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New York City’s efforts to help overweight kids lose weight has paid off in an unexpected way – the most obese children have taken off the most weight.

Officials found an almost 10 percent drop in rates of severe obesity from 2006 to 2011 among the city’s public school students, while general obesity rates fell by 5.5 percent.

“It’s good news,” said Christine Johnson of the New York City Department of Health and Mental Hygiene, which conducted the study. “We found that severe obesity has gone down in New York City schoolchildren in kindergarten to 8th grade. It’s decreasing even faster than the rate of decline we see overall in obesity.”

What’s behind it? The study just looked at numbers but Johnson says New York’s been pressing schools to help kids exercise more and eat better food.

"It’s decreasing even faster than the rate of decline we see overall in obesity.”

“The Department of Education has a program called Move to Improve,” Johnson told NBC News. “It trains public school teachers on ways to incorporate fitness into classrooms.”

For instance, the youngest kids may be encouraged to stand up at their desks and walk in place as they imagine taking a nature walk. Older kids might dance. “It’s a lot of fun,” Johnson said.

Schools have also been forced to improve the food they’re serving – most New York public school kids get free meals at school, and this sets norms for what they eat outside school, Johnson said.

For the study, the health department’s Sophia Day and colleagues looked at the annual weight and height data taken on nearly 950,000 school children aged 5 to 14 in public schools for the 2006-2007 school year and compared the 2010-2011 school year.

“Severe obesity among NYC public school students in grades K–8 decreased 9.5 percent from the 2006–07 school year (when it was 6.3 percent) to the 2010–11 school year (to 5.7 percent), and obesity decreased 5.5 percent (from 21.9 percent to 20.7 percent),” they wrote in their report, published in the Journal of the American Medical Association’s JAMA Pediatrics.

“We have done a lot in schools. But kids are exposed to a lot when they walk out the door."

Overall, the numbers are small but they reverse a 40-year trend. The rate of severe obesity had more than tripled from the 1970s. Nationally, nearly 6 percent of U.S. kids are severely obese. And studies show that severely obese kids are not only nearly certain to become obese adults, but they are already suffering the dangerous health effects such as heart disease and diabetes.

Obesity in kids is not measured the same way as in adults. Children are considered overweight if they are in the 85th to 95th percentile of the body mass index, or BMI, for all children their age. Anything over the 95th percentile is obese, and class 2 severe obesity is at 120 percent. Morbid obesity, or class 3, is defined as 140 percent.

So an average 10-year-old boy who is 4 and a half feet tall would be considered obese at 95 pounds and would be severely obese at 115 pounds.

Johnson said clearly much more needs to be done. “We have done a lot in schools. But kids are exposed to a lot when they walk out the door,” she said. The city wants to work next to encourage supermarket chains to open more stores in neighborhoods where convenience stores are often the only place to shop. The city has also started a program called Health Bucks, which distributes $2 vouchers for people to use at farmer’s markets, to encourage people to eat more fresh fruits and vegetables.

Data from very large national surveys show that all types of obesity are on the rise in American children, but some other studies have begun to show drops in extreme obesity among the very youngest kids – who may be the easiest to influence, Johnson said.