by Karin Zeitvogel

WASHINGTON - More US children are becoming extremely obese at a younger age, putting them at risk of dying decades younger than normal-weight children and of suffering old-age illnesses in their 20s, a study warned Thursday.

Extreme childhood obesity, which was defined only last year by the Centers for Disease Control and Prevention (CDC), affects 7.3 per cent of boys and 5.5 per cent of girls, the study by leading US health care provider Kaiser Permanente showed.

That translates into more than half a million children in California alone, where the study was carried out, being classified as extremely obese, or having a body mass index (BMI) - calculated by dividing a person's weight in kilograms by their height in meters squared - greater than 35.

A 12-year-old boy standing 1.52 metres tall and weighing 82 kilograms would be considered to be at the lower limit of extreme obesity under the CDC criteria.

He would be carrying around 50 percent excess body weight, "which is really a lot," said Corinna Koebnick, a researcher at Kaiser Permanente's department of research and evaluation and lead author of the study.

"This study is all about health, not about looks," Koebnick told AFP.

"Extreme obesity is a serious health problem, and we want these children to have long and happy lives.

"But without major lifestyle changes, these kids face a 10 to 20 years shorter lifespan and will develop health problems in their 20s that we typically see in 40- to 60-year olds," Koebnick said.

"We know that obesity in children or adults leads to a higher rate of diabetes, cardiovascular disease and fatty liver disease, and these are serious issues that we can expect extremely obese children to face at a higher rate and probably much earlier than other children."

Researchers looked at the health records for 2007 and 2008 of 711,000 children and teens in California, aged two to 19, for the study, which is the first to provide a snapshot of just how prevalent extreme obesity is in US children today.

"Seven percent of boys and five percent of girls are extremely obese, and in some ethnic subgroups, we found as many as 12 percent of kids were extremely obese. I think those are scary results," Koebnick said.

Around 12 percent of black teenage girls and Hispanic teen boys were extremely obese, the study published online in the Journal of Pediatrics, found.

Boys tend to become extremely obese younger than girls - at age 10 rather than 12 - but girls have two peaks of extreme obesity, the second coming at 18 years of age.

In addition to finding a worryingly high level of extreme obesity in US children, the researchers found that 37 per cent of US kids were overweight - defined by the CDC as having a BMI greater than 25 - and 19 per cent were obese, or had a BMI over 30.

And the trend seems to be for kids to become extremely obese rather than just moderately obese or overweight, Koebnick said, although she stressed that more research needs to be done to confirm the trend.

Kaiser Permanente is a founding member of the non-profit Partnership for a Healthier America, which was set up earlier this year to generate support for First Lady Michelle Obama's campaign to push back childhood obesity in a generation.