While the link in adults between exercise and weight has long been a source of obsession, research now suggests that children as young as eight who don't exercise also get fat.
The connection between children's activity rates and weight has been a controversial issue but researchers who fitted children with padlocked pedometers for six days say they have shown children who are more active are less likely to be overweight - and a high heart rate in children, often taken as a measure of fitness, was actually found in the inactive, heavier children.
"Our results conclude for the first time that a significant relationship between activity, fitness and fatness in children exists, and that children who were more active were less likely to be overweight," said Dr Roger Eston, the senior lecturer in human physiology who led the research at the University of Wales, Bangor.
His team fitted child volunteers aged eight to 10 with a padlocked belt pouch containing a pedometer and a triaxial accelerometer, a device that measures activity and movement. The belts were worn from morning until bedtime, and a heart monitor was also put in place.
There are no figures for obese children in Britain, but in Europe one in five middle-aged adults are classed as obese, and in the US 300,000 people a year die of diseases linked to their obesity.
An increase in fatty diets and a decline in manual activity have combined to create generations of heavyweights who eat too much and exercise too little. The world's fattest man died last year weighing 73 stone in his mid-40s. It was estimated that such an accumulation of fat needed an excess equal to only a small bar of chocolate a day.