Australian researchers have discovered it's a lack of exercise, rather than eating too much, that is causing obesity in children.
Canberra researchers followed 734 children for four years from when they were eight years old until they were 12, and found the overweight and lean children consumed roughly the same amount of calories, but the lean ones did more exercise.
Study leader Dr Richard Telford told ninemsn his team of researchers carefully measured the children's exercise levels with pedometers and body composition using DXA scans.
Parents and teachers helped the children fill in diet diaries twice during the study.
"We measured everything carefully and found that the children who were fatter at each of those age groups were not eating more than children who were lean. The problem was that they were moving around a lot less," said Dr Telford, who is a research fellow at Canberra Hospital's clinical trials unit and adjunct professor at the Australian National University's school of medicine.
"What is driving childhood obesity is a lack of physical activity, rather than the dietary component."
Dr Telford said good nutrition is important for children's general health, but when it comes to reducing childhood obesity, the best thing they can do is move more.
"Internationally, it's recommended that children should be taking 10,000 to 12,000 steps per day but these children were on average taking about 15 to 20 percent less than that," he said.
"So rather than getting on the back of kids about watching what they eat, we should be finding ways to really allow those kids to move around more. Kids on average should be increasing their physical activity by 20 percent."
The children were recruited from outer Canberra suburbs, where household incomes were in line with the Australian average.
"I wanted to see what was driving body fat and body composition in normal, community-based children," Dr Telford said.
"They're not children with problems with morbid obesity. They are just general kids who participate in sport and have no health problems."
The study found boys tended to be 15 percent more active than girls.
"Boys were leaner than girls all the way through and the increased amount of physical activity accentuates that," Dr Telford said.
These children will continue to be followed so researchers can assess how their childhood behaviour impacts their health in later life.
"This is part of a big project and we want to follow as many participants as we can up until age 80," Dr Telford said.
"For the researchers who are around then and writing that up, they'd be able to see how their quality of life affected by what they did when they were children and teenagers."