Some questions have pat answers -- such as, "Does this dress make me look fat?" (Of course not.)
More tricky is the question of whether a person's address can make her fat. Many urban planners and health researchers think it can. In study after study, they have all but concluded that urban sprawl -- malls miles away, homes too far away for people to walk to shops, schools and parks -- contributes to obesity.
But University of Toronto economist Matthew Turner, author of a new and controversial study on the topic, contends that this answer, like an unflattering dress, just doesn't fit the body of evidence.
He acknowledges that in the last three years, roughly a dozen studies have taken statistical snapshots of where people live and how heavy they are -- most reporting that people who live in sprawling neighborhoods tend, on average, to be fatter.
"It's widely observed that people are heavier in sprawling neighborhoods than in nonsprawling neighborhoods," Turner says. But, he adds, it doesn't mean the sprawl is to blame.
He points out (as do, for that matter, the authors of these earlier studies) that the studies can't prove that living amid sprawl leads to obesity -- because they are just snapshots and don't report changes over time.
"There are two possible explanations," he says. "One is that sprawling neighborhoods cause people to be heavy. The other is that people who are predisposed to be heavy are attracted to sprawling neighborhoods."
Turner believes that the latter is the case.
He and his colleagues tested the two theories in a study released online last week that has not yet been published in a journal. The researchers examined data collected on almost 6,000 young men and women living throughout the United States.
Most subjects had moved at least once during the six-year period (1988 to 1994) that the study examined. That allowed the researchers to compare each person's weight before and after relocation.
The researchers also knew the subjects' addresses -- so they could record if the people had moved away from, or into, a sprawling neighborhood. On satellite images, they drew a 2-mile circle around each person's home and calculated the average distance between buildings as a measure of sprawl. They also noted the average density of retail establishments, which reflects mixed commercial-residential development and hints at how easy it might be for people to walk to restaurants and stores.
The team found that people's weight did not increase significantly when they moved from neighborhoods that had low sprawl to high sprawl. Nor did weight change when people moved between areas with different densities of shops and stores.
Though health research typically undergoes a quality-control procedure known as peer review before being published, Turner says that's not standard in economics.
Matthew Kahn, a Tufts University economist who has questioned some aspects of anti-sprawl rhetoric, applauds the new study.
"This is powerful evidence challenging the conventional wisdom," he says. "Turner and his coauthors, I believe, are the first researchers comparing the same guy over time to see whether he gets fat when he suburbanizes."
Public health and urban planning experts asked about the study acknowledge the scientific value of comparing data from different points in time.
But most express little praise for the study -- and numerous criticisms.
"I'm kind of skeptical of their conclusions," says Nicholas Freudenberg, a public health researcher at Hunter College in New York City.
For one thing, he and other critics say the study might have been too small or too brief to identify an effect. For another, "They don't present any evidence for their hypothesis that obese people prefer to live in sprawling areas," he says.
And, he adds, "Even if their [conclusion] is true, their study doesn't provide a reason for public health folks to abandon efforts to reduce sprawl." Cutting sprawl could lessen fuel use and air pollution, help prevent car crashes and reduce social isolation, he says.
Other researchers point to the many studies that -- even while imperfect -- have looked at the link between obesity and urban sprawl.
"Overwhelmingly," says Reid Ewing, an urban planner at the University of Maryland, those studies have found "evidence of an association between the built environment and obesity."
Some researchers say the new study may have missed the boat because it ignored important aspects of the urban landscape.
"I think they measured sprawl pretty poorly," says Russell Lopez, a professor of environmental health at Boston University.
"The study fails to account for density that's vertically arranged," adds Lawrence Frank, an urban planning professor at the University of British Columbia in Vancouver. He notes that the study's manner of calculating sprawl might rate a residential tower that's adjacent to an empty lot as more sprawling than a cluster of single-family homes.
Even horizontal density may not reflect a community's walkability, says Ross Brownson, an epidemiologist at St. Louis University School of Public Health.
If a neighborhood is densely populated but contains barriers to physical activity -- such as an absence of sidewalks or a dearth of parks within walking distance of homes -- some people may have difficulty exercising as much as they should, he says.
"If you live in the suburbs, yes, you're going to drive more," Kahn says. "But you have more access to public parks, to basketball courts." Whereas, he adds, "If you're afraid of walking around in the city, you might walk less when you move to the center city."