Courtemanche compared 20 year's worth of government health surveys to fuel prices. The percentage of American adults considered obese doubled to nearly one-third between 1979 and 2004, and Courtemache says 8 percent of the increase can be attributed to low fuel prices. "Cheaper gas during that period made us fat," he told Wired.com. "Presumably, the opposite would be true."

His number-crunching suggests a permanent $1 increase in gasoline prices could cut the obesity rate by 10 percent within seven years. The number of Americans who are overweight, but not obese, could fall by 7 percent in that time, he says, as people shun cars in favor of walking, biking or taking mass transit. Granted, you don't burn many calories sitting on the bus, but you burn quite a few walking to and from the bus stop, he says. People also tend to eat out less frequently, opting instead to cook their own meals. "These results suggest that the recent spike in gas prices may have the 'silver lining' of reducing obesity in the coming years," he writes in the paper.

Some doubt rising fuel prices will do much to shrink waistlines. Dan Sperling, head of the Institute of Transportation Studies at UC Davis, says people don't change their behavior when gas prices rise, they simply buy more fuel-efficient cars. Nevertheless, evidence suggests they're riding bikes more often.

National sales figures aren't available yet, but bike shops in cities from Los Angeles to Chicago to New York have seen sales rise 20 to 40 percent in the past month or so. Those who aren't buying new bikes are digging that old Schwinn out of the garage. "Bike shops are doing record business fixing up old bikes," Tim Blumenthal, executive director of the national bicycle-advocacy group Bikes Belong, tells Wired.com. "It's off the chart."

That said, Blumenthal doesn't expect a big increase in the number of people biking to work. Most people live too far away from the office, he says, and don't have anywhere to clean up or stash their bikes once they arrive. But Blumenthal expects increasing numbers of people to hop on a bike for that run to the store, or the gym, or any of the other short trips he says make up 40 percent of the trips Americans make each day. "There's a general perception that (high gas prices) are here to stay, and people are changing their habits," he says.

Courtemanche can soon test his hypothesis. The national average for a gallon of gas was $3.22 when he wrote his paper last year. It's now at $4.10. He's planning a follow-up study.

Photo: richardmasoner/Flickr