may be more than just a health risk. It may also, apparently, be keeping kids out of school.

Researchers from Penn and Temple University have, for first time, documented a surprising link between childhood obesity and increased school absenteeism. The study of more than 1,000 fourth, fifth and sixth graders in Philadelphia city schools found heavier children missed 20 percent more school days than their normal-weight peers. The findings suggest that as the rate of childhood obesity in the United States continues to increase, so too will school absenteeism.

Race, socioeconomic status, age and gender have been the four main indicators of increased absenteeism among school children. However, body-mass index, or BMI, was a better indicator of poor classroom attendance, according to this study.

“What we showed is, more so than race, age, sex and socio-economic status, body-mass index is the best predictor of being absent from school,” says Andrew B. Geier, lead author and a graduate student in Penn’s Department of Psychology.

Over the last 25 years, the percentage of children aged 2 to 5 years who are overweight or obese more than doubled to 13.9 percent. The rate of obesity increased from 6.5 to 18.8 percent for those aged 6 to 11 years, and it more than tripled to 17.4 percent for kids aged 12 to 19 years, according to the Centers for Disease Control and Prevention.

“Excess weight gain among adolescents and children in the United States is a serious problem. And it’s getting bigger,” Geier says.

Previous studies have shown students who miss school suffer academically, but never before has the link between obesity and school attendance been examined.

“Absenteeism really sets you up for so many negative things. The more you’re absent from school, the more likely your risk of using drugs, HIV, teen pregnancy, not attending college, not graduating high school, having a lower income and increased use of public services,” Geier says.

Researchers looked at the link between weight and absenteeism in 1,069 students at nine public schools in Philadelphia. Absentee data were gathered by homeroom teachers over the entire academic year, and student weights and heights were recorded in the second semester of the term as part of an ongoing obesity prevention trial.

For the analysis, students were grouped by weight into one of four categories: underweight, normal-weight, overweight and obese. Obese children missed an average 12.2 days, compared to 10.1 days for normal-weight students.

“I wasn’t surprised by the main finding that overweight kids miss more school,” Geier says. “I was surprised by the fact that BMI is the No. 1 predictor because it is totally absent from the literature.”

Since the study was made public, Geier says he’s received numerous calls from school administrators across the country wanting to know how to put the information to use.

“If you’re going to create some interventions that are really going to help kids, you need to know what’s causing (absenteeism),” Geier says.

One question the study did not attempt to answer was: Why are overweight kids missing more school? Geier believes psychosocial factors like bullying are more to blame than health concerns. “Children at these ages are very resilient. For the most part, you don’t see the health issues associated with obesity, like Type 2 diabetes, at that age. So there must be psychosocial factors that are keeping them from going to school,” Geier says.

Geier’s predicts his study will be a springboard for further research on the effects of obesity and school performance.

“Are they more likely to be absent on gym class days? That would really help separate if they’re missing school for physical or psychosocial reasons.”