A new culprit has been named as the reason for Americans becoming a nation of fatties: technology.
While most people thought it was down to the advent of 'super-sized' fast food, new research places the blame squarely on a double dose of innovation.
Researchers now suggest that 60 per cent of the extra pounds Americans have put on may be caused by a decline in the physical demands of work brought about by the arrival of computers and the like.
The other 40 per cent is due to technological innovation in agriculture which has driven down real food costs.
This double whammy has left 60 per cent of Americans overweight and a quarter technically obese.
The researchers, economists Darius Lakdawalla of Rand Corp and Tomas Philipson of the University of Chicago, described the situation as a "rising epidemic of obesity".
They pointed out that, in past decades, strenuous jobs meant that workers were in effect paid to exercise. Now workers with more sedentary jobs pay to exercise at the gym.
It is this lack of exercise, rather than an increase in calories, that has led to the weight gain, they say, which is no longer music to the ears of the US fast-food companies as they are now facing lawsuits from some litigious fatties.
"We used microdata from a variety of sources to quantify the importance of job-related exercise in weight determination, and found it to be significant," Lakdawalla and Philipson wrote in their paper for the National Bureau of Economic Research, a private research organisation.
They point out that an average 6ft-tall man today weighs 30 pounds more than he did in the early 1900s.
Because of the fundamental changes in agricultural production and lifestyle, the researchers did not offer much hope that Americans would lose the additional pounds.
But Americans should not try and go back to having the waistlines of previous generations.
Instead, they said, the country's policy makers should admit defeat in the fight against flab and should improve treatments for heart disease and diabetes, which are caused in part by obesity.
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