Recently I wrote a guest post on a friend's blog that plotted the percentage of obese people in a country and the percentage of trips that are human powered (walking + bicycle). Anecdotally Western countries with a lot of biking and walking appear to have a slimmer populace and this is indicated by the data.
The relationship is not causal as there are many things that were not controlled in the studies that went into the graph -- you can't say that an obese country like the US would become thinner if people became more physically active, although it seems to be common sense. You can say, from other studies, that walking and biking for transportation generally would increase the level of physical activity. In any event a carefully controlled study would be very interesting.
The chart I originally did broke some rules of good graphical presentation largely to make a visual point and hopefully make people think. Here is a more conventional plot of the same data.
(finding these data isn't easy as the studies they are from can be very different. Some Northern European countries - notably Denmark and Sweden - are probably in the "green" area)
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