Examining the relationship between the food environment and adult diabetes prevalence by county economic and racial composition: an ecological study

Type 2 diabetes mellitus (“diabetes”) has increased among adults ages 20 and older in the United States (U.S.) from 5.5 million in 1980 to 1.91.9 million in 2014 [1]. Currently, 11% of U.S. adults have been diagnosed with diabetes, with racial/ethnic minorities being disproportionally affected [1]. Left untreated or poorly managed, diabetes can cause kidney damage, blindness, and vascular insufficiencies leading to lower-limb amputations [2]. Complications from diabetes have led to increasingly high healthcare costs. The total estimated costs associated of diabetes in the United States in 2012 was $245 billion, including $176 billion in direct medical costs and $69 billion in reduced worker productivity [3].

Diabetes risk is influenced by genetics, age, obesity, physical inactivity and poor diets [4]. Consumption of foods with higher amounts of added sugars and fat combined with lower intake of foods with fiber such as fruits, vegetables, and whole grains can increase the risk of diabetes [2]. Dietary behaviors are affected by multiple levels of influence including individual (genetics and personal health beliefs); interpersonal (social networks and supports); community (environmental characteristics); and societal (public policies and systems) [5]. One potential community-level influence on diet which has been the subject of many recent studies is the food environment [6‐8]. The food environment is defined as the distribution of food sources within a community, including the number, type, location, and accessibility of retail food outlets [9]. The most common retail food outlets include convenience stores, full-service restaurants and fast-food restaurants.

While some studies have shown that greater access to healthier retail food outlets and lower access to less healthier retail food outlets is associated with more favorable dietary behaviors [6], others have shown conflicting results [10]. In Sallis and Glanz’s (2009) systematic review focusing on the community food environment, the presence of grocery stores or supermarkets in communities increased the probability of having a healthier diet [11]. However, a longitudinal study involving more than 5000 young adults found that having geographic access to more supermarkets was unrelated to fruit and vegetable consumption [12]. Other studies looking at more intermediate health outcomes, such as obesity, have also been mixed [13, 14]. Measuring diet and obesity can be complicated since studies often rely on time-consuming self-reported measures, such as dietary recalls [15, 16]. Additionally, some studies have critiqued the reliability of using BMI as a proxy for weight-related diseases. Since BMI is based on height and weight, it cannot account for differences between fat mass and lean body mass (i.e., muscle). Additionally, BMI threshold categories are based predominately on Caucasians living in Europe and the United States, which can be problematic since some races/ethnicities have higher percentages of body fat, so while they have a normal BMI, they might actually be overweight or obese [17, 18]. Therefore, examining the effect of diet on more proximal, objectively measured disease outcomes, such as diabetes, may provide greater insight into the role of the food environment and health. In general, self-reported diabetes diagnosis has been shown to be a reliable measure [19, 20].

A national study examining retail food outlets and the prevalence of diabetes found that fast food restaurants and convenience stores were positively associated with the prevalence of diabetes and farmer’s markets were negatively associated with them [21]. In another study, county-level data from South Carolina revealed that fast food restaurants were negatively associated with the prevalence of diabetes, yet convenience stores were positively associated with them [22]. In both studies, grocery stores and supercenters were not significantly associated with diabetes prevalence. Mixed findings from these studies could be due to differential effects of the food environment on high and low poverty counties, as well as counties with varying racial/ethnic populations, geographic location, or methodological models used to examine associations. [6‐8, 10, 13]. The food environment may play a stronger role in determining the diets and health of lower-income and racial/ethnic minority populations [23, 24]. These populations are more likely to have access to nutrient-deficient foods such as sugar sweetened beverages and salty snacks sold by convenience stores and fast food restaurants and less access to fresh fruits and vegetables sold by larger chain grocery stores or farmer’s markets [25‐27]. Yet, the relationship between the retail food environment and health outcomes by socioeconomic status and/or race have not been well documented. One study found that with each additional supermarket in a census tract, fruit and vegetable consumption increased by 32% among African American residents, but only 11% for white residents [6]. Another study focusing on racial disparities in obesity prevalence as explained by the retail food environment found that the food environment explained a greater proportion of obesity prevalence in counties with very high and very low proportions of African American residents [28].

