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Access to public transport and childhood obesity: A systematic review

Summary The lack of access to public transport is generally considered to be a risk factor for childhood obesity by discouraging active transport and thus physical activity. To explore the association between access to public transport and childhood obesity, we have conducted a systematic literature search in the Cochrane Library, PubMed, and Web of Science for studies published before January 1, 2019. A total of 25 cross‐sectional and two longitudinal studies conducted in 10 countries were identified. Inconsistent findings were identified arising from a great variety of sample characteristics, definitions of exposure (ie, access to public transport), and outcome variables (eg, obesity), and analysis methods. While over half of the studies showed null associations between access to public transport and childhood obesity, we have observed more positive than negative associations among the rest of the studies. These observations suggest that an increased level of access to public transport may have a health‐promoting effect and hence prevent the development of childhood obesity. However, this conclusion needs to be further corroborated in future research on the basis of large‐sample health surveys, in situ observations, and comparative analyses among different study areas.


Summary
The lack of access to public transport is generally considered to be a risk factor for childhood obesity by discouraging active transport and thus physical activity. To explore the association between access to public transport and childhood obesity, we have conducted a systematic literature search in the Cochrane Library, PubMed, and Web of Science for studies published before January 1, 2019. A total of 25 crosssectional and two longitudinal studies conducted in 10 countries were identified. Inconsistent findings were identified arising from a great variety of sample characteristics, definitions of exposure (ie, access to public transport), and outcome variables (eg, obesity), and analysis methods. While over half of the studies showed null associations between access to public transport and childhood obesity, we have observed more positive than negative associations among the rest of the studies. These observations suggest that an increased level of access to public transport may have a health-promoting effect and hence prevent the development of childhood obesity.
However, this conclusion needs to be further corroborated in future research on the Fei  basis of large-sample health surveys, in situ observations, and comparative analyses among different study areas.

K E Y W O R D S
built environment, obesity, physical activity, public transport

| INTRODUCTION
Obesity is