Children are more likely to become overweight or obese during summer vacation than during the school year, a U.S. study suggests.

A boy eats an ice cream in the 32 degree Celsius heat on the first weekend of the summer school holidays at the swimming pools in Kriens July 10, 2010.

The national study of more than 18,000 school children found that from the autumn start of kindergarten to the spring semester of second grade, the prevalence of obesity increased from 8.9 to 11.5 percent. During that same period, the proportion of overweight kids climbed from 23.3 to 28.7 percent.

None of these gains in numbers of obese or overweight kids happened during the school year, however. All of the increases in what’s known as body mass index (BMI) – a measure of weight relative to height – occurred during summer vacations.

“The structured nature of the school day, with its scheduled exercise periods and limited opportunities to eat, helps students maintain a healthy BMI,” said study co-author Paul von Hippel, a health policy researcher at the University of Texas, Austin.

“By contrast, we speculate that many non-school environments are relatively unstructured and unsupervised, allowing children to indulge in sedentary activities and excessive snacking,” von Hippel added by email.

To assess the influence of school attendance on child obesity, researchers examined data on kids’ height and weight at the beginning and end of each academic year from the fall of 2010 until the spring of 2013.

At the start of kindergarten, 23 percent of the children were overweight and 9 percent were obese, researchers report in the journal Obesity.

During each of the two summers in the study, the proportion of overweight and obese kids increased by approximately one percentage point per month.

There were no meaningful increases in the numbers of overweight or obese kids during the academic year.

This doesn’t prove summer vacations cause obesity or that school attendance prevents it, but the findings do suggest major risk factors for putting on too much weight during childhood exist outside of schools, the authors conclude.

During vacations, risk factors for weight gain include getting less sleep, watching more television and exercising less than during the school year, the researchers speculate.

“When comparing weekends to school days, studies have often reported that children sleep less and have more variable sleep, are less physically active, more sedentary, and consume unhealthier diets on weekends,” said Dr. Paul Collings of the Bradford Teaching Hospitals Foundation Trust in the U.K.

“If summer vacation can be considered a very long weekend, then this will take its toll,” Collings, who wasn’t involved in the study, added by email.

Although the study doesn’t offer evidence of problems like excessive snacking or insufficient sleep and exercise during the summer, it still makes sense for parents to keep an eye on these things, said Dr. Jonathan Mitchell of the Children’s Hospital of Philadelphia.

“Parents should be thoughtful about the potential for rapid weight gain to occur during the summer,” Mitchell, who wasn’t involved in the study, said by email.

Because just arranging child care during the summer can be a struggle, other problems may seem less pressing, said Dr. Eliana Perrin of the University of North Carolina at Chapel Hill.

“That being said, if there’s ways to find routine, decrease calorie-dense and nutrient poor foods for children, help their children get better sleep, help them avoid screen time, and get out and be active – even if the weather is hot – that would be great,” Perrin, who wasn’t involved in the study, said by email.

Ideally, parents can promote healthy habits throughout the year, said Dr. Inyang Isong of Boston Children’s Hospital. This can include encouraging kids to get at least an hour of exercise a day, limiting screen time to less than two hours a day, and keeping televisions and tablets out of kids’ bedrooms, said Isong, who wasn’t involved in the study.

“Collectively adopting these healthful lifestyle behaviors, as a family, can go a long way, as parents are role models for their children,” Isong added by email. “As the saying goes, your children will become who you are; so be who you want them to be.”

Source: Reuters