For parents, the data suggest a need to be extra vigilant about what their children eat during the summer. But that is easier said than done. While many school districts have nutritionists involved in lunch planning, relatively few summer camp programs do. Camps tend to focus on food safety  making sure the food doesn’t spoil in the hot sun. And parents who want to pack lunches struggle with the same problem, often resorting to packaged products that will hold up in the hot weather.

“Camp food is terrible,” said Susan B. Roberts, director of the energy metabolism laboratory at the Friedman School of Nutrition Science and Policy at Tufts University. “The problem is that they are doing what is easiest  the lowest common denominator for what kids like, and on top of that usually it has to be not something that goes bad and is no work to prepare.”

Despite the food, children who go to camp at least have the advantage of daily organized physical activity. Those who do not, particularly those in low-income families, often spend summers at home with little supervision or structure to their day. They end up watching television or playing video games and grazing on food all day.

“During the year, television viewing habits are more limited because of school and homework, but in summer, all bets are off,” said Dr. David Ludwig, director of the obesity program at Children’s Hospital Boston. “They are lying around all day long with little supervision, watching TV and playing video games. The mythical childhood of summer at the beach is becoming increasingly rare, certainly for children in the inner city.”

Weight gain aside, even brief exposure to certain foods can shape tastes and preferences for the rest of the year. Not only do children eat more when watching television, for instance, but they are also exposed to numerous commercials for sugar- and fat-laden foods and snacks.

“They’re being exposed to a huge number of food commercials for highest-calorie lowest-quality products,” Dr. Ludwig said. “Those effects have been demonstrated to alter food choice.”

Parents concerned about those choices can take a lesson from the schools. Ask baby sitters to create more structure in the summer day, schedule lunches and snacks at regular times when possible and encourage children to take a “recess” outdoors a few times a day. And parents of children in day camp still have control over breakfast and dinner and all meals on weekends.

“There’s not a lot of data on children’s summertime activities,” said Paul von Hippel, a former Ohio State researcher who was the lead author of last year’s study. “But I think what our data show is that it’s good to keep kids busy during the summer  just busy enough so they’re not eating all the time.”