By Sabrina Tavernise, New York Times

PHILADELPHIA — After decades of rising childhood obesity rates, several American cities are reporting their first declines.

The trend has emerged in big cities like New York and Los Angeles, as well as smaller places like Anchorage, Alaska, and Kearney, Neb. The state of Mississippi has also registered a drop, but only among white students.

“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, which reported a 5.5 percent decline in the number of obese schoolchildren from 2007 to 2011.

The drops are small, just 5 percent here in Philadelphia and 3 percent in Los Angeles. But experts say they are significant because they offer the first indication that the obesity epidemic, one of the nation’s most intractable health problems, may actually be reversing course.

The first dips — noted in a September report by the Robert Wood Johnson Foundation — were so surprising that some researchers did not believe them.

Deanna M. Hoelscher, a researcher at the University of Texas, who in 2010 recorded one of the earliest declines — among mostly poor Hispanic fourth graders in the El Paso area — did a double-take. “We reran the numbers a couple of times,” she said. “I kept saying, ‘Will you please check that again for me?’”

Researchers say they are not sure what is behind the declines. They may be an early sign of a national shift that is visible only in cities that routinely measure the height and weight of schoolchildren. The decline in Los Angeles, for instance, was for fifth-, seventh- and ninth-graders — the grades that are measured each year — between 2005 and 2010. Nor is it clear whether the drops have more to do with fewer obese children entering school or currently enrolled children losing weight. But researchers note that declines occurred in cities that have had obesity reduction policies in place for a number of years.

Though obesity is now part of the national conversation, with aggressive advertising campaigns in major cities and a push by Michelle Obama, many scientists doubt that anti-obesity programs actually work. Individual efforts like one-time exercise programs have rarely produced results. Researchers say that it will take a broad set of policies applied systematically to effectively reverse the trend, a conclusion underscored by an Institute of Medicine report released in May.

Philadelphia has undertaken a broad assault on childhood obesity for years. Sugary drinks like sweetened iced tea, fruit punch and sports drinks started to disappear from school vending machines in 2004. A year later, new snack guidelines set calorie and fat limits, which reduced the size of snack foods like potato chips to single servings. By 2009, deep fryers were gone from cafeterias and whole milk had been replaced by one percent and skim.

Change has been slow. Schools made money on sugary drinks, and some set up rogue drink machines that had to be hunted down. Deep fat fryers, favored by school administrators who did not want to lose popular items like French fries, were unplugged only after Wayne T. Grasela, the head of food services for the school district, stopped buying oil to fill them.

But the message seems to be getting through, even if acting on it is daunting. Josh Monserrat, an eighth-grader at John Welsh Elementary, uses words like “carbs,” and “portion size.” He is part of a student group that promotes healthy eating. He has even dressed as an orange to try to get other children to eat better. Still, he struggles with his own weight. He is 5-foot-3 but weighed nearly 200 pounds at his last doctor’s visit.

“I was thinking, ‘Wow, I’m obese for my age,’” said Josh, who is 13. “I set a goal for myself to lose 50 pounds.”

Nationally, about 17 percent of children under 20 are obese, or about 12.5 million people, according to the Centers for Disease Control and Prevention. The CDC defines childhood obesity as a body mass index at or above the 95th percentile in comparison to children of the same age and sex in the center’s growth charts, a reference population based on children in the United States primarily from the 1960s and ’70s.

Obese children are more likely to be obese as adults, creating a higher risk of heart disease and stroke. The American Cancer Society says that being overweight or obese is the culprit in one of seven cancer deaths. Diabetes in children is up by a fifth since 2000, according to federal data.

“I’m deeply worried about it,” said Francis S. Collins, the director of the National Institutes of Health, who added that obesity is “almost certain to result in a serious downturn in longevity based on the risks people are taking on.”

Obesity affects poor children disproportionately. Twenty percent of low-income children are obese, compared with about 12 percent of children from more affluent families, according to the CDC. Among girls, race is also an important factor. About 25 percent of black girls are obese, compared with 15 percent of white girls.

Some experts note that the current declines, concentrated among higher income, mostly white populations, are still not benefiting many minority children. For example, when New York City measured children in kindergarten through eighth grade from 2007 to 2011, the number of white children who were obese dropped by 12.5 percent, while the number of obese black children dropped by 1.9 percent.

But Philadelphia, which has the biggest share of residents living in poverty of the nation’s 10 largest cities, stands out because its decline was most pronounced among minorities. Obesity among 120,000 public school students measured between 2006 and 2010 declined by 8 percent among black boys and by 7 percent among Hispanic girls, compared with a 0.8 percent decline for white girls and a 6.8 percent decline for white boys.

“The needle is actually moving,” said Gary D. Foster, director of the Center for Obesity Research and Education at Temple University.

He first noticed the change while conducting a study of middle school students. Even children who made up the control group that did not take part in anti-obesity measures had a weight drop of nearly 4 percent, compared with 5.5 percent for those who did.

Here at William H. Ziegler Elementary in Northeast Philadelphia, where most students qualify for free or reduced-price lunch, the day begins with a nutrition tip over the loudspeaker. Teachers give out colorful erasers and stickers instead of Tootsie Rolls. Fund-raising events feature fruit smoothies instead of chocolate.

Some students had never seen broccoli or cauliflower, so Jill Dogmanits, a sixth-grade teacher, started taste tests to acquaint students with those vegetables and healthy snacks like hummus, fresh pineapple and whole-wheat bagels.

But school is only part of the day. Children buy an average of 350 calories worth of snacks in corner stores every day, according to a study by Foster’s center at Temple University. About 640 corner stores are now part of a program of stocking healthier food, according to the Food Trust, a nonprofit group that runs it.

“Parents tell their kids, ‘Take this money and go buy a snack,’” said Josh, as children streamed into a store across from his school where crayon-colored sugar drinks called Hugs sell for 25 cents and generic soda is 40 cents.

Donald F. Schwarz, a pediatrician who is the city’s health commissioner, said: “I think we are beginning to turn the tide with the many things that have gone on now for a decade.”

It is too early to tell whether the trend will hold.

“I’d like to see another year of measurement before I go out and party over this,” said Mary Currier, Mississippi’s state health officer.

And some public health experts say that without broader policy actions like a soda tax, which Philadelphia tried but failed to pass in 2010 and 2011, deeper change will be difficult. Still, new data from Philadelphia — from more than 20,000 children in first through sixth grades — show a further 2.5 percent obesity decline from 2011 to 2012, Foster said.

Josh lost weight this summer, exercising outside with his stepfather, an Army reservist. But now that it’s cold he has gained some back. Still, he believes he can influence others. His 2-year-old cousin now asks for bananas instead of chips at the corner store. Josh takes full credit.