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