This piece in Care2 Make A Difference quoting the American Journal of Preventative Medicine says using public transport and ditching the car promotes good health by reducing obesity. http://www.care2.com/causes/access-to-public-transit-reduces-obesity.html
That is also anecdotally true anywhere. Just look at rail and bus networks and people who are regular commuters. The levels of obesity are markedly lower there. That is my observation in India, where even if you isolate factors such as lack of adequate nutrition, a cohort of particular income is likely to have comparable results. Sadly, we have few empirical studies to prove this, despite the rising motorisation for over a generation.
In any case here is another paper by Besser et al from 2005, that pretty much makes the same case. So what we need to do is campaign for walking spaces and good, comfortable and affordable public transit systems.