Young children who are overweight or obese tend to eat more calories per day than their peers of normal weight, but that's not the case in older children, a U.S. study suggests.

Among children over age nine, overweight or obese ones ate fewer calories a day compared to children with a healthy weight.

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All the children and teens in the U.S. study in Pediatrics had their height and weight measured, but physical activity levels weren't tracked. (M. Spencer Green/Associated Press)

"One explanation for this would be that increased caloric intake in early childhood is related to obesity's onset, but other mechanisms, such as differences in energy expenditure, may be more responsible for maintaining weight through adolescence," Asheley Cockrell Skinner, an assistant professor of pediatrics at the University of North Carolina and her colleagues, wrote in Monday's issue of the journal Pediatrics.

The study was based on a survey of 12, 648 children age one to 17.

Parents filled in detailed questionnaires about what children under age six ate in the previous 24 hours. Kids from six to 11 reported themselves with help from an adult such as a parent if needed. Those over age 12 reported for themselves.

All participants had their height and weight measured.

Researchers calculated body mass index (BMI) percentiles. Sex, race, ethnicity and income were taken into account in the analysis.

In the study, overweight children aged seven to nine reported consuming fewer calories a day than their healthy-weight peers.

Eating more not a full explanation

The study's authors didn't investigate why. They speculated that increased caloric intake in early childhood becomes a pattern that leads to earlier onset of obesity, which in turn becomes self-perpetuating.

Another potential explanation is that overweight children are less physically active than others.

"This is in line with other research that obesity is not a simple matter of overweight people eating more — the body is complex in how it reacts to amount of food eaten and amount of activity," Skinner said in a release.

It's also possible that dietary intakes were reported inaccurately. The researchers called self-reported intakes the main limitation of the study.

 "Reducing childhood obesity may require early education on appropriate levels of energy intake and, in later childhood, a focus on non-calorie-reducing interventions such as increases in activity," the study's authors concluded.

Physical activity levels were not measured in the study.

The research was funded by the U.S. National Institutes of Health.