About 5 percent of U.S. children and teens are "severely obese," and the numbers are rising, according to a new statement from the American Heart Association.

Although recent data suggesting that the rate of childhood obesity has started to level off, "a worrisome trend has emerged in the form of severe pediatric obesity," the researchers wrote in their study published Sept. 9 in the journal Circulation.

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"Severe obesity in young people has grave health consequences," said study author Aaron Kelly, a researcher at the University of Minnesota Medical School in Minneapolis. "It's a much more serious childhood disease than obesity."

Severely obese children have higher rates of Type 2 diabetes and cardiovascular issues at younger ages, including high blood pressure, high cholesterol and early signs of atherosclerosis – a disease that clogs the arteries. [10 Ways to Promote Kids' Healthy Eating Habits]

Treating children and teens with of severe obesity is challenging, the researchers said. Many treatments that are commonly used with some success in overweight and obese children, such as lifestyle changes, are less effective in those with severe obesity.

The researchers recommended using a standard definition for severe obesity in youth; they define children over age 2 as severely obese if they have a body mass index (BMI) at least 20 percent higher than 95 percent of other children of the same age and gender. The researchers also said any child with a BMI of 35 or higher should be considered severely obese.

Based on this definition, a 7-year-old girl of average height weighing 75 pounds, or a 13-year-old boy of average height weighing 160 pounds, would be defined as severely obese.

Most experts recommend treating severely obese children first with the least intensive treatment options such as lifestyle changes, the researchers said. More intensive treatments such as medication and potentially surgery should be considered after other treatments have failed.

Increased funding will be needed for research into whether new medications and other treatments, including surgery, are safe and effective in treating children with severe obesity, the researchers said.