For overweight Americans relief is on the way, in the shape of ever-higher petrol prices. Getting out the car to drive downtown for a super-sized plate full of fatty fast-food is the highlight of the day for many Americans. The result is a public health crisis with four out of 10 American adults already overweight or heading that way.
After consuming mountains of chips, fried meat and baked goods all washed down with corn-sweetened soft drinks, overweight Americans then worry which best-selling diet book will help them see their toes again. It turns out that higher petrol prices can slim down more than the wallets of the overweight.
The ever-rising cost of filling up their cars is prompting millions of Americans to pack their own lunch and walk to the bus.
The statistics are dramatic: they show that when petrol prices have risen in the US, obesity has shown a corresponding fall of as much as 10 per cent according to a new study, A Silver Lining? The Connection between Gas Prices and Obesity.
The study's author, Charles Courtemanche, from the University of Washington, St Louis, said his inspiration came when he was filling up his car: "I was pumping gas one day, thinking with gas prices so high I may have to take the Metro," he said.
After figuring out that he would get an extra 30 minutes of exercise per day by walking to and from the Metro, he correlated statistics for obesity and petrol prices in America.
American obesity rates began to rise sharply in the early 1980s. Part of this has been blamed on an overworked population demanding convenience foods – prepared, packaged products and restaurant meals that contain more calories than home-cooked meals.
According to Marion Nestle, of New York University, the arrival of the Reagan administration in 1980 brought government subsidies for farmers who grew more food. Fast-food companies reacted by serving larger portions and inventing snacks. The calories available per capita nearly doubled to 3,900 a day and a crisis was born.
But, to his shock, Mr Courtemanche found that 13 per cent of the rise in obesity between 1979 and 2004 could be attributed to the falling price of petrol. "First, if a person uses public transportation, such as subways, buses, trolleys or rail services, the need to move to and from the public transit stops is likely to result in additional walking, again decreasing weight," he said.
"Second, since the opportunity cost of eating out at restaurants rises when the price of gas increases, people may substitute eating out to preparing their own meals at home, which tend to be healthier."
Now petrol prices are on the up again, reaching a record high of $3.22 (£1.60) per gallon in May 2007, and so according to the theory, obesity levels should now be falling. "The recent spike in gas prices may have the 'silver lining' of reducing obesity in the coming years," the study said. It calculated that an increase of $1 per gallon in real gasoline prices would reduce US obesity by 15 per cent after five years. That would save 16,000 lives and $17bn a year, according to the research.
Mr Courtemanche said he had already received hate mail for suggesting that high petrol prices are good for Americans. "One person yelled at me: 'So now I'm supposed to be happy about gas prices!'" he said.
Some 59 million Americans are overweight. Almost 65 per cent are either obese or overweight, 10-30lb over a healthy weight, with greater chances of developing diabetes, heart disease, and some types of cancer. And it is not just adults. Some obese children in the US knock back two or three bottles of cola a day, equivalent to 1,000 calories.
Most US cities do not have good public transport networks, although they are improving. As more people react to the "sticker shock" of paying more than $3 a gallon by opting to walk, take the bus, or even cycle, planners anticipate pressure for better public transport.