This week’s Forbes magazine features a story on Americas Most Sedentary Cities. Translation: which cities in America have the highest levels of obesity, and why?

The answer to “why?” is a complex but understandable interaction between several factors: suburban sprawl, lifestyles of convenience, and some local factors that vary city-by-city.

It turns out that transportation planning and the larger context of urban planning — or lack thereof — are contributing mightily to our lack of exercise and communities’ ‘walkability.’ Research is finding that the lack of greenspace and parkland in a community contributes to sedentary lifestyles and, thus, obesity.

The Forbes team researched the 20 most sedentary, and thus most “sloth-y,” cities. In spots #1 and #2 are Memphis and New Orleans (I know, I know, I’m a foodie, too…just fantasizing about the wonders of BBQ and Paul Prudhomme as I type the names of those two offending food meccas).

Forbes analyzed 2006 data from the CDC’s Behavioral Risk Factor Surveillance System including body mass index (BMI) and physical inactivity. The team married data from Nielsen on the number of hours of TV watched per week by metro area.

While obesity has spread as quickly as urban sprawl — a major contributor to the epidemic — the factors that cause obesity can be very specific to a community. Forbes found that in New Orleans after Katrina, a lack of supermarkets has led to citizens relying on small convenience stores for junk food-laden daily fare.

Memphis’s combination of southern cuisine and most-watched hours of TV, coupled with a higher proportion of people who don’t exercise, has led to its high obesity profile.

The British organization SmarTrans has done a stellar job organizing some of the latest research into the relationship between suburban sprawl and health. Take a look at the article, Pathways to obesity: Identifying local, modifiable determinants of physical activity and diet, in the November 2007 issue of Social Science and Medicine. Mai Stafford and her colleagues found lower levels of obesity in areas with more swimming pools and supermarkets.

Health Populi’s Hot Points: City and transportation planning play large, and often unrecognized, roles in supporting public health. These planners set the environmental context for daily living. These functions are generally siloed away from health policy makers. In focusing on the public’s health, we should make sure to include all relevant stakeholders at the table who influence and positively promote healthy lifestyles. The case of obesity in the US is a prime example of the complex interactions between lifestyle, local and household economies, and community environment.