Nearly 7 percent of sixth graders across the U.S. are severely obese, reveals a new study appearing online in the Journal of Adolescent Health.

“Severe obesity in children requires attention because it is associated with high rates of risk for diabetes and cardiovascular disease as children get older,” said Marsha Marcus, Ph.D., lead study author.

Researchers invited 6,365 middle-school children to health screenings at 42 middle schools in diverse U.S. locations. Half of the children were minority students or eligible for free or reduced lunch.

Clinicians define severe obesity differently in children than they do in adults. In children, the measurement used is a percentile of body mass index (BMI) for age and sex. For this study, researchers considered children with a BMI in the 99th percentile to be severely obese -and 6.9 percent of students fell into this category.

“Ethnic children appear to have higher rates of severe obesity, but rates for all children were high,” said Marcus, who heads the Eating Disorders Program at Western Psychiatric Institute and Clinic, University of Pittsburgh Medical Center.

Marlene Schwartz, Ph.D., deputy director of the Rudd Center for Food Policy and Obesity at Yale University, said, “The findings of this study are alarming because it shows there are even more children than we realized at medical risk due to excess weight.” She has no affiliation with the study.

Schwartz, who has worked with children in both clinical and research settings, added, “In both situations, I have been surprised that some children who are severely obese according to their BMIs do not always look heavy. As a culture, we have become used to seeing heavier children so the visual norm has shifted. Because of this, many children are at risk of not receiving help since their parents do not see them as obese.”

While no simple answer exists, “Parents need to talk to their children’s doctors and seek treatment for children with severe obesity,” Marcus said.

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More information: Marcus MD, et al. Severe obesity and selected risk factors in a sixth grade multiracial cohort: The HEALTHY Study. J Adol Health online, 2010.