Concern has been mounting about childhood obesity for the last 30 years. Approximately 17 percent of American children ages 2-19 are obese according to data from the Centers for Disease Control and Prevention.

As the numbers inch upward, obesity has become part of the national conversation, with aggressive advertising campaigns in major cities and a push by Michelle Obama to encourage kids to eat healthier foods and get exercise through sports and outdoor play.

New data suggests these efforts are paying off. New numbers from the Robert Wood Johnson Foundation show that in Philadelphia, New York City, Mississippi and California, childhood obesity is actually declining.

“It’s been nothing but bad news for 30 years, so the fact that we have any good news is a big story,” said Dr. Thomas Farley, the health commissioner in New York City in an interview with the New York Times. New York City reported a 5.5 percent decline in the number of obese schoolchildren from 2007 to 2011.

Researchers aren't sure what is behind the declines, whether the drops have more to do with fewer obese children entering school or currently enrolled children losing weight, reports the New York Times. One thing, however, is certain: In places where declines occurred, broad obesity-reduction policies have been in place for a number of years.

Comment on this story

For example, Philadelphia has been implementing policies to fight childhood obesity for years. Sugary drinks like sweetened iced tea, fruit punch and sports drinks started to disappear from school vending machines in 2004. Schools set fat and calorie limits for snacks and reduced the size of junk food snacks to single servings. Deep fryers were removed from school cafeterias and whole milk was replaced with skim.

"Philadelphia’s measured success could inspire other cities to start experimenting with obesity reduction policies," according to Aviva Shen for ThinkProgress. "These policies could help shift the currently untenable status quo," she added.