Is obesity really caused by suburban sprawl?

Suburban sprawl is often linked with rising obesity - for example, see this submission (PDF 290KB) to last year’s Urban Growth Boundary Review from Kelvin Thompson, Labor Member for the Federal seat of Wills, or this article in the Sydney Morning Herald. The customary argument is that because the incidence of obesity is lower in the inner city where densities are higher, it follows that low density outer suburban development is the cause, or at least a very significant contributor, to obesity.

At first glance this seems to make some sense. For example, only 1.1 per cent of workers in Melbourne’s outer suburbs walk to work, compared to 12.9 per cent in the inner city.

But for all its faults, is it reasonable to put the blame for obesity on sprawl or would we better off focusing our energies on the real issues associated with sprawl rather than being distracted by sideshows? No, it isn’t reasonable to blame sprawl.

The key reason is that what goes in our mouths is more important than how much we exercise. 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.

The inner city has a lower incidence of obesity primarily because the residents eat better. And they don’t eat better because of higher density but because they have higher incomes than residents of the outer suburbs and, importantly, higher levels of education. They are more likely to know about the importance of good eating and they are more likely to be able to afford to eat better food. They also have smaller households on average so it’s easier to cook healthy food at home rather than go out for fast food.

Possibly most important of all, they are much more likely to be single and young, with a strong incentive to manage their food intake carefully in order to look good.

This misunderstanding of the demographics of the inner city comes up in other contexts, too. For example, the fact that inner city residents have a higher per capita ecological footprint than suburban residents has been used to condemn density when in fact the former’s environmental profligacy is due to their higher incomes - richer people tend to consume more of almost everything. The correct interpretation is that inner city households have an appalling environmental performance despite living in smaller houses - it would be worse, given their smaller household size, if they lived in McMansions.

Calories out is the other key variable in the obesity equation. We know inner city residents walk more than outer suburban residents but much of the reason for that, as I argued here, is because they live close to the enormous concentration of jobs and attractions in the CBD and inner city, rather than because of population density. As soon as you move from the inner city (about a 5km radius) to the inner suburbs (about 5-10km radius), the proportion of workers who walk to work drops from 12.9 per cent to 2.1 per cent.

Replicating the inner city’s density of destinations in the suburbs in a way that significantly affects walking levels seems unlikely (let’s not forget that less than 10 per cent of Melbourne’s population lives within 5km of the CBD and that they have 28 per cent of the metropolitan area’s jobs).

But it’s going to be hard to offset that Big Mac just by walking to your local pub or restaurant. I’m not aware of any data that suggests outer suburban residents play significantly less sport or undertake less formal exercise than their inner city counterparts. In fact outer suburban workers are more likely to have a job that involves physical effort, like a trade, than inner city professionals. If there are significant differences in per capita exercise levels, I suggest it would have more to do with demographic factors than with density.

Densities are rising in new developments in the outer suburbs, so can we expect a fall in obesity? I doubt it. It’s highly unlikely the new residents will have jobs within walking distance because the great majority of Melbourne’s jobs are dispersed across the suburbs at low density.

There’s a quaint notion they’ll walk to the corner store rather than drive to the district shopping centre. Structure plans accordingly make provision for convenience stores - for example, the draft structure plan for Toolern, Melton, has three plus a large activity centre. I don’t buy that.

Why would households burdened by mortgage payments and childcare costs forgo the cheaper prices available by driving to bigger centres, in favour of the higher cost at a local convenience store? And what does “convenience” really mean anymore? The use-by date on fresh milk at my Safeway is around 10 days and UHT milk is ubiquitous. There’s obviously a market for stores like 7-11, but last time I looked mine didn’t sell fresh vegetables or fruit, the very things that do go off quickly. Now that smoking is out of favour, I don’t see much hope for small walking-based convenience stores in new estates.

I think many more kids could walk or cycle to school both in the outer suburbs and everywhere else. But the key constraints there aren’t housing density (although I do wonder if school density is lower in the outer suburbs?). They’re traffic and over-wrought perceptions of “stranger danger”.

All in all, the current levels of walking (2.1 per cent) and cycling (4.5 per cent) to work that apply to the inner suburbs seem like a reasonable, but ambitious, target for the outer suburbs, given that job density is higher in the inner suburbs.

As I’ve argued elsewhere, sprawl appears to have only a small independent effect on obesity. A recent study by Zhenxiang Zhao of the University of Illinois, Effects of Urban Sprawl on Obesity (PDF 485KB), 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%”.

Sprawl has its downsides but over-egging the pudding doesn’t help rational debate.