Do more fast food restaurants make the locals fat?

Distribution of BMI for people living in US towns adjacent and non adjacent to interstate highways (note there are 2 curves)

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 States of the US. The ‘treatment’ group lives within five miles of an interstate highway where the density of fast food restaurants is high. The ‘control’ group live between five and ten miles 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 student’s 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 five miles 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 – herehere and here). And as always, correlation doesn’t mean causation.

9 Comments on “Do more fast food restaurants make the locals fat?”

  1. Mahyar says:

    Having lived in the US for years, I can tell you that it ain’t proximity to highways that causes fatness, because obesity is everywhere. It’s lifestyle. Did you watch that Jamie Oliver show at the fattest school district in the US? People just don’t know how to cook anymore and are genuinely taken aback at the thought of eating unprocessed food.

    It’s interesting when visiting more affluent, less insular parts of the country like NY and LA, where obesity seems less of a problem.

    Maybe a similar relationship exists here. Your next article on obesity perhaps…

  2. chris says:

    “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.”

    Is this just an attack on the nanny state? I am surprised anyone believes seat belts have not saved lives, or smoking restrictions have not reduced lung cancer rates. More efficient engines have led to bigger and heavier cars and more cyclist fatalities – but that is a different effect.

    Does the absence of pokies in a town cause a community to be less disadvantaged, or was it the strong community that resisted the parasites in the first place? Cause and effect are not easy to discern, but correct experimental design can help. The above study is unconvincing.

    A person’s family and peers will have a bigger influence on what they eat.

    Even at a typical workplace one will find a range of healthly to junk eaters, with corresponding exercise rates.
    Jamie Oliver is to be congratulated for trying to educate people about the food industry. But does that make him part of the nanny state?

    • Alan Davies says:

      I don’t read it as an attack on the ‘nanny state’. I think its saying that policy-makers should make the effort to fully understand what causes problems rather than jumping to obvious ‘solutions’ (e.g. alcopops tax).

      PS Note also in relation to the seatbelts study that it relates to US drivers who seem to have a markedly different attitude to seatbelts than we do.

      • chris says:

        We can make a law that houses must be built with the ability to cross-ventilate. But that is no guarantee the occupants will open the window when the cool change comes, or close the blinds when the sun is hot. But we should at least give them the opportunity. Then so long as some percentage do make use of the opportunity, the law is a success, despite it being an inconvenience to developers.

  3. Interesting. The BMI density match looks too close. I’d expect differences even if their really isn’t any causal relationship between restaurant availability and BMI. Was the study peer reviewed? it doesn’t look like it. But, accepting it at face value, perhaps both populations are effectively saturated? The highway asymmetry bothers me a little also.

  4. […] This post was mentioned on Twitter by David Gillespie, Yolande Pritchard. Yolande Pritchard said: RT @gillespi: Do more fast food restaurants make the locals fat?: –> I am living proof, yes, and more than once […]

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