Candy, soda, chips, and other junk food available at schools doesn't cause weight gain among children-at least for middle school students, a new study has suggested.

The study relies on data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-1999, which follows a nationally representative sample of students from the fall of kindergarten through the spring of eighth grade (the 1998-1999 through 2006-2007 schools years).

"We were really surprised by that result and, in fact, we held back from publishing our study for roughly two years because we kept looking for a connection that just wasn't there," said Jennifer Van Hook, a Professor of Sociology and Demography at Pennsylvania State University and lead author of the study.

Van Hook and her co-author Claire E. Altman, a sociology and demography doctoral student at Pennsylvania State University, used a sub-sample of 19,450 children who attended school in the same county in both fifth and eighth grades (the 2003-2004 and the 2006-2007 school years).

They found that 59.2 per cent of fifth graders and 86.3 per cent of eighth graders in their study attended schools that sold junk food.

But, while there was a significant increase in the percentage of students who attended schools that sold junk food between fifth and eighth grades, there was no rise in the percentage of students who were overweight or obese.

In fact, despite the increased availability of junk food, the percentage of students who were overweight or obese actually decreased from fifth grade to eighth grade, from 39.1 per cent to 35.4 per cent.

"There has been a great deal of focus in the media on how schools make a lot of money from the sale of junk food to students, and on how schools have the ability to help reduce childhood obesity," Van Hook said.

"In that light, we expected to find a definitive connection between the sale of junk food in middle schools and weight gain among children between fifth and eighth grades. But, our study suggests that-when it comes to weight issues-we need to be looking far beyond schools and, more specifically, junk food sales in schools, to make a difference," she said.

According to Van Hook, policies that aim to reduce childhood obesity and prevent unhealthy weight gain need to concentrate more on the home and family environments as well as the broader environments outside of school.