% obese persons in Australia by age group: 1995 vs 2004–2005 (source: Preventative Health Taskforce)
A common view among politicians, the media, planners and health professionals is that urban sprawl is a key cause of the modern obesity epidemic. Higher population densities and more walkable neighbourhoods, many argue, are an essential strategy for fighting this scourge of the affluent lifestyle, e.g see here and here.
The trouble is both propositions are dubious. There are good reasons to pursue higher densities in Australia’s major cities, but addressing obesity isn’t one of them. It’s unlikely tightening the belt on sprawl will have much, if any, impact on the average BMI of Australians.
It’s not really surprising this view of sprawl is so entrenched. After all, there’s plenty of evidence showing suburbanites are generally fatter than inner city residents. But correlation is not causation, and households aren’t randomly distributed across cities.
An alternative explanation is suburbanites aren’t fatter because they live in the suburbs, but because people who are more likely to be fat self-select into the suburbs. Conversely, people who are likely to be thin self-select into denser neighbourhoods like the inner city. Changes in urban form will accordingly have little if any impact on obesity.
This fascinating study published in the leading Journal of Urban Economics, Fat city: questioning the relationship between urban sprawl and obesity, supports the alternative explanation. The research team, which included internationally prominent academics Henry Overman, Diego Puga and Mathew Turner, examined changes in the geographic location of 6,000 respondents in the US between 1978 and 1994.
They had access to a remarkable data base which, among other important variables, provided the precise street addresses, height and weight of respondents at intervals over a 16 year period. They were able to track changes in the location of individuals and construct measures of the neighbourhood density and walkability of each address.
As expected, the authors found residents of more sprawling neighbourhoods are indeed heavier on average than people who live in less sprawling neighbourhoods (although they found this applied to men but not women). However their results strongly suggest urban sprawl does not cause weight gain:
Rather, people who are more likely to be obese (e.g., because they do not like to walk) are also more likely to move to sprawling neighborhoods (e.g., because they can more easily move around by car). Of course the built environment may still place constraints on the type of exercise that people are able to take or the nature of the diet that they consume. The key point is that individuals who have a lower propensity to being obese will choose to avoid those kinds of neighborhoods. Overall, we find no evidence that neighborhood characteristics have any causal effect on weight.
Observers of Australian cities who argue that density and walkability affect obesity usually compare the inner city with the suburbs e.g see here. But the two populations are not the same. For example, compared to suburban populations, inner city residents are more likely to be young, single, have no dependants, have a higher level of education and enjoy a higher income.
It’s easier to be thin when you’re young (see exhibit!), haven’t had a baby, don’t have kids badgering you for fast food, are well informed about nutrition, can afford good food and have the time to cook. These sorts of factors are more likely to explain why on average inner city residents are thinner, not the fact they live at higher densities (I’ve expanded on this line of thinking before, here, here and here).
The authors of the article are aware their conclusions contradict the received wisdom on the connection between sprawl and obesity. However they point out their findings are consistent with other studies showing that sorting rather than causation is the primary mechanism that drives observed differences within cities on many socioeconomic variables. They conclude:
It follows immediately from our results that recent calls to redesign cities in order to combat the rise in obesity are misguided. Our results do not provide a basis for thinking that such redesigns will have the desired effect, and therefore suggest that resources devoted to this cause will be wasted. The public health battle against obesity is better fought on other fronts.
And equally, the battle for higher densities, improved public transport and shorter commutes is also better fought on other fronts. Of course this is a US study so the usual caution in extrapolating from another culture to Australia should be exercised – I’ve discussed the relationship between obesity and urban form and infrastructure in the Australian context on a number of occasions before (see links above and also here, here, here, and here).
17 Comments
Average BMI is a TERRIBLE indication of obesity. I don’t care if every doctor in the world uses it. I’m 180cm, and I weigh 90kg, giving me a BMI of 27.3, technically either overweight or obese (depending on who is interpreting this measure). There’s a slight problem there in that my bf% is around 4-6% (depending on where in a training cycle I am).
It’s time we stopped using a measure that was developed at the start of the 19th century as a measure for modern humans.
Interesting stuff. I can’t help but feel that there’s something missing in the logic of the research though, or at least in what is drawn from its conclusions. While I can see that the perceived causal relationship between urban design and obesity might be misguided or over-pronounced, it seems difficult to believe that urban design that fosters sedentary lifestyles isn’t at least an enabling mechanism that perpetuates obesity. I agree that moving to the inner-city won’t cause someone to the magically become thin, but surely the incentive to be more physically active (closer shops, parks etc.) and the disincentive to drive for all trips (traffic, parking costs etc.) must have some impact on the physical activity of those living in more ‘walkable’ neighbourhoods.
Interesting. But a bit of a chook and egg scenario innit?
People who like to avoid walking are drawn to the burbs… where they raise kiddies who also don’t like to move and it all goes on ad infinitum.
