In discussions of urban design and obesity, walkable neighborhoods are often touted as a way of combatting America's weight problem. But a new University of Washington study indicates that the best predictor of whether or not people will be overweight is to know their zip code. A description of the report in Newsweek's online edition sums it up this way:

In a study published in the September issue of the journal Social Science & Medicine, University of Washington researchers found that adults living in ZIP codes with the highest property values were the slimmest, and those living in ZIP codes with the lowest property values were the fattest.

In other words, obesity is, at least in part, a class issue. It appears to be due to the availability of healthy foods: People who can afford to shop at Whole Foods have an advantage over those who don't have health-conscious, low-cost markets handy. A major indicator of obesity is property value. In an interview with Newsweek's Karen Springen, the study's lead author, Adam Drewnowski, director of the Center for Public Health Nutrition and professor of epidemiology and medicine at UW, had this to say:

If we looked at the median prices of residential properties, for each $100,000 of added value, obesity rates go down by 2 percent. In Mercer Island, the median house price is around $1.5 million and the obesity rate is around 5 percent. In the worst areas, in south King County, the median asking price is $270,000 and the lowest asking price is about $10,000 for a manufactured home. The obesity rate there is around 28-30 percent.

This is a suburb-to-suburb comparison. The difference isn't walkability, it's socio-economic. But the difference is also evident within dense, walkable cities, according to the report's lead author:

The department of health noted that if you go from the Upper East Side [of New York City] over to East Harlem, the obesity rates quadruple, from 7 percent to about 28 percent, and the rates of diabetes go up seven times, from 2 percent to 14 percent. You are, in fact, in a different world.

The equation for obesity is much more complicated than the notion that urban lifestyles offer more fitness than suburban or rural ones. It suggests that weight problems are connected to social equity and class. In other words, a bike path may not be as good a solution as making organic produce available to the poor, though neither is likely to change the bigger picture on its own.