However, there are too many external factors that are wrapped up in such a relationship between countries, including social conditioning (the more obese people in your social network, the greater your chance of becoming obese), food consumption patterns (use of high fructose corn syrup, GMOs, etc), gasoline and car taxes (high gas and car taxes keep car ownership low), potential distance to be traveled (smaller socially perceived distance horizons imply a shorter travel distance), etc., etc. Therefore, a much better analysis would be a pre-test vs. post-test analysis of people in a city before (pre-test) and after (post-test) the installation of a public transportation network. Furthermore, the public transportation network needs to have wide-spread social buy-in. In other words, not buses, which get caught in the same traffic snarls as cars do, making their use less beneficial than driving (for those who have cars). In other words, a more conclusive study should try an apply an experimental design. Yet, how can one just do that?
Well, it works when you do a pre-test and post-test of a newly installed public transportation network in a major United States city: Charlotte, NC. The study in the American Journal of Preventative Medicine showed:
using light rail for commuting was associated with reductions in body mass index (BMI) over time. Specifically, LRT reduced BMI by an average of 1.18 kg/m2 compared to non-LRT users in the same area over a 12-18 month follow-up period. This is equivalent to a relative weight loss of 6.45 lbs for a person who is 5'5. LRT users were also 81% less likely to become obese over time.Public transportation seems to work, folks. Still, it would be interesting to look at how these rates change as the network expands, as well as including how quickly people in currently unserved areas change their BMI as they get served by an ever-growing transportation network. (My thought is that the rate of BMI decrease will be faster as the network size increases, although the rate of decrease will be tangentially limited.)
UPDATE: In a recent study by Trust for America's Health, Michigan is rated as being the 10th fattest state in the nation, among adults (but 41st fattest among children). I don't know what their methodology was, but it seems to match up with some other things I've seen elsewhere. Anyway, if people want to use the evidence of the Charlotte, NC study to link up with obesity rates in the state, then it might be a new and interesting way to organize efforts to actually get more movement on regional light rail in Southeast Michigan. Of course, with Michigan being the traditional car capital of the world, this could still be a non-starter...