Either way the fact that the burbs allow people to live on the sofa in between drives to the mall and the bottle shop tends to underline the point that the suburban lifestyle – the sprawling developer estate – is not healthy.
Lazy acres.
Here’s a guess: look at the suburbs that correspond with high quantities of fast food restaurants. Not all suburbs do. Inala is one of the poorer areas of Brisbane, but it’s relatively low in the BMI, because there are lot of Vietnamese eateries there. A mate of mine works in Logan City – a couple of km east – and complains of all the “fatties” there. But there are zillions of McChucks and Kentucky Fried Mistakes all along the Pacific Highway.
mmmm… contrarian just for the sake of being contrarian? Alan Davies, and his blog has form in this regard.
It is not a straightforward cause and effect… whereby suburbs where you need to get in a car to do anything = fat people + walkable streets and neighbourhoods (mostly inner city built before the car became king) = non-fat people.
However, it has been rigorously researched and documented that environmental factors make it easier for people to walk – its common sense really. The Heart Foundation has done some great research. Walking is a critical feature in our evolution, it provides a definition of humanity. Places that hugely reduce our need to walk take away the incidental exercise that our bodies need and have evolved to do.
Sounds to me like Alan Davies is actually involved in Culture Wars. I wonder if the US research he relies on has got some Big Oil, Republican sponsors? How dare those greenie types tell us we can’t get in our cars and develop more and more sprawling housing estates where the level of incidental walking is almost zero?
Another developer / real estate lobbyist? can we do better than this Crikey?
It all makes sense.
I just need to put on 50kg and suddenly McMansions will look gorgeous.
Willie C, go easy on the ad hominem attacks. From Wiki: “An ad hominem….is an attempt to negate the truth of a claim by pointing out a negative characteristic or belief of the person supporting it. Ad hominem reasoning is normally described as a logical fallacy”. I’ll let your comment stand but in future please stick to the substance.
Comment still stands Alan, are you involved in Culture Wars? Will this be citied by some politican or development lobby as reason to turn over more land for ex-urban sprawl?
It is a direct result of globalisation: rich countries now have access to cheap high energy high carbohydrate food and we spend too much time working with a view to buying more stuff that we don’t need. Because we have less time we think less about cooking healthy meals. Eating healthy meals takes time and money. Low SES suburbs, and rural areas have the highest rates of obesity.
I’m not sure what this research actually offers in practical value? I could just as easily argue that fat people eat McDonalds and therefore there is no causative relationship between McDonalds and weight gain.
Now, perhaps we could argue this is social engineering but the perfect analogy I can think of is building design. In the 50s building were built with lifts and crappy, out of the way staircases. Modern buildings now often put nice staircases front and centre and put the lifts down the back. Now: lazy people are still going to take the lift. However doing this with the staircase will increase the proportion of people who walk up the stairs. That little piece of social engineering has a net benefit to the users of the building. Similarly, when we build new houses building them in a walkable manner will increase the amount of people who walk, and therefore benefit the users of that city.
You’re probably right, time poor people with kids tend to move to the suburbs because a sedentary lifestyle is quicker and easier. That doesn’t change the fact that these suburbs, just like the fast food they’re more likely to eat, is a causative mechanism in their weight gain. Urban policies that reduce the cost of living in “walkable” areas will still encourage these same people to walk more often.
Jonathan Nolan: The value of the research is it suggests that if they have a choice, people who have “fat” lifestyles will tend to opt for sprawling neighbourhoods like the outer suburbs of Australian cities.
Sure, if we suspend political reality and assume those same outer suburban settlers were denied that choice and were instead obliged to live at higher densities (say townhouses), then I agree it’s likely they would walk more.
But I don’t think it’s obvious it would be a lot more. For one thing, their diet wouldn’t change and that has a much larger impact on weight than exercise (it takes an hour and a half’s walking, or an hour’s cycling, to burn off the calories in just one Big Mac).
For another, outer suburban residents overwhelmingly work in dispersed suburban locations which are hard to service with decent public transport, so they’ll continue to drive.
I’d also like to know what a centre within walking distance of every outer suburban home would look like – are outer suburban residents really drawn to cafe society as much as their counterparts in the inner city are? Why wouldn’t they continue to drive to the mall where they get economies of scale and the benefits of comparison and complementary shopping?
As I say in the post, there are good arguments for promoting higher densities, but arguing obesity is over-egging the pudding. Planners and architects frequently attribute too much behavioural power to their works.
And obesity itself would be more efficiently addressed by non-design strategies.
I don’t think you need to suspend reality or deny people choice. This isn’t about forcing everybody into higher density. It’s about trying to increase the proportion of people who make that choice. This can be done through policies that make such housing cheaper, better designed etc. I’m not saying we should force people to walk to the shops, but pricing parking to reflect it’s actual cost might lead many people to make that choice. That’s just one example among many.
