By Steve Tawa, Stephanie Stahl

PHILADELPHIA (CBS) - A new study on child obesity indicates that students gain less weight if they go to schools in states with stronger laws about what foods can be sold outside their breakfast and lunch programs.

It’s said to be the first national look at the effectiveness of “competitive food laws,” by which states regulate vending machines, snack areas, and beverages that compete with the meals programs.

In the study, 6,300 children in 40 states were tracked over a three-year period.  Researchers compared their body mass index (“BMI”), a measure of weight relative to height, when they were in fifth grade and then again when they were in eighth grade.

Dr. Virginia Stallings, director of the nutrition center at Children’s Hospital of Philadelphia, liked what she saw in the research.

“It was very well designed,” she tells KYW Newsradio.  “It involved many states, not just one school district, and the quality of the measurements on the children were very high standards.”

The states were classified as having strong, weak, or no laws governing competitive foods in 2003 and 2006.

Students living in states with strong laws all three years gained less weight than kids in states without those laws.