Here is an interesting paper by Washington University economist Charles Courtemanche, "A Silver Lining? The Connection between Gas Prices and Obesity," recently posted on SSRN.

Rising gasoline prices have become a major concern in the U.S. over the past few years. An increase in the cost of driving causes people to consider substitutes, such as walking and taking public transportation, both of which are likely to result in additional physical exertion. Since exercise is known to decrease body weight, it is possible that the recent escalation in gas prices will have the silver lining of reducing America's obesity rate. This has the potential to significantly improve public health since obesity, which is widely believed to be one of the leading causes of preventable deaths in the U.S., can lead to a number of diseases, such as heart disease and diabetes. I find that a $1 increase in real gasoline prices will, in the long run, lower the prevalence of obesity in the U.S. by about 1/3 and save approximately 40,000 lives and $40 billion per year. I also find direct evidence that this effect occurs through increasing low-intensity exercise, such as walking. Additionally, I estimate that 3-8% of the rise in obesity in the U.S. between 1979 and 2004 can be attributed to declining real gas prices during the period.