Examining analyses by income and race can help researchers better understand how the relationship between food environment and diabetes prevalence varies for more vulnerable populations. Building a general understanding of this relationship requires research that uses large, national level data. Therefore, this study uses national data to better understand the impact that food environments have on diabetes prevalence. The purpose of this study was to examine the relationship between access to retail food outlets and the prevalence of adult diabetes by county economic and racial composition.

This study examined the relationship between access to retail food outlets and the prevalence of adult diabetes by county economic and racial composition. Surprisingly, full-service restaurants were associated with decreased diabetes prevalence regardless of income or race. This may be due in part to high poverty counties having less access to full-service restaurants, which might be more expensive to eat at, therefore reducing their ability to purchase meals there. Additionally, full-service restaurants in low poverty counties may be more likely to serve healthier, upscale meals. One study found that a higher density of full-service restaurants was associated with lower weight status. The authors hypothesized that customers sought seeking healthier foods might be more likely to eat at full-service restaurants [35].

Consistent with other research, convenience stores were associated with higher diabetes rates in high and low poverty counties across all racial compositions except for high poverty/low-minority areas. High minority counties had the least access to grocery stores and farmer’s markets (regardless of poverty level). Access to farmer’s markets was only associated with decreased diabetes rates in low poverty/low-minority counties – even though they only had the second highest density farmers’ markets. Only low poverty/low-minority counties benefited from having a higher density of grocery stores. This may be due to these counties having greater access to grocery stores than any other county type.

One possible explanation for the lack of statistical significance in this study among high poverty counties is that at lower income levels, availability of healthy food makes little difference since it is still unaffordable for many [36, 37]. Conversely, wealthier counties with disposable income may have more opportunities to spend money on food. While previous studies have found supermarkets and grocery stores to be negatively associated with county-level diabetes rates, they did not look at the interaction between race and poverty [24, 38]. One study that did examine race found there was a greater positive correlation between grocery stores and fruit and vegetable among African American residents than white residents [6]. While we did not find similar positive benefits for access to grocery stores among high-minority counties, this may signal that including the association of both race and poverty at the county-level may better reflect the effects of retail food outlets on health than just race or poverty status alone [35].

Consistent with previous research, this study found that high poverty and high-minority areas had lower access to grocery stores and farmer’s markets [24, 27]. Bower and colleagues (2014) found that high poverty communities had fewer supermarkets regardless of race/ethnicity, but at equal levels of poverty, predominantly African American had the fewest supermarkets [27]. Furthermore, lower socioeconomic communities had greater access to convenience stores and fast food restaurants than higher socioeconomic communities [10, 13]. For this study, high poverty/high-minority populations had the greatest access to convenience stores and fast food restaurants and the least access to grocery stores and farmer’s markets among any other county type. Conversely, counties with low poverty/low-minority populations had the greatest access to grocery stores and farmer’s markets than any other county type – nearly triple the density of farmer’s markets than counties with high poverty/high-minority populations.

This study investigated the relationship between access to retail food outlets and diabetes prevalence by county economic and racial composition. Overall, our results suggest that the food environment is central when examining diabetes prevalence, specifically in wealthier counties with a low proportion of minority residents. Some studies have begun to focus on innovative methods for addressing healthy food access in disadvantaged neighborhoods including building supermarkets [39], hosting mobile farmer’s markets [40], and assisting smaller food stores to carry healthier food items [41]. More studies are needed to examine these relationships, and if possible, future studies should focus on longitudinal study designs so researchers can track individuals over time. However, this study’s finding is still particularly important given recent efforts nationwide to increase food access in lower-income and minority communities. These efforts may not effectively improve disease rates if they cannot address other food access issues such as cost of healthy food, quality of healthy food (i.e., fruits and vegetables) and education to help low-income individuals learn how to prepare and cook with healthier foods [26]. Public health researchers and policymakers should recognize that increasing availability of healthier food outlets may not be enough to change diet and health outcomes.