Does sprawl cause obesity?Posted: March 15, 2010
I’m unconvinced by the argument that suburban sprawl is obesity’s best friend. I’m equally suspicious that higher density living is justified as a sensible response to obesity, as this story in the Sydney Morning Herald, How City Living Fights the Waistband Sprawl, contends.
It’s not that I doubt there’s a correlation between obesity and distance from the city centre. The SMH story reports University of NSW researchers as finding that “those living in the outer suburbs were 30 to 50 per cent more at risk of being overweight and 40 to 60 per cent less likely to be physically active than their inner-city counterparts”.
Nor do I doubt that the physical environment might have some role. After all, 13% of inner city residents in Melbourne walk to work compared to just 1% in the outer suburbs.
But how much of this difference is due to low density living? Is the relationship causal?
Even at first glance, weight gain seems to me to be much more sensitive to what you eat than what you do (or don’t do). For example, you have to walk the dog for an hour and a half, or cycle for an hour, to burn off the calories in just one Big Mac. Isn’t it likely that all those suburban families eat more fast food than inner city latte sippers?
So differences in diet are probably a much more significant factor explaining obesity than low density living.
Addressing diet directly would therefore seem to be a much more efficient way to tackle obesity, rather than indirectly by tweaking something as large and complex as land use polices. After all, there could be very serious unintended consequences on non-trifling matters like access to home ownership, property values and neighbourhood amenity.
In any event there may be other reasons why the suburbs exhibit higher levels of obesity than the inner city. It might be, for example, that fatter people like to drive rather than walk and hence choose to live in the suburbs where it is easier to park and the streets are less congested. This is what is called a ‘selection’ problem.
A recent study by Zhenxiang Zhao of the University of Illinois, Effects of Urban Sprawl on Obesity, examined data on 53 large metropolitan areas using a methodology designed to minimise selection effects. The author concludes that “overall, my results suggest that urban sprawl did cause an increase in obesity, but its effect was relatively modest….a 1% decrease in the proportion of the population living in dense areas increased the prevalence of obesity by 0.1% to 0.2%”.
This difference is large enough to warrant attention, but it is a far cry from the sensationalism of the SMH article. Zhenxiang Zhao’s study did not attempt to identify the cause of the difference and nor did it use any measure of income, but my sense is that in Australian cities, which are significantly different from US cities in some ways, it has much more to do with social and economic factors than with density.
The areas Australian obesity researchers classify as the densest – in and around the inner city – are very different from the suburbs, especially those on the urban fringe. In particular, incomes, property values and educational status are much higher in the inner city on average than in the rest of the metropolitan area.
These affluent ‘knowledge workers’ are more likely to know about the importance of diet, to avoid fast food, to exercise and, if they have children, to ensure their kids participate actively in sporting and recreational activities. Although McDonalds’ outlets are as ubiquitous in the inner city as they are in the suburbs, inner city households are better able to afford higher quality foods like vegetables, fruit and lean cuts of meat and less likely to feel the need to ‘feed four for $20 at Maccas’.
Adults in the inner city are also younger and more likely to be unmarried or unattached. Of course they’re going to worry more about their appearance and hence their weight than suburban fogies who’re more likely to have bodies feeling the depredations of child birth and age.
And anyway, density is a much over-rated explanation for why obesity varies between inner and outer areas. While dwelling density is higher in the inner city, the difference in population density between inner city and suburban Melbourne is modest.
This is because households in the inner city tend to be smaller on average due to higher proportions of one and two person households, whereas suburban households tend to be larger because they often have dependents.
Consider thisanalysis by the Public Transport Users Association. It shows that the dwelling density of Kings Park, a recent subdivision near St Albans, is double that of inner city North Fitzroy, but the population density of the two suburbs is the same.
The inner city also has a singular advantage that is independent of density – proximity to the CBD. This makes walking to public transport or direct to work a more viable option. The CBD is by far the largest, densest concentration of jobs in the metropolitan area and is the focus of a comprehensive radial tram and train network.
This advantage cannot be replicated readily in the suburbs because there isn’t a single suburban centre that comes even remotely close to the size and density of the CBD.
The importance of proximity to the CBD in explaining the special character of the inner city is demonstrated by the fact that walking’s share of work trips plummets from 13% in the inner city to just 2% immediately one locates in the adjacent inner suburbs. This share is only marginally better than the outer suburbs.
The clear message here is that there are real dangers in extrapolating the conditions in the inner city to the suburbs. This applies to many issues, not just obesity, and is a consistent misunderstanding in much of the debate on urban policy. The inner city and suburbs are different countries.
Moreover, the inner city’s special character is the result of much more than just dwelling density. Increasing density in the suburbs at the expense of the fringe would therefore be unlikely to come even close to recreating the conditions of the inner city.
We already know that residents of medium density housing in the suburbs have much the same rates of car ownership and car use as their neighbours in detached houses. There’s every reason to think they’ll exercise just as much and eat just as much too.