Body mass index in elementary school children, metropolitan area food prices and food outlet density
Introduction
Childhood overweight has increased rapidly over the past two decades,1, 2 and concerns about childhood obesity have reached the highest levels of policy making in the USA, as well as in the UK and other industrialized countries. Legislation considered by the US Congress includes the ‘Improved Nutrition and Physical Activity Act’, or ‘IMPACT’, which earmarks funds for programmes that create ‘opportunities for students to make informed choices regarding healthy eating behaviours’ and increase after-school physical activity. Although it is generally believed that local food supply characteristics, such as relative prices of different types of foods and outlet density, affect what children eat, there has been no study of national scope on the relationship between the change in children's body mass index (BMI) and local food supply characteristics in the USA nor, as far as we know, in other countries.
We examined how the variation in real prices (i.e. US$ prices relative to cost of living) of different types of foods across localities and the variation in food outlet density in children's neighbourhoods relates to changes in BMI between kindergarten and third grade. The data analysed were from the Early Childhood Longitudinal Study—Kindergarten Class (ECLS-K), a nationally representative dataset for the USA that followed a cohort of kindergarten children from the 1998–1999 school year, merged with zip-code-level data on food outlets from the Census and metropolitan-level data on food prices from the American Chambers of Commerce Researchers Association (ACCRA).
Differential availability and affordability of foods have been suggested as important contributors to income and racial/ethnic disparities in obesity rates. The poor tend to live in areas with fewer large supermarkets and more convenience and small stores, which can result in higher prices for comparable items.3 US studies ranging back to the 1960s have consistently found that large suburban supermarkets have the lowest prices, and small grocery stores and convenience stores have the highest prices.3 For some neighbourhoods, the opening of a supermarket can be a major event. The announcement of plans to build a large supermarket in downtown Los Angeles, an area that has been without a large supermarket for decades, warranted an article in the Los Angeles Times4 several years before this supermarket is likely to open. Higher food prices (as well as limited product selection) at convenience stores and small stores could lead to consumption of foods with higher energy density, which are much cheaper for the same amount of calories than fresh fruit or vegetables.5, 6 At the same time, the energy density of a diet has also been implicated as a cause of obesity.5, 6, 7 Drewnowski and Specter concluded that the lower prices per calorie for energy-dense foods contribute to higher obesity rates among lower-income populations.5
Economic models suggest that lower food prices will increase consumption. Economists typically assume that this will automatically increase caloric intake. One study attributed the largest share of weight gain among adult Americans to an increase in the number of restaurants per capita over time, and a secondary role to the relative prices for all foods.8 However, insights from the nutrition field suggest that increased consumption of low-energy foods might actually lower caloric intake.5, 6, 7 Increased fruit consumption, for example, has been linked to a lower BMI.9 However, economic studies of obesity have not yet distinguished between types of food, such as price changes for fresh produce compared with meats, which is a central hypothesis in the nutrition field.5
Thus, it is not clear whether geographic variation in prices or supply can account for any noticeable differences in weight across populations. This paper will focus on weight gain among children. We tested whether prices for different types of food at the metropolitan level, and the type and density of food outlets (grocery stores, convenience stores, fast-food restaurants and full-service restaurants) in a child's neighbourhood (defined by home and school zip codes) are associated with differential rates of BMI change. We also tested whether effects differ by race/ethnicity, gender, poverty status or children at risk for becoming overweight.
Section snippets
Methods
The individual-level data came from the ECLS-K, which surveyed a nationally representative cohort of kindergarten children from over 1000 schools in the USA during the 1998–1999 school year. The sample was selected using a multistage cluster sampling design, where schools were selected first, followed by selection of children within schools. The ECLS-K is a panel dataset that collected baseline data in the autumn of kindergarten (Wave 1) followed by additional waves in the spring of
Results
Table 1 shows BMI changes for boys and girls. The average gain between spring of kindergarten and third grade for all children in the analysis sample was 2.15 BMI units and the median gain was 1.5 units. Even in kindergarten, the median (50th percentile) BMI in the ECLS-K sample was about 0.5 units heavier than the 50th percentile of the growth chart. The growth charts are based on historical representative data for the USA collected prior to the obesity epidemic and are widely used by
Discussion
This paper describes the relationship between BMI change among elementary school children and metropolitan area food prices for different types of foods and the density of food outlets in the neighbourhood. We found that relative food prices are associated with changes in the BMI and obesity rates, and the relationship is significant and robust for fruit and vegetable prices: higher fruit and vegetable prices predict greater BMI increase. At the extreme, price differences across metropolitan
Acknowledgements
This research was funded by the US Department of Agriculture, Economic Research Service, Grant No. 43-3AEM-3-80116.
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