Fast Food Makes You Fat. Duh!

Excessive eating - not a lack of exercise - is behind childhood obesity, a groundbreaking study shows.

It is the first conclusive evidence behind the spiralling trend, challenging the federal government's refusal to restrict food advertising aimed at children, Fairfax newspapers report.

The study of 5,407 students from 90 public schools in NSW, commissioned by the state government, contradicts the "urban myth" that children are increasingly inactive and hooked on sedentary pastimes, the newspapers say.

They are exercising much harder than in the 1990s, and yet still stack on more weight.

Michael Booth, the University of Sydney doctor who conducted the statewide survey, found 23 per cent of girls and 26 per cent of boys were overweight or obese in 2004 - up from 20 to 21 per cent in 1997.

But the children, aged four to 16, were exercising harder.

More than 80 per cent in years 6 to 8 were adequately physically active - spending at least an hour a day in moderate or vigorous activity - compared with less than 60 per cent in 1997.

"This flies in the face of what I think was an urban myth that children were inactive," said Dr Booth, who was so surprised that he ordered a recalculation.

"Like everyone else, I believed the urban myth."