Regulating the foods and beverages sold outside of federal meal programs at schools may help curb the child obesity epidemic, suggests a U.S. study released Monday.

Researchers found that fifth graders in states with strong "competitive food laws" packed on fewer pounds than did kids in states with no such legislation.

"I definitely see this as encouraging evidence that the laws can have a positive impact," said Daniel R. Taber of the University of Illinois at Chicago, who worked on the study. "But we need to recognize that it is not going to influence all students."

Childhood obesity has soared in recent decades. In 2008, more than a third of U.S. youths were either overweight or obese, according to the Centers for Disease Control and Prevention.

The new findings come as the U.S. Department of Agriculture is updating its standards for competitive foods sold at schools across the nation as required by the Healthy, Hunger-Free Kids Act of 2010.

A host of programs are geared toward stemming the obesity tide by providing healthier school foods. Just last week, one study showed half as many adolescent students as in 2006 can still buy high-calorie sodas at school.

But so far there's little evidence that such programs work, Taber said.

So he and his colleagues tapped into an earlier study following 6,300 students in 40 states from 2003 to 2006 (fifth to eighth grade). They compared the children's body mass index (BMI), a measure of weight in relation to height, with the competitive food laws in each state.

Students whose states had consistent, strong laws throughout the study period gained 0.44 BMI points less than children whose states did not regulate the foods sold at schools.

It's hard to translate that into pounds, because children are growing. But the researchers also found that states with strong laws in 2003 saw five percent fewer students remain overweight and eight percent fewer remain obese in eighth grade than did states without competitive food laws.

The researchers classified laws as strong or weak based on a scoring system from the National Cancer Institute. A strong law, said Taber, is one that is mandatory and contains specific rules, such as how much fat and sugar foods may contain.

To be as effective as possible, Taber said the rules should be consistent throughout high school, have strong language and be as specific as possible.

"The key is really in the details," he told Reuters Health.

The study is published in journal Pediatrics.