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I’ve always felt there’s something dubious about the “food deserts” hypothesis that’s so captured Michelle Obama’s imagination. Now a story in the New York Times suggests my instinct might be close to the mark.

Food deserts are places that have good access to fast food outlets selling high-fat, high-sugar junk food, but poor access to supermarkets and grocery stores offering healthier food options. The hypothesis, based on research like this 2007 study, is that poor childhood health outcomes like obesity, most especially in poor neighbourhoods, are in part a direct result of the low density of supermarkets.

That theory’s never sounded convincing to me. While I don’t doubt there’s some accessibility effect, my feeling is there are other factors that explain much better why fast food is so attractive to poorer communities.

One is lots and lots of people like the taste of fast food. That’s in large measure because it’s laced with stuff we’ve evolved to crave. That was OK when sugar and fat were hard to get, but it’s bad for us when it’s plentiful.

Fast food is also just plain convenient – it requires no preparation, so it saves time and effort. That’s likely to be very attractive to a poor, single mother who might be working two jobs.

And although it’s not as cheap as buying the basic ingredients and cooking at home, fast food is affordable. It’s much cheaper relative to incomes (including low incomes) than in the past. And it’s extraordinarily cheap given it requires no labour input or skill from consumers.

Bear in mind too that not all lower socioeconomic households have the high level of education that alerts the middle class to the health dangers of fast food. Indeed, some might not feel they have the sort of positive future that makes it worthwhile to sacrifice a bucket of fries now for possible health benefits in the medium to longer term.

A study done in 2004 by Burdette and Whitaker lends support to my doubts. It examined the relationship between the body weight of 7,020 Cincinnati preschool children and three environmental factors – the proximity of the children’s residences to playgrounds; proximity to fast food restaurants; and the safety of the children’s neighbourhoods. The authors found:

There was no association between child overweight and proximity to playgrounds, proximity to fast food restaurants, or level of neighborhood crime. The association between child overweight and playground proximity did not differ by neighborhood crime level.

A new study published earlier this year also supports my reservations. An and Sturm examined daily servings of fruits, vegetables, juice, milk, soda, high-sugar foods, and fast food taken by 13,450 children and adolescents in California.

They compared their consumption against the density of businesses in their neighbourhood, 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 authors found no robust relationship between food environment and consumption.

I don’t have access to the full article, so here’s what the New York Times reported last month about the An and Sturm study:

Dr. Sturm found no relationship between what type of food students said they ate, what they weighed, and the type of food within a mile and a half of their homes. He has also completed a national study of middle school students, with the same result — no consistent relationship between what the students ate and the type of food nearby. Living close to supermarkets or grocers did not make students thin and living close to fast food outlets did not make them fat.

So I’m inclined to the view that food deserts aren’t an important part of the explanation for child obesity in poor neighbourhoods. Even if there were abundant supermarkets and grocery stores in these areas, I doubt it would make a lot of difference.

But here’s a surprise. A story published last month in the New York Times, reporting on new research by Dr Helen Lee of the Public Policy Institute of California, suggested that the basic premise of food desert theory might be false. She finds low income areas don’t in fact have a significant shortfall in supermarkets and grocery stores.

Dr Lee studied a national sample of 11,400 children over the period from kindergarten to fifth grade and compared changes in body weight against their access to various food establishments. This is an important study because it’s both longitudinal and national.

She finds the density of fast food convenience stores in predominantly minority and poor neighbourhoods is indeed higher than in more affluent and majority areas, but so is the density of supermarkets, large scale grocers, corner stores and full service restaurants.

Poor areas have nearly twice as many supermarkets and large-scale grocers per square mile as non-poor areas. She says:

These findings suggest that availability of supermarkets is not obviously limited in low-income or residentially minority areas. In fact, for some measurements, poor and minority neighbourhoods had greater availability (per land area) of supermarkets.

Overall, she also finds that “differential exposure to food environments bears little relationship to the key outcome of interest, childhood obesity development” (there’s a full ungated copy of her paper here).

She cautions against putting too much weight on local availability anyway. Noting that car ownership has increased among disadvantaged groups, she points to a study which found that:

low-income consumers are more likely to drive (using their own car or borrowing one) to a “supercenter” (a low cost, very large retail goods store), which is often further away from their local supermarket. They also tend to spend a greater share of their food budget at these low-cost outlets compared to other families

Dr Lee suggests targeting limited food availability is not a promising childhood obesity prevention strategy. It appears “that the more relevant factors for understanding unhealthy weight gain are the socio-demographic characteristics of children and families and proximate behaviours within the home context.”

So all in all there’s a lot of room for doubt about the food deserts theory. Certainly the Director of Yale University’s Rudd Center for Food Policy and Obesity, Kelly D. Brownell, seems unconvinced. She told the New York Times that “it is always easy to advocate for more grocery stores….But if you are looking for what you hope will change obesity, healthy food access is probably just wishful thinking.”