It’s commonly taken for granted that a disproportionately large number of fast-food stores in an area is a key reason why the local population often has high rates of obesity.
The California Centre for Public Health Advocacy, for example, argues that “there is growing scientific evidence that what people eat — and their likelihood of being obese — is influenced by the food environment in which they live”.
More people are going to fast-food restaurants where the food is high in calories and portions are large. Many respectable studies have shown a clear correlation between average body weight and eating out. According to one, growth in the number of restaurants accounts for as much as 65% of the rise in the percentage of Americans who are obese.
Various policies have been proposed to break the evident connection between fast-food restaurants and obesity. These include proposals to limit the number of fast-food restaurants, “fat taxes”, limits on fast-food advertising and mandatory calorie counts on menus.
But do fast-food restaurants really cause obesity? As with many things, appearances may be deceiving. Correlation doesn’t mean causation.
This study, Are Restaurants Really Supersizing America?, by UC Berkeley researchers Michael Anderson and David Matsa, poses this question: “do more restaurants cause obesity or do preferences for greater food consumption lead to an increase in restaurant density?”
It is not obvious, they say, that the empirical link between eating at restaurants and obesity is causal. “If consumers’ lifestyles”, they argue, “are increasingly conducive to excess energy intake and positive energy balance, the increasing prevalence of restaurants may simply reflect a greater demand for calories.”
They compare indicators of body mass for two rural populations in eleven US states. The “treatment” group lives within eight kilometres of an interstate highway where the density of fast-food restaurants is high. The “control” group live between 8-16 kilometres from the highway where accessibility to restaurants is lower.
They find that there is no causal link between obesity and restaurant food — the effect on BMI of restaurants is minimal at best (see chart).
The distributions of BMI in highway and non-highway areas are virtually identical, and point estimates of the causal effect of restaurants on obesity are close to zero and precise enough to rule out any meaningful effects.
But they also find that notwithstanding that more calories are consumed in a typical restaurant meal than at home, lowering restaurant prices does not increase obesity. There are two reasons:
First, there is selection bias in who eats at restaurants; people who eat at restaurants also consume more calories when they eat at home. Second, when eating relatively large portions at restaurants, people tend to reduce other calorie consumption at other times during the day. After accounting for these factors, eating a meal at a restaurant is associated with only 24 additional calories.
The authors suggest that the same outcome may apply to other targeted obesity intervention programs. They point to two recent studies of school-based programs where improvements to the nutritional content of cafeteria food had no effect on students’ weight because there was no control over what students ate outside of school.
This may also apply to other targeted interventions where consumers have other ways of compensating. The authors cite a study that contends mandating automobile safety devices does not reduce traffic fatalities because motorists respond by driving less carefully. Another study found that smokers react to cigarette taxes by smoking fewer cigarettes more intensively.
In the case of obesity, consumers can choose from multiple sources of cheap calories. Restricting a single source — restaurants — is therefore unlikely to affect obesity, as confirmed by our findings. This mechanism may also underlie the apparent failure of so many targeted obesity interventions … despite their ineffectiveness, such policies have the potential to generate considerable deadweight loss.
They measure the potential deadweight loss of policies targeted at restaurants in the US at up to $33 billion annually.
There are some possible shortcomings in this research. It might be that behaviour in rural areas does not translate to urban areas as easily as the authors assume. It might be that the people who live more than eight kilometres away from fast food restaurants work or go to school closer to the highway.
Even so, it sounds a cautionary note not to over-sauce the physical environment angle (an issue I’ve discussed specifically in the context of obesity before — here, here and here). And as always, correlation doesn’t mean causation.
*This story first appeared on Melbourne Urbanist.
Although Mr Davies article is so bland as to seem inoffensive, dig a tad deeper and O contraire – big food versus local community looms on the horizon -with a big fat belly accompanying it. One only has to undertake a little research to see that this ‘study’ and its boosters forms part of a small section of the food culture wars raging away in states.
It’s all about some basic dichotomies and disparities between wealth, class and access to healthy and safe food. i.e. local versus corporate food production, marketing and distribution; factory farming v/s family farming, mom and pop diners v/s chains. I know that sounds a bit glib, but hey-I’m only writing a letter not a 600 word article.
Whether it be road builders, construction companies or mass food producers Mr Davies always seems to come out on the side of the big lobby group in his articles. Talk about ingrained bias
Fred 76
Interesting article and comment, but taking the article at face value, the food available for purchase at US supermarkets should also come into the equation somewhere. US supermarkets are pretty crappy and it difficult to find unprocessed food at any of them especially in the working class areas. Whole Vegetables and fruit are rare.
Notice how kids in crowds and people generally are more vibrant in Sydney when there is a slight southerly breeze and therefore clean air in the otherwise soupy sydney airshed?
What is the correlation of air pollution to obesity?
Which raises another policy issue from left field – cash for clunkers is supposedly economic madness and poor environmental payoff? Really? About 600 or 1000 people in sydney die prematurely every year from air pollution.
So the coalition support premature death of old people and the sick? Who knew?
eli greg is typical of the surprisingly widespread inability to understand there really is a difference between correlation and causation. But it helps shield True Believers from uncomfortable material, so that’s probably comforting for him.
Tom McLoughlin, it’s un-related to the original topic, but since you raised it, you might consider investigating the effect of wind in general on both kids in school playgrounds and horses in large paddocks — regardless of the wind’s direction or ‘cleanliness’.
On the other hand, there’s probably little point in suggesting anyone do anything. In a world where so many ‘educated’ commenters can distinguish between correlation and causation only if they don’t like the conclusion suggested by an ‘argument’; where factor analysis isn’t to be mentioned; and where postmodern dogma (when convenient) tells us all opinions are equally ‘valid’, it all too often ends up becoming an intellectually irrelevant quagmire.
8 to 16 kilometres (5 to 10 miles) may be a deterring distance when crossing a city, but I would suggest that it is a minimal distance to many rural people. I can’t see that those distance variances in rural communities should create any difference in results, even if the restaurant effect is valid.
Possibly a comparison of rural communities with a restaurant culture, and a rural communities without fast food restaurant chains would be more effective, but I can’t see that an 8 km boundary would have any reason to be an effective difference when coillecting results.