WASHINGTON (Reuters) – Extreme obesity among American children is much worse than previously believed, putting them at greater risk of serious health problems as they age, US researchers said.

A study of more than 700,000 children and teens in southern California found that more than six percent, or 45,000, were extremely obese and more boys than girls were far too heavy, the researchers reported in the Journal of Pediatrics.

"This study is unique because it is the first time that we've had a large up-to-date snapshot of what's happening with obesity in our children," co-author Dr. Amy Porter of Kaiser Permanente health care system said in a video statement.

"The prevalence of obesity in children is much higher than we ever thought it was" Porter said. She said the study also showed that extreme obesity was rising in almost every group.

It found that seven percent of boys and five percent of girls were extremely obese, as were more than two percent of all children under five years old.

Co-author Corinna Koebnick, another researcher with Kaiser Permanente, said the results of the broad multiethnic study, with estimates for racial subgroups by age and sex, probably applied across the country.

"Children who are extremely obese may continue to be extremely obese as adults, and all the health problems associated with obesity are in these children's futures," Koebnick said in a statement.

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

US first lady Michelle Obama is leading an administration effort to fight childhood obesity focused on improving nutrition in homes and in schools.

Two-thirds of American adults are overweight or obese and a third of children are obese, increasing the risk of heart disease, diabetes and other chronic illnesses, and adding about $150 billion a year to US healthcare costs.

The researchers sought to determine how many children in a section of Southern California were extremely obese under a new US Centers for Disease Control and Prevention definition.

Previous research based on a federal health survey suggested that 3.8 percent of children were extremely obese. Doctors do not define obesity in children the same way they do for adults.

Obese children are defined as those whose weight is above the 95th percentile for their age and height and extreme obesity is 1.2 times that measurement.

In the study, researchers looked at health records of 710,949 children and teens aged two to 19 enrolled in a managed health care plan in 2007 and 2008.

The group was almost evenly split between girls and boys and about half were Hispanic.

They found the heaviest children were black teenage girls and Hispanic teenage boys. Asian-Pacific Islanders and white children had the lowest percentage of extreme obesity.

Arkansas Senator Blanche Lincoln, chairwoman of the Senate agriculture committee, introduced legislation that would set national nutrition standards for food sold in US schools and set aside money for school gardens.