On average, respondents who lived in areas with the lowest densities of supermarkets around their home (less than 0.5 supermarkets per square mile) rated the availability of healthy foods 17% lower than those in areas with the highest densities of supermarkets. The relationship between supermarket density and perceived availability of healthy foods was stronger in North Carolina and weaker in Maryland and New York. Among persons living in areas with no supermarkets, having higher densities and a larger variety of smaller stores within close proximity of the home was associated with improved perceived availability of healthy foods only in North Carolina.
Our results are consistent with previous studies that have shown that areas served by supermarkets have better availability of healthier food items.4,16,25,26,28
Sloane et al. inventoried selected markets in the Los Angeles metropolitan area and found that low-fat dairy, whole grain products, and lean meats were significantly less available and of lower quality and less variety in areas of high African-American concentration27
possibly because of the lack of supermarkets in these areas.12,13, 15,16,19,33,34
A survey of stores in New York also reported that only one in three smaller neighborhood stores sell reduced fat milk compared to nine in ten supermarkets, and less than a third carry fresh produce compared to 91% of the supermarkets.19
Similarly, mean quality of fresh produce was significantly lower in the predominately African-American, low-SEP community where supermarkets are less likely to be located than in the racially heterogeneous, middle-income community.16,28
In a recent study by Jetter et al. in neighborhoods served by smaller grocery stores, access to whole-grain products, low-fat cheeses, and lean ground meat was limited with 64% of all items unavailable in small grocery stores.26
Adjustment for population density did not substantially alter our results, suggesting that the number of people a store services may not be as relevant as simply having a store in spatial proximity. The association between supermarket density and perceived healthy food availability became weaker as the size of the area for which supermarket densities were calculated increased. This is consistent with the fact that survey respondents were asked to report on healthy food availability within a mile of their home, and hence, stronger associations are expected for the 1-mile window than for the larger windows.
With the exception of North Carolina, we found no clear evidence that residential proximity to smaller grocery stores or variety in smaller stores were correlated with better perceived availability of healthy foods in the absence of supermarkets. In fact, in Maryland, lower densities of smaller stores and less variety in smaller stores were actually associated with better perceived availability. This may have to do with the nature and quality of these small stores in the specific areas we studied, many of which may offer few, if any, affordable and high-quality healthy food items.25–28
However, the fact that healthy food availability was positively associated with the presence of smaller stores in one site suggests that efforts to improve the healthy foods available in small stores may be one avenue toward improving the local food environment in poor and minority neighborhoods, often characterized by the presence of many small stores but no large supermarkets. The presence of many small stores may also have important advantages in terms of walkability, social interactions, safety, and community development generally, which have other potential health benefits.
There was some evidence of regional variation in the relationship between store densities and perceived food availability across the three sites studied. For example, supermarket density was most strongly associated with perceived availability of healthy food in North Carolina. In addition the density and variety of smaller stores were significantly associated with perceived availability of healthy foods only in North Carolina. The spatial proximity of food stores may be more relevant in less densely populated areas like North Carolina than in areas with higher population densities that will also have generally higher densities of stores overall. In addition, different store types may contribute differently to healthy food availability in different regions and cultural contexts. For example, smaller grocery stores and ethnic grocers could be a primary source of healthy foods in some areas, whereas large supermarkets may be the primary source in other areas. Our results point to some of these differences, but additional work contrasting a broader range of areas is needed.
Although perceived availability and density of supermarkets were positively associated, associations were generally not very strong. The observed increase of 17% in the perceived availability score when comparing the bottom to the top tertile of supermarket densities is equivalent to a difference of 1.25 points on the absolute scale (compared to a standard deviation of 3.1 points and an interquartile range of 3.0). Measurement error in both GIS and perceived measures may have resulted in weak observed associations (bias toward the null), even if supermarkets are an important source of healthy foods. Important measurement error in GIS-derived densities may be introduced by error in the identification and classification of stores.35–38
Although we defined supermarkets using methods analogous to those used in prior work,12,15,30
measurement error is clearly a possibility. Our measure of the density of smaller stores pools together a heterogeneous group of stores that may differ substantially in the healthy foods they offer. Perception-based measures may also be subject to measurement error. Respondents were asked about resources within 1 mile around their home, which may introduce misclassification if respondents misestimate the geographic bounds of 1 mile. Also, reports of availability are necessarily based on perceptions that are influenced by various subjective experiences and personal behaviors, including personal preferences, awareness of food shopping in the neighborhood, and knowledge/ability to recognize low-fat and “high-quality” fresh produce. Perceptions of availability may also be influenced by different cultural, economic, and regional contexts. Another potential limitation of these analyses was the survey response rate of 46.5%. Although information is not available on non-respondents, the sample was approximately representative of the areas from which it was drawn.39
Another potential reason for the weak associations is that both measures may be imperfect proxies for the construct of healthy food availability. Supermarket densities (even when measured with little error) may simply not capture important variability across areas in the availability of healthy foods. These stores may not be the only source of healthy foods, and supermarkets in different areas (e.g., in low-income vs. high-income areas) may be very different in terms of the healthy foods offered.17,19
In addition, even in the absence of measurement error related to resident’s knowledge of the area, the small number of items and the types of items included in the survey scale used to characterize perceptions have important limitations when characterizing healthy food availability. Additional work to develop more appropriate scales is needed.
Despite the many challenges in measuring both the presence of stores and the perceived availability of healthy foods, our results indicate that characterizations of the local food environment based on perceived measures are associated with GIS-based characterizations of the local food environment. These results also suggest that both measures may provide complementary information. Additional work is needed to determine to what extent the actual healthy food availability observed in stores (for example, by systematic raters or observers40
) correlates with the availability reported by residents and how availability varies by the sociodemographic characteristics of residents. Exploring ways to combine survey measures with locational data may help to create more reliable and valid characterizations of the food environments.41
Future research on how the local food environment is related to diet may benefit from complementary approaches to measurement of the environment.