Exercise can help overweight kids slim down, but a new study suggests they might be at risk for leg, ankle and foot injuries in the process.
That could be because their legs are supporting extra weight, researchers said.
The finding makes sense intuitively, but there had not been conclusive, thorough studies done among children before, lead author Eva Jespersen said.
Jespersen is a physiotherapist and doctoral student at the Centre for Research in Childhood Health at the University of Southern Denmark in Odense.
She and her colleagues studied 632 kids, ages seven to 12. They measured the kids' body fat percentage and body mass index (BMI), a ratio of weight in relation to height, then followed them for the next two and a half years.
The kids all went to physical education class for at least an hour and a half each week. Parents reported on children's injuries, which were then confirmed by a doctor.
There were a total of 673 leg, ankle and foot injuries during the study period.
Kids who were determined to be overweight or obese based on both their body fat and BMI numbers were most likely to sustain a lower body injury, according to results published in the British Journal of Sports Medicine.
But the differences compared to normal-weight kids were small.
Children with a healthy BMI had 4.4 injuries for every 1,000 "athletic exposures." (An athletic exposure was defined as one and a half hours of physical education class or one afterschool sports practice.) That compared to 5.3 injuries for every 1,000 exposures among kids with an overweight or obese BMI.
The injury rates were similar when the researchers separated healthy and heavy kids based on body fat percentage.
"In the bigger picture the need to get all kids more active, and particularly overweight children, is imperative," Malachy McHugh told Reuters Health in an email.
McHugh is the director of research at the Nicholas Institute of Sports Medicine and Athletic Trauma at Lenox Hill Hospital in New York City. He was not involved in the Danish study.
"The fact that this data is coming from a country noted for its relatively active and fit population is not insignificant - it is likely stronger effects would be seen in children in countries with lower overall physical activity and higher overall BMI," he said.
Ten percent of kids in the study were overweight based on BMI and only one percent was obese.
The results don't mean that overweight kids should avoid sports, Jespersen said.
"Physical activity is associated with numerous health benefits, including lowering the levels of overweight and obesity," she said. "Instead of avoiding sports, emphasis should be put on preventive measures" to lower the risk of injury.
McHugh agreed. "The long term consequences in terms of poor health for habitually sedentary individuals far outweighs the small elevation in risk of injury for overweight children," he said.
Heavier kids and their parents should try to choose activities that promote physical fitness, balance training and motor coordination, which will help protect against injury, Jespersen said.