Appearing to buck national trends, the prevalence of severe obesity among school children in New York City was down by almost 10 percent in the 2010-11 school year from 2006-07, researchers reported on Thursday.

Earlier research had shown a decline in overall obesity among New York City public school children, but the prevalence of severe obesity had not been studied. Public health experts have worried that while “easy” cases of obesity were being addressed, more and more children might be moving from being merely obese to extremely so, putting them at risk for heart disease, diabetes and other illnesses.

In New York City, at least, that appears not to be happening.

The study, published in the journal Preventing Chronic Disease, was based on height and weight measurements of 947,765 children attending public schools in kindergarten through eighth grade.

Severe obesity fell from 6.3 percent of the children in the 2006-07 school year to 5.7 percent in 2010-11, according to the researchers, who were led by Sophia Day of the New York City Department of Health and Mental Hygiene. The change represents a 9.5 percent decrease.

“I think there is potentially a glimmer of hope here,” said Asheley Cockrell Skinner of the University of North Carolina School of Medicine, an expert on childhood obesity who was not involved in the study.

Childhood obesity is based on body mass index (BMI), which is weight (in kilograms) divided by height (in meters) squared. But unlike adult obesity, which starts at a BMI of 30, and extreme obesity (a BMI of 40 or more), childhood obesity is not defined by a set number.

Extreme or severe childhood obesity, for instance, is understood as having a BMI at least 20 percent greater than 95 percent of children. For a boy standing 4 feet, 6 inches (1.4 meters) tall, 95 pounds (43 kilograms) is obese and 115 pounds (52 kilograms) is severely obese.

In the New York study, the prevalence of severe obesity was highest among boys, minorities and poor children. Although prevalence declined in every group, the greatest decrease was among white students and wealthy students.

“Wealthier families have access to things poor families do not,” said Skinner - everything from soccer camp to safe neighborhood playgrounds.

Last month, Skinner and Dr Joseph Skelton of Wake Forest School of Medicine in Winston-Salem, North Carolina, reported in JAMA Pediatrics that 8 percent of 2-to-19-year-olds are severely obese, according to data from 2011-2012. That was up from 6 percent in 2005-06.

It is not clear what might explain the progress against severe childhood obesity in New York, but “the city likes to be out in front in trying new things” such as healthier school lunches, Skinner said.

“There’s not a lot of evidence for most of these policies, so it’s good to see that at least they’re not doing harm,” such as causing kids dissatisfied with healthy school lunches to wolf down pizza and ice cream on their way home, he added.

(Reporting by Sharon Begley; Editing by Leslie Adler)

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By Sharon Begley