Subway development and obesity: Evidence from China

Highlights

• The mean BMI of Chinese migrant workers increased by about 1.0 kg/m² from 2008 to 2018.

• Living near a subway station slows the increase in BMI for Chinese migrant workers.

• Subway and neighborhood planning efforts should consider community health benefits of living near subway stations.

Abstract

Introduction

A correlation between reduced obesity and living near a subway station has been found in developed countries. However, the causal nature of this relationship still cannot be established because most existing literature relies on cross-sectional data. The current subway boom in developing countries gives us a chance to address this concern by obtaining a temporal ordering of the shift in exposure to subway stations and the change in rates of being overweight or obese. This study applies an analysis strategy with repeated cross-sectional data about migrants in Chengdu, China, to explore the change in body mass index (BMI) related to living near subway stations among lower- and middle-lower income population.

Method

The body mass index (BMI) data come from a repeated cross-sectional study on health-related issues of migrant workers, a typical low-income population, in Chengdu, China. The first survey (1,015 individuals) was conducted in 2008 when no subway system existed in that city, and the second survey was conducted in 2018 (1,797 individuals) after a subway system had been built and matured. The analysis strategy using the propensity score in the difference-in-difference model (DID) was used in this study.

Results

The respondents’ mean BMI increased by about 1.0 kg/m² over the 10 years. However, compared with people who live outside the 400-m subway station buffer zones, those who live inside the buffer zones have a significantly reduced BMI increase of 0.545 kg/m². The results for those living inside the 800-m subway station buffer zones are similar to those for the 400-m setting, with a significantly reduced BMI increase of 0.434 kg/m².

Conclusions

This study provides evidence about the effects of living near subway stations on reducing obesity, specifically for lower- and middle-lower income population. The results will help policymakers decide whether to establish subway stations to cover low-income neighborhoods, from a public health perspective.

Introduction

Being overweight or obese is associated with diabetes, cardiovascular diseases, cancers, and musculoskeletal disorders (Jiang et al., 2012; Lauby-Secretan et al., 2016; The Emerging Risk Factors Collaboration, 2011). Since 1980, the epidemic of obesity has increased globally—a total of 107.7 million children and 603.7 million adults globally were obese (body mass index, BMI, ≥30) in 2015 (GBD 2015 Obesity Collaborators, 2017; Roberto et al., 2015; Roussy et al., 2011). To address this situation, efforts are being made to reduce the global overweight and obesity trend. Researchers have suggested that high food and calories combined with low physical activity levels is the primary driver of obesity (Swinburn et al., 2011). Additionally, studies have also recognized that changes in the environment, such as the much higher availability of “fast food” and the increased use of automobiles, could be one reason for changes in dietary and physical activity patterns (Sturm, 2008).

As such, the subway—a vital component of many urban built environments and a feature of modern cities—has been linked to people being overweight or obese in this century. In 2007, a cross-sectional study investigating 13,102 adults in New York City suggested that the density of subway stations is significantly inversely associated with BMI (Rundle et al., 2007). This significant association has also been found in other subgroups. For example, studies from Massachusetts and New York City both indicated that children's adiposity is inversely associated with the density of subway stations (Lovasi et al., 2011; Oreskovic et al., 2009). Advocates argued that living close to subway stations might promote physical activity by changing commute styles and encouraging leisure activity, thus affecting energy balance. Studies have provided evidence indicating that living near subway stations, such as when the distance from the household to the closest subway station is less than 800 m, can reduce the use of automobiles, and the commute choice shift from car to public transit can result in more physical activity (Lachapelle et al., 2011; Morency et al., 2011; Zhang et al., 2017). The existing literature also has found the positive influence of subway stations on diverse land use and broadening the market for local retail businesses (Calvo et al., 2013; Zheng et al., 2016), which both can contribute to more leisure activity (Bourdeaudhuij et al., 2005; Lin, 2018).

However, other researchers also remind us that we should not ignore the association between living near subway stations and an energy-dense diet. The rapid change in the fast-food industry in urban areas (Wang et al., 2016) may negate the relationship between living near subway stations and the trend in being overweight or obese. Some research suggests that living near a high density of retail stores and restaurants that accompany living near subway stations might encourage leisure actives, but others point out that proximity to these consumer amenities may also cause people to eat more high-energy-dense food (Fleischhacker et al., 2011; Ms et al., 2016; Reitzel et al., 2014). Moreover, studies have indicated that there are many ads in subway stations around the world for less-healthful food (e.g., candy, chips, sugary cereals, frozen pizzas, energy drinks, hard alcohol, and beer) but no ads promoting more-healthful food or beverages (Elisabeth et al., 2011; Lewis, 2012; Lucan et al., 2017). Exposure to this advertising may influence people's behavior and thus lead to weight increase; studies have indicated that for every 10% increase in food ads, there is a 5% increase in the population being overweight or obese (Lesser et al., 2013). With all these study findings, it might be easy to question whether living near subway stations can mitigate trends in being overweight or obese.

To address the controversy about the relationship between living near subway stations and people being overweight or obese, longitudinal and experimental studies should be conducted. These study designs can avoid incorrect assumptions when estimating the causal effects because they can include temporal ordering of the shift in exposure to subway stations and the change in rates of being overweight or obese. However, most existing literature relies only on cross-sectional data to explore this relationship (Lovasi et al., 2012). In addition to not capturing the temporal ordering of built environment exposure and health outcomes, the existing literature focusing on this relationship is subject to the influence of neighborhood preferences and the selection process (Lovasi et al., 2012). For instance, if physically active people self-select into more walkable communities with easy access to subway stations, the observed relationship will be confounded by the individuals’ inclination and may not correctly reflect the effects of living near subway stations on being overweight or obese.

The good news is that the subway boom in developing countries over the past decade may offer an opportunity to address the causal link between living near subway stations and mitigating the trend of an overweight or obese population. In China, the expansion of subway construction was a slow-moving process until 2000s when China became the host of the 2008 Olympic Games. In the 10 years since the Games, more than 20 systems and 100 lines have been built. Each year from 2010 through 2015, China created roughly 232 miles of subway lines The country now has subways in 31 cities, with a total of 133 lines covering some 2,700 miles (Lu et al., 2016). The newly built urban railway systems bring about a natural experiment by dividing residents into two groups: those living near subway stations (experimental group) and those living far away from subway stations (control group). This natural experiment gives researchers a chance to estimate the effects of living near subway stations on the likelihood of an individual being overweight or obese by using the difference-in-difference model (DID), which is often used by public health researchers to explore causal relationships (Wing et al., 2018). Moreover, a new strategy using propensity scores in DID has been developed in recent years (Stuart et al., 2014). This strategy can allow researchers to implement the DID model with not only longitudinal data but also properly repeated cross-sectional data to address the causal effects of interest.

Accordingly, this study used propensity score weighting in a DID model with data from China to explore the causal inference about living near subway stations and the trend in individuals being overweight or obese, specifically lower- and middle-lower income population. We focused only on lower- and middle-lower income population because some studies have pointed out that income level may influence the relationship between living near metro/subway stations and commute choice (Li and Zhao, 2017; Wang and Zhou, 2017). Additionally, the association between weight status and the number of high density of fast-food restaurants near subway stations may also vary among high- and low-income population (Oreskovic et al., 2009). In the next section, we introduce our data and methods. The results are subsequently reported. Finally, we summarize and discuss the results.