Are food deserts relevant to obesity?

Last year, we wrote about the increasing attention garnered by research on “food deserts“–regions of America where people lack easy access to nutritious foods. Like many folks, we were lulled into the idea that life in such deserts would contribute to obesity–by leaving people without other options to eat healthy produce rather than pre-packaged junk food. But recent research has led to us to ask: were we wrong?

The White House and local governments have introduced major initiatives to address food deserts in the country, like promoting supermarket subsidies to convince corporations to build stores in poor neighborhoods, and helping food stamp recipients gain eligibility to purchase produce from local farmers’ markets. The USDA has also kept track of food deserts around the country, mapping these areas intensively and monitoring access to food in multiple regions.

A pair of recent research studies, however, have been widely publicized as questioning the link between food deserts and obesity, and thereby questioning whether these national initiatives are worth their effort. The first study, by Roland Sturm (a prominent obesity researcher at the RAND Corporation), was a statistical study that correlated data from 8226 children (aged 5–11 years) and 5236 adolescents (aged 12–17 years) from the 2005 and 2007 California Health Interview Survey. Sturm and colleagues analyzed the correlations between daily servings of fruits, vegetables, juice, milk, soda, high-sugar foods, and fast food, as compared to the “food environment”, which was measured by counts and density of businesses, distinguishing fast-food restaurants, convenience stores, small food stores, grocery stores, and large supermarkets within a specific distance (varying from 0.1 to 1.5 miles) from a respondent’s home or school.

The most notable problem with this methodological approach is that it is an observational study of correlations between business density and self-reported intake of various foods. Self-reported food intake has been noted for years to be a poor measure of actual intake, as people tend to overestimate their intake of healthy foods and underestimate their intake of unhealthy foods. Secondly, this methodology is predisposed to “ecological bias“–that is, there are several reasons why food environments may be related to different levels of reported consumption, for example that an area is urban and therefore will have a high density of supermarkets and all sorts of other stores but also have a high number of people in poverty who eat junk food perhaps because it’s cheaper–and therefore it may look like all sorts of correlations exist (or lack of correlations, as too many competing variables wash each other out). Sturm and colleagues pointed this out; they state: “No robust relationship between food environment and consumption is found. A few significant results are sensitive to small modeling changes and more likely to reflect chance than true relationships. This correlational study has measurement and design limitations. Longitudinal studies that can assess links between environmental, dependent, and intervening food purchase and consumption variables are needed. Reporting a full range of studies, methods, and results is important as a premature focus on correlations may lead policy astray.”

However, a number of politically-extremist commentators have mis-interpreted this “non result” with the idea that food deserts are “not real” or a “myth“. They have therefore misused this study’s lack of correlation between supermarket density and self-reported intake of fruits and vegetables as indicating that there is no such thing as a food desert or that lack of access to healthy foods is irrelevant to nutrition or obesity.

A second study has also been recently misinterpreted and mis-publicized in the same way. Helen Lee of the Public Policy Institute of California published a paper in which a database of obesity prevalence among a non-nationally-representative panel of kindergarteners was correlated to food availability measures from a business establishment database (the National Establishment Time Series). She found that children who live in residentially poor and minority neighborhoods are statistically more likely to have greater access to fast-food outlets and convenience stores, but–more surprisingly–these neighborhoods also have access to other food establishments including large-scale grocery stores. Differential exposure to food outlets was not independently correlated to weight gain over time in this sample of elementary school-aged children. Dr. Lee concluded that “It may thus be important to reconsider whether food access is, in all settings, a salient factor in understanding obesity risk among young children.”

As opposed to this nuanced conclusion, the paper was again recycled into biased headlines like “The Jig is Up on Food Deserts“. Actually, a correct interpretation of Dr. Lee’s study is that it shows a potential age-variability in the effect of food deserts. That is, several other studies already provided substantial support for the idea that food deserts are relevant to obesity in a number of contexts. This appears to be particularly true among adolescents. A study analyzing repeated cross-sections of national survey data found that availability of supermarkets was associated with lower obesity among teenagers, whereas increased density of convenience stores was associated with higher obesity. An independent study found that high schools with a fast-food restaurant within walking distance had significantly higher obesity prevalence among its student population compared to schools without a fast-food outlet nearby.

By contrast, Dr. Lee’s study was conducted to glimpse into the eating patterns of much younger children. Adolescents have more mobility and agency in food purchasing decisions, making walkable proximity to junk food versus supermarkets a potentially relevant issue. On the other hand, younger children must often rely on their parents to access food, and previous studies show that parents may rely on factors like stores near they work, in addition to other factors such as product diversity, price ranges, and brand loyalty. Dr. Lee’s findings do suggest that availability of supermarkets may not be as limited in low-income or residentially minority areas in her study as in other areas, which diverges from much of the earlier literature probably because she uses different datasets, different food industry categorizations, and different measures of retail food access and composition than most studies. Furthermore, heterogeneity in food accessibility between regions was not captured in Lee’s study, which aggregates the country into an average. It is also important to note that “proximity” to a store as defined in these studies can be hard to define–accessibility of a food store for urban dwellers may have nothing to do with distance but rather with how close a store is to a major bus or subway line, while in rural areas “proximity” may be determined by access to a car.

Nevertheless, Lee and Sturms’ studies are useful. They suggest that there are important other aspects of obesity to consider besides food deserts–for example, as Robert Lustig has pointed out, the critical availability and marketing of high-sugar products. In other words, it may not be that eating insufficient produce has little to do with obesity or is not the major issue determining obesity; rather, not cutting-down on other less-healthy calories is the issue, and these calories are available in both supermarkets and convenience stores. The problem, of course, is that it is politically simpler to suggest that we should subsidize American farmers and distribute more fruits and vegetables than it is to suggest that we should regulate calorie content and enforce rules on food producers and marketers. Regardless, these studies are also isolated to only studying obesity as the outcome variable of interest. If we are also interested in broader aspects of nutrition besides just body mass, such as appropriate intake of vitamins and minerals and healthy fibers that reduce cholesterol, for example, access to fruits and vegetables still remains highly relevant.

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