The relationship between exercise and diet is far more complicated than you suggest because exercise is an appetite suppressant.
Poor access to public transport is one of the criticisms of dispersed suburbs, so you do have a chicken-egg type situation there.
Addressing obesity might be more efficiently addressed by non-design strategies but you would have to pay a few million for a study to find out the answer to that. I’m not sure you’ve provided enough evidence to back up that claim.
There is a lot of evidence that shows that habits formed at a young age frequently stick around much later in life. For this reason, having more people in suburbs that are not car dependent would have a positive impact on reducing obesity. Kids that grow up in suburbs with few footpaths, no bike lanes and long distances to get anywhere that are serviced by poor public transport are unlikely to get into any transport habit apart from driving.
Additionally there is plenty of evidence that the rise in sedentary lifestyles has contributed greatly to the rise in obesity, claiming that diet is almost entirely to blame is simply ignoring half the evidence. It may take an hour of cycling to burn off the calories in a Big Mac, but someone is also considerably less likely to want to eat a Big Mac in the first place if daily exercise is part of their routine, food cravings are altered by exercise. Similarly increased exercise has been shown to be the most important factor for people to maintain weight-loss (see ‘Exercise in the treatment of obesity: Effects of four interventions on body composition, resting energy expenditure, appetite, and mood.’).
Your general point that planners may over attribute urban form to obesity is probably true, but claiming that sedentary lifestyles have not contributed much to the rise in obesity is just as far from the truth.
IkaInk: I’m not saying sedentary lifestyles are irrelevant, I’m saying that diet “has a much larger impact on weight than exercise”.
Here’s what Lennert Veerman from the School of Population Health at UQ has to say on the exercise/diet issue (article accessible here):
In relation to the argument that exercise suppresses appetite, that might hold for vigorous exercise, but I’m doubtful it’s true for the sort of low level activity, i.e. more walking, implied by higher densities. Do you have any further info on that aspect?
Unfortunately, unless you have more qualifications under your belt than I realise, neither of us are health experts. I’m sure we could go tit-for-tat listing different articles, by equally respected experts on whether diet or lifestyle has been the larger contributor (and a quick scan of just a few dozen results in Google Scholar shows that plenty of highly cited, peer reviewed articles published in respectable journals have argued both points of view), but that sort of argument isn’t going to get us far.
Instead I’d argue that the causes of the obesity epidemic are complex and a large range of solutions will be necessary to fight the problem. I do believe that better urban form can at least help people become more active, or at the very least give more people the option to be more active as part of their regular travel, and as a result I believe that better urban form can and should be part of the range of solutions.
IkaInk: “Instead I’d argue that the causes of the obesity epidemic are complex and a large range of solutions will be necessary to fight the problem.”
OK, but let’s try to understand both the costs and the benefits of each solution and establish if the latter exceed the former.
The cause of obesity is simply poor hunting and gathering techniques. Obesity is positively correlated to time spent in motor car and to consumption of processed food. Yet our centres strategies create communities where driving is the method of travel and the supermarket is the staple (poor) source. When you investigate communities and cultures where the source environment reduces the need to drive, then the obesity stats change markedly. This is not because of exercise (weak correlation) but because the source environment changes to fresh and healthy food. If you mix walking and driving to hunt and gather, then you buy different food (you have to as it changes your view of food). Simple illustration, USA 38% obese, New York City 18%, Greenwich Village 9% (Greenwich Village has similar socio-ecs to USA). You simply cannot drive to a supermarket in New York (there aren’t any), so this changes the supply environment to fresh food, deli’s and small dry goods stores. Americans drive 10,000kms per annum to shop, we drive 6,000, Europeans drive 2,000. There is a constant pattern here.
The other correlation is mall floorspace (limited data unfortunately but it shows the same trend): USA 50% of all retail floorspace is in malls = 38% obese, Aust 30% in malls = 25% obese, France 7% in malls = 7% obese, Italy 11% in malls = 11% obese.
USA % of health budget on obesity related diseases = 9%, Australia = 2%, France <1%. If we had the same obesity level as France we would save $2.7 billion per annum.
The problem we have is that the market model (price and quantity) leads to overconsumption and is derived from efficiencies of scale for retailers such as supermarkets (who need 10,000+ person catchments). If we want to be healthy then we need a finer grain of centres with smaller catchments and broken up components of the supermarket (butcher, baker, deli etc). This changes the hunting and gathering environment. We need good urban form to deliver it and yes some density.
By the way USA gym membership = 18% (highest in developed world), France = 5.4% (lowest in developed world). Gym membership is positively correlated with obesity!! If we don't get enough exercise out of every day life and we're buying and eating the wrong food – then we feel the need to join a gym to at least try and get that weight off. Sad but true! Agree with comments above BMI technique now over 100 years old -when we discover that pretty much the entire All Black team is obese it is clear we need a different measure .