Is there a robust relationship between neighbourhood food environment and childhood obesity in the USA?

Summary

Objectives

To examine the robustness of the relationship between neighbourhood food environment and youth body mass index (BMI) percentile using alternative measures of food environment and model specifications.

Study design

Observational study using individual-level longitudinal survey data of children in fifth and eighth grades merged with food outlet data based on student residential census tracts.

Methods

The relationship between food environment and BMI was examined with two individual outcomes (BMI percentile in eighth grade and change in BMI percentile from fifth to eighth grade) and three alternative measures of food environment (per-capita counts of a particular outlet type, food environment indices, and indicators for specific combinations of outlet types).

Results

No consistent evidence was found across measures (counts of a particular type of food outlet per population, food environment indices, and indicators for the presence of specific combinations of types of food stores) and outcomes to support the hypothesis that improved access to large supermarkets results in lower youth BMI; or that greater exposure to fast food restaurants, convenience stores and small food stores increases BMI.

Conclusions

To the extent that there is an association between food environment and youth BMI, the existence of more types of food outlets in an area, including supermarkets, is associated with higher BMI.

Introduction

Healthy people 2010 aimed to reduce the proportion of obese children and adolescents in the USA to 5% by 2010.¹ However, the actual rates are three times higher. The latest figures show 16.9% of those aged 2–19 years in the obese category [at or above the 95th percentile of the body mass index (BMI)-for-age growth chart] and 31.8% in the overweight category (at or above the 85th percentile).² This has led to a flurry of governmental activities in the past year, including a $400 million healthy food initiative,³ the establishment of the White House Childhood Obesity Task Force,⁴ and a new strategic plan that makes obesity prevention a priority in the Department of Health and Human Services.⁵

A recurring theme in these governmental activities is the role of the food environment, in particular the notion of ‘food deserts’, where access to healthy and affordable food is limited, especially in disadvantaged neighbourhoods.⁶ ‘Food deserts’ are often identified by the absence of large supermarkets and the presence of a large number of fast food restaurants and/or small food outlets.7, 8 The hypothesis linking the food environment to obesity argues that greater access to fast food outlets, convenience stores and small grocery stores results in lower diet quality and overeating, whereas greater access to large supermarkets has a preventive effect due to its provision of various healthy products, mainly fruits and vegetables. This hypothesis is a central theme of the White House Childhood Obesity Task Force report⁷ and the $400 million healthy food financing initiative.³

Evidence on how the food environment relates to obesity is still developing and, at this point, is more tentative than presented in the media and policy arguments.9, 10, 11 The Task Force's recommendation on increasing the number of supermarkets in order to reduce childhood obesity only references a single study that associated nearby chain supermarkets with lower adolescent BMI and convenience stores with higher BMI.¹²

Reflecting the relatively early stage of this field, existing studies are heterogeneous with many unique measures and approaches that preclude meta-analyses.⁹ In any new field of investigation, early results do not always hold up, or require some modification that is only detectable through replication, a central principle of the scientific method. The absence of reproducibility of original findings stems from factors such as lack of statistical power in individual studies, selection bias and publication bias.13, 14

This study contributes to the literature on obesity epidemiology by examining the robustness of the relationship between neighbourhood food environment and youth BMI percentile in the USA using alternative environment measures and model specifications. Three commonly applied measures of the food environment are adopted, namely the per-capita counts of a particular type of food outlet in the neighbourhood,12, 15, 16 the food environment indices (i.e. ratio of certain types of food outlets vs other or all outlets),17, 18, 19 and indicators for the presence of specific combinations of types of food stores.²⁰ Alternative measures may reflect dissimilar assumptions on consumer behaviour and/or capture different characteristics of the food environment, so results can differ. However, if results are highly sensitive to the choice of measure, more caution should be placed on the interpretation of results and policy implications. Aside from replicating cross-sectional analyses with different food environment measures, changes in BMI over time were also studied. Almost all published studies have used cross-sectional outcomes (i.e. obesity or BMI at a point in time), and thus a longitudinal outcome measure should serve as an important complement. If certain environmental factors lead to obesity, results should hold when examining the change in BMI. As this study investigated youth, the outcome measures were BMI percentile (based on growth chart) and change in BMI percentile (from fifth to eighth grade).