The authors said the findings put the onus on parents to encourage exercise, limit snacks and turn off the television when school is out to help fight an obesity epidemic among children.
Examining government data collected from 1998 to 2002 on 5,380 U.S. children, researchers found the average weight gain over the summer, as measured by body mass index, was triple that during the kindergarten school year.
It was double during the following school year when the children attended first grade.
"In school, there are only certain times you can eat. When not in school there are many times to eat," co-author Brian Powell, an Indiana University sociologist, said in a telephone interview.
The trend likely continues through adolescence, he said, though high school students might get more free time to snack while in school.
Among children who were already overweight or racial groups with higher incidence of childhood obesity, such as blacks and Hispanics, the gap between weight gain in summer and the school year was greater, said the study published in the American Journal of Public Health.
Roughly 17 percent of U.S. children are obese and millions more are overweight -- conditions that tend to persist into adulthood when diabetes, heart disease and other health risks emerge.
Powell credited schools with doing a decent job of serving nutritious lunches. Even in schools with vending machines offering fatty snacks, the portions tend to be considerably smaller than what is available outside school, he added.
But he warned that schools that cut physical education periods and exercise-related extracurricular activities to trim budgets may be undermining children's health.
