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Urban sprawl not cause of human sprawl

November 01, 2006

As health-spending on obesity-related illnesses continues to rise in the United States, many suggest that urban planning geared towards active and healthy living could be an important tool to curb obesity.

But does urban sprawl really cause human sprawl? Not according to research conducted at the University of Toronto, the London School of Economics and Universitat Pompeu Fabra in Spain. In the recently released working paper, Fat City: Questioning The Relationship Between Urban Sprawl and Obesity, researchers find no evidence that urban sprawl affects weight.

As is often reported, the researchers find that people living in sprawling neighbourhoods tend to be heavier than those living where development is compact and there are many shops and amenities within walking distance. However, this is not because sprawling neighbourhoods cause people to gain weight. These populations are heavier because individuals more at risk for obesity tend to live in such places. "Someone who does not like to walk is more likely to be obese and is more likely to live where one can easily get around by car," says U of T economics professor Matthew Turner, one of the study's authors. "Thus, the finding that people in sprawling neighbourhoods are heavier does not imply that sprawl causes obesity."

The researchers matched a recently available satellite image of the United States to confidential survey data that reports the weight and address of a sample of nearly 6,000 individuals for six years. Since about 80 per cent of the people in the sample changed residences during that period, researchers could check whether people gained weight when they moved to a more sprawling community. "If you think that sprawl causes people to gain weight, then people who move from compact to sprawling neighbourhoods should gain weight. They don't," says coauthor Professor Diego Puga of Universitat Pompeu Fabra.

This means that plans to redesign the environment will not lead to cities that cause people to be thin; rather, they are likely to create cities to which thin people move. "Our results provide a basis for thinking that 'smart growth' type designs will not cause people to be thinner. This means policy-makers who try to combat the obesity epidemic by encouraging these designs are wasting tax dollars," says Turner. "The public health battle against obesity should be fought on other fronts."

Other experts hailed the research as significant in fighting popular misconceptions about the causes of obesity. Matthew Kahn, an economics professor at Tufts University and author of Green Cities: Urban Growth and the Environment, said the researchers employed statistics to challenge conventional wisdom. "They used sophisticated econometrics to take a more careful look at whether suburbanization does indeed make us fatter," says Kahn, who was not part of the study. "Hopefully their methods will be adopted by public health researchers seeking to establish causality rather than simply reporting raw correlations."

University of Toronto