The researchers followed more than 19,000 students from grades five through eight in 1,000 private and public schools.
"What we found basically is, there is no relationship between going to a middle school that sells junk food and gaining weight," says researcher Jennifer Van Hook, PhD, professor of sociology and demography at Penn State University.
The message, she stresses, is not that junk food is OK, but that schools are probably not the main source of kids' junk food intake. The study is published in the Sociology of Education.
Junk Food in Schools: A Short History
Public health experts have been concerned about junk food in schools for many years. Obesity now affects 17% of U.S. children and teens ages 2-19, according to the CDC. Advocacy groups have directed efforts at getting junk food out of schools. Some previous studies have found a link between junk food sales in schools and children's weight, although Van Hook says the studies were small and not conclusive.
One scientifically sound study, she says, did show a link between the availability of junk food in school and weight gain in high school students.
Van Hook’s study was what’s called an observational study, which means it cannot prove cause and effect, but only a link or association. Still, she expected to find a link, she says, but did not.
Junk Food in Schools: Focusing on Middle Schoolers
Van Hook and her colleagues looked at data on almost 20,000 children from the National Center for Education Statistics.
It was collected from the 1998-1999 school year, when the students were in kindergarten, until they were in eighth grade in 2006-2007.
The researchers looked at the children's body mass index, or BMI, at the beginning of the fifth grade and again in the eighth grade.
They considered whether the schools offered what Van Hook calls "competitive" food in addition to school-served food. These foods run the gamut from candy and ice cream to low-fat yogurt.
At the fifth-grade mark, 59% of the schools offered junk food. At theeighth-grade mark, 86% did.
The researchers considered race and ethnicity, socioeconomic status, and other factors that might affect unhealthy weight gain.
Children in schools with junk food sales did weigh a bit more by eighth grade than the kids in schools without. However, the difference was slight and not significant from a statistical point of view, Van Hook says.
In the eighth grade, 35.5% of kids in schools with junk food were overweight, while 34.8% of those in schools without it were.
In the report, the researchers write that their study provides "no support'' for the idea that the junk foods sold in school increase the risk of obesity in the age range studied.
"No matter how we looked at it," Van Hook tells WebMD, "we found absolutely no difference between the group of kids exposed and those who were not."
Van Hook says the story may change, however, by high school. She suspects the structure of middle school, which limits the chances a child has to buy junk food, may play a role explaining her finding. By high school, students have more unstructured time and more money.
Van Hook says she is not against efforts to rid schools of junk food. However, she says her study finding suggested that ''we can't count on schools to solve the problem of childhood obesity." The school setting and exposure to junk food, she says, may represent only a small part of the problem.
Efforts directed only at removing junk food from schools may be misdirected, Van Hook tells WebMD. Those efforts should be extended to the community and to the home.
Parents can educate children about healthy diets early, she says. Habits are formed early and may also affect a child’s response to junk food later.
Junk Food in Schools: If It's Around, Will Kids Eat It?
The lack of information about each child's intake is a problem of the study, says obesity expert Marion Nestle, PhD, MPH, the Paulette Goddard Professor of Nutrition, Food Studies, and Public Health at New York University. "I don't think this study can tell you much unless you look at actual food consumption, which this study was not designed to do," she says.
"It makes sense to think that if junk food is around, kids will eat it, and there is plenty of evidence to back that up," Nestle says.
"Junk food has no place in schools," she says. "Schools set an example for what kids should eat."
Kids need a consistent message about healthy eating, agrees Ginny Ehrlich, MPH, the CEO of Alliance for a Healthier Generation, which advocates for healthier food choices in schools. Kids are the first to point out the hypocrisy of learning about healthy foods in nutrition class and then being exposed to junk foods at school, she tells WebMD.
''In school environments, where kids spend probably the most time next to being at home, it's really important that those messages about healthy eating and physical activity are reinforced and promoted," she says.