Current theory says that a city's walkability promotes health and will impact the fight against obesity. The claim is that America's weight problem can be helped by making cities more pedestrian-friendly. It should follow, then, that our most dense and walkable cities are where the skinny people are, right? Well, not really.

Check out Web sites like Walk Score and you'll see a familiar list of boutique cities listed as having the most walkable neighborhoods: San Francisco, New York, Boston, Chicago, Philadelphia, Seattle, Portland, Washington, D.C. The car-dependent suburbs tend to make Americas fat and sedentary. Good urban design promotes fitness. Right?

Not so fast. A list of U.S. counties compiled by Money magazine ranks regions by body mass index. The list reveals something different. America's skinniest counties don't appear to be urban but rather rural and suburban. They include places in rural Oregon (Bend), Montana (Bozeman, Missoula), and Tennessee. But many of the most populous counties encompass affluent suburbs famed for their office parks and hot tubs.

Of the big, walkable cities, only San Francisco County, which consists primarily of the city of San Francisco, made No. 2 on the list. But No. 1 was the upscale, granola-munching Bay Area suburb of Marin County. Among the other skinniest counties were Boulder County, Colo.; San Mateo and Santa Clara counties in the middle of the Silicon Valley burbs; Washington, D.C., suburbs like Montgomery County, Md., and Fairfax County, Va.; and wealthy arts enclave Santa Fe County, N.M. The issues of obesity are complex, involving behavior, genetics, diet, social class, race, and many other factors. But the Money list suggests that affluent suburban living doesn't necessarily produce chubby couch potatoes, and walkable cities don't guarantee herds of lithe citizens.

In fact, according to the American Heart Association, a body mass index of more than 25 means a person is overweight. By that measure, only five counties on Money's top 25 list have populations with body mass index numbers that indicate their residents are not overweight. Residents of Marin County weigh in with an index of 24.48. (San Francisco County, Williamson and Maury counties in Tennessee, and Boulder County round out the top five that are not overweight.) Every other county has a number over 25 that shows "increased risk for heart and blood vessel disease."

It may be galling to urban planners to realize that some SUV drivers are in better shape than the folks who inhabit urban villages. But being realistic suggests that tackling the obesity "epidemic" is complicated and won't be solved by simply turning cities into jungle gyms for fatties.