The key to tackling obesity, a suduy on the obesitychild.jpg epidemic, is tackling over-eating and not the lack of exercise. the reason they give is that - physical activity could not fully compensate for excess calories.
"There is no evidence that a marked reduction in physical activity has been a contributor to this epidemic in the United States," study leader Boyd Swinburn told AFP on the sidelines of an international obesity conference in Amsterdam, where the research was unveiled.
"The increase in energy intake... virtually explained all of the weight gain."
Swinburn, a professor at the health faculty of Australia's Deakin University, said American children had grown on average four kilogrammes (nine pounds) heavier over the past three decades with adults putting on an extra eight kgs (17 pounds).
The study calculated what Americans should weigh today based on their current, higher food intake, and comparing this to their actual weight.
If they weighed more than projected, this would suggest a drop in physical activity.
In fact, researchers found that American adults weighed less than could be expected from their diet, "which means that if anything over that period of time, the adults had been increasing their physical activity, not decreasing," said Swinburn.
Among children, the tests yielded a 100 percent match, leading researchers to conclude that changes in physical activity had had no impact whatsoever on America's children growing fatter.
The findings would "probably be similar" for other developed countries, Swinburn said.
For the US population to return to its leaner, 1970s self, children would have to cut their intake by about 350 calories a day -- equal to one can of fizzy drink and a small portion of French fries, and adults by about 500 calories -- the equivalent of a Big Mac burger.
Alternatively, children would have to walk for an extra two-and-a-half hours a day, and adults for nearly two hours, said Swinburn.
"Getting everybody to walk an extra two hours a day is not really a feasible option for countering the epidemic," he said.
"We need to limit our expectations of what an increase in physical activity can achieve."
Swinburn stressed that the findings did not seek to negate the value of physical activity for weight control and overall health.
"But if we want to influence the underlying drivers (behind obesity), we have to have our eye much more on the energy intake side than on the physical activity side."
In short, Americans must eat less, he said.
The World Health Organisation estimates that in 2005, about 1.6 billion adults were overweight, of which at least 400 million were obese.
The conference was organised by the European Association for the Study of Obesity.