Topeka ? Nobody knows why for sure, but girls in Kansas have been gaining weight at an alarming rate.

From 2003 to 2007, the percentage of Kansas girls 10 to 17 years old who were obese nearly doubled, a new federal study shows.

The 91.4 percent increase in the obesity rate was the greatest experienced by any state and was too large to be considered a statistical fluke, the study’s lead researcher said.

“I think that it is definitely real,” said Gopal Singh, an epidemiologist with the Health Resources and Services Administration. “But why, that is quite puzzling.”

Indeed, the jump is a head-scratcher for experts who work with overweight kids in Kansas. Some were skeptical about the numbers.

Singh’s study, published this month in the journal Archives of Pediatrics & Adolescent Medicine, used data from telephone surveys that asked tens of thousands of parents and guardians about the health of their children. More than 40,000 children from the 2003 and the 2007 survey were included in the study.

Nationwide, the researchers found statistically significant weight gain among girls. The prevalence of overweight among girls increased 9.4 percent and the prevalence of obesity by 17.6 percent. Changes among boys were smaller and not considered significant.

Among states, only Arizona, with a 90.9 percent increase in obesity among girls, approached Kansas.

But some experts said they weren’t seeing the same kind of numbers.

“Our own data wouldn’t support any sudden feeding frenzy among Kansas girls,” said Ann Davis, a University of Kansas Hospital psychologist who runs weight reduction programs for children at the hospital and in rural elementary schools.

But Davis has noticed that Kansas parents seem more willing now to acknowledge that their children have a weight problem, something particularly uncomfortable to do when the child is a girl.

In years past, Davis would be able to recruit only a few families from a rural school to take part in her program. Now she’s overwhelmed by the dozens who want to sign up.

“I’ve seen a tremendous shift in that, that my child is overweight or obese and I need to take action,” she said.

That greater recognition of weight problems may make for more accurate reporting by parents who are surveyed about their children, Davis suggested.

It’s not that a lot more girls are tipping the scales in the Sunflower State than in the rest of the country. In Kansas, 16.3 percent of girls were obese in 2007, not significantly higher than the national rate of 13.5 percent, the study found.

But in 2003, just 8.5 percent of Kansas girls were obese. The national rate that year was 11.5 percent.

The obesity rate for Kansas boys didn’t change significantly and at 16.2 percent in 2007 was similar to the national rate of 19.2 percent.

Obesity among Missouri boys and girls also held fairly steady, with rates comparable to the national numbers, according to the study.

“I don’t know any factor that would have girls gaining more weight than boys in the state,” said Mark Thompson, director of the Kansas Coordinated School Health Program, which works with school districts to develop nutrition and physical activity programs.

Thompson cited surveys of Kansas high school students that found overweight and obesity levels had remained fairly constant from 2005 through 2009.

Thompson said there was “a possibility there is a fluke” in Singh’s data.

“It’s not because of a statistical anomaly,” Singh countered.

Most of the increase in obesity in Kansas may have been among girls who were below high school age, he said.

About one in three children in the United States is now considered overweight or obese, which is raising concerns about their future health and even their longevity. Many children already are developing diabetes or showing early signs of heart disease that typically are found in adults.

First lady Michelle Obama recently initiated the “Let’s Move” campaign against childhood obesity. A government report last week offered 70 recommendations, including healthier food at schools and getting children to exercise more, to combat weight gain.

People who are obese are well above normal weight and have large amounts of body fat. Obesity is usually measured by a calculation based on height and weight called the body mass index, or BMI.

For example, a 12-year-old girl who is 5 feet tall would be considered to be at a healthy weight at 110 pounds and obese at 130 pounds.

Singh’s study found wide variations in obesity rates among states, even after accounting for ethnic and economic differences.

Kids in Kansas, for example, were twice as likely to be obese as kids in Oregon, which had the lowest obesity rate.

Differences in the availability of parks and playgrounds and in state policies promoting healthy weight among children may play a role, Singh said.

State policies could be a factor in Kansas, said Sarah Hampl, a pediatrician who directs weight management services at Children’s Mercy Hospital.

She pointed to a 2009 report by the Trust for America’s Health that noted which states had nutrition standards for foods available to children at school or policies for measuring students’ BMIs.

“Notably, Kansas was one of the only states in the nation that doesn’t have this kind of legislation,” Hampl said.