Here's an interesting study [PDF] from Washington University in St. Louis in which economist Charles Courtemanche demonstrates a causal relationship between the price of gasoline and obesity rates in the United States. According to this study, an additional $1 in real gasoline prices would reduce obesity in the U.S. by 15% after five years, and that 13% of the rise in obesity between 1979 and 2004 can be attributed to falling real gas prices during this period. Courtemanche provides evidence that increased gas prices will result in more exercise for Americans as well as fewer restaurant visits. He writes:
If the price of gas rises, the cost of driving also rises, which may affect body weight in two ways. First, people may substitute from driving to walking, bicycling, or taking public transportation. Walking and bicycling are forms of exercise, which increase calories expended. 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 increasing calories expended. Second, since the opportunity cost of eating out at restaurants rises when the price of gas increases, people may substitute from eating out to preparing their own meals at home, which tend to be healthier. Income effects may also lead people to eat out less in an effort to save money to pay for the increased cost of gas.
Courtemanche notes that the reduced obesity rates can save 16,000 lives and $17 billion per year in health costs, partially offsetting the pain of paying higher gas prices.