As for the $64,000 question of why obesity numbers fell - the answer to which could save the nation billions in health-care costs and increased productivity - Schwarz and others were cautious. They also were careful to use words such as hopeful to describe the local finding.
"It chips away at and it stops what is a consistent trend upward, which [has been] the frightening part," Schwarz said.
Physicians at the Robert Wood Johnson Foundation, which has made fighting obesity its top goal, were more jubilant.
"Philadelphia is a positive deviant, a crucial proof of the concept that communities can reduce obesity rates - and do so in a way that helps to close the disparities gap," James S. Marks and Risa Lavizzo-Mourey wrote in commentary accompanying the article in the journal Preventing Chronic Disease.
In an interview with the journal's editor posted, along with the full paper, at www.cdc.gov/pcd, Marks added that "we're hopeful that, as more and more communities make changes like Philadelphia and some of the others have made, we will see these through the nation as a whole."
The few major jurisdictions that have measured declines in the last few years - Arkansas and California, New York City and now Philadelphia - all have taken various actions to reduce children's waistlines.
The Philadelphia School District, for example, was among the first to remove all sodas and drinks with extra sugar from vending machines, in 2004. Districtwide snack standards were developed in 2006; in 2009-10, the district began offering free breakfasts to all students, discontinued the use of fryers, and switched from milk with 2 percent fat to 1 percent.
The city government, meanwhile, banned trans fats in restaurants, mandated nutrition labeling on chain-restaurant menus, and developed incentives for corner stores to offer more fresh produce. Even Mayor Nutter's two failed attempts to pass a soda tax generated intense public discussion of obesity and health.
In the absence of evidence that genetic changes suddenly added layers of fat beginning in the 1970s, the "environment" - price, advertising, purchasing habits, and myriad other factors - is clearly the culprit in the obesity epidemic, said Gary D. Foster, director of Temple University's Center for Obesity Research and Education.
"So if we blame the environment for the last 30 years," said Foster, who was not involved with the new study, "we've got to credit the environment" for the improvement.
Because the jurisdictions actively tackling obesity are the same ones carefully measuring rates, it is impossible to say for sure that the trend isn't already happening nationwide, Foster said. He added: "But Philadelphia is among the most aggressive cities in the country for changing the environmental factors that affect obesity."
At its core, obesity is simply an excess of calories in over calories out, although differences in metabolism mean that some can remain skinny while eating the same ice cream cones that cause others to balloon.
Over the last decade or so, however, health researchers have proved what many dieters have long known: Changing habits isn't easy. As a result, researchers now say encouraging dozens or even hundreds of small changes is the best way to make a difference. The Institute of Medicine earlier this year endorsed the science behind that concept, and recommended several of the broad-based measures that Philadelphia has pushed for several years.
Even something as basic as getting children to make fewer visits to corner stores can make a difference, said Foster, who published a paper several years ago finding that Philadelphia children consume an average of 360 calories a visit. Every 3,500 calories equals about a pound.
For the new study, the Philadelphia Department of Public Health researchers analyzed height and weight data of more than 100,000 public school students.
Rates of obesity, which for children is defined as the 95th percentile of body mass index (a calculation based on height and weight), dropped in all grades but declined the most in kindergarten through fifth grade (down 6 percent) and grades six through eight (down 4.7 percent). A 2.4 percent drop in grades nine through 12 was not statistically significant.
Rates of what the researchers termed "severe" obesity followed a similar pattern but more so: down 7.7 percent overall. Still bigger drops were measured among African American males (down 13.8 percent) and Hispanic females (down 10.2 percent).
Asked whether there was any way to read the findings other than good news for the city, Schwarz pointed out that one in five Philadelphia school children was still obese, and one in 12 severely obese.
"This is not 'obesity is over,' " he said. "This is, 'We have some hope that we can make a difference,' but we have a very long way to go."
Contact Don Sapatkin at 215-854-2617 or firstname.lastname@example.org.