Studies show that there is no apparent relationship between a store’s mix of products and its customer’s body-mass index. And limited experimental evidence suggests that improving the supply of fresh foods seems to have limited impacts on food-consumption patterns. Preliminary results of a study of consumers in a Philadelphia neighborhood that got better supermarket access didn’t find that they ate more fruits or vegetables, or lowered their body-mass index—including those who patronized the new store.

Physical proximity alone is not likely to be a strong explanation of variations in diet. Judged by proximity to grocery stores, nearly all of rural America is a food desert. Nathan Yau at FlowingData uses Google Maps data to construct a compelling map of how far it is to the nearest grocery store across the entire nation. The bleakest food deserts are the actual deserts of the American West, in Nevada and Wyoming.

City dwellers, particularly those in the biggest, most dense cities tend to live closest to supermarkets and have the best food access. WalkScore used its data and models to develop some clear images of who does (and doesn’t) have a good grocery store nearby. They estimate that 72 percent of New York City residents live within a five-minute walk of a grocery store. At the other end of the spectrum, only about 5 percent of residents of Indianapolis and Oklahoma City have that convenience. When it comes to walking, this data shows the real food deserts are in the suburbs.

There are other ways of measuring food access and mapping food deserts. The U.S. Department of Agriculture and PolicyMap have both worked to generate their own maps of the nation’s food deserts. They account for both neighborhood income levels and how far it is to the nearest grocery store.

While it’s clear that income plays a big role in food access, it’s far from clear how to combine income and proximity to define food deserts. The USDA uses an overlay to identify which low-income neighborhoods have limited food access. PolicyMap has a complicated process that compares how far low-income residents have to travel to stores compared to higher-income residents living in similarly dense neighborhoods.

But combining neighborhood income and physical proximity muddles the definition of food access. First, and most important, it acknowledges that income, not physical distance, is the big factor in nutrition. Both of these methods imply that having wealthy neighbors or living in the countryside means that physical access to food is not a barrier. Second, it is an individual’s household income, not her neighbor’s income, that determines whether she can buy food. Third, these methods implicitly treat low-income families differently depending on where they live. For example, PolicyMap excludes middle-income and higher-income neighborhoods from its definition of “limited supermarket access” areas—and therefore doesn’t count lower-income families living in these areas as having poor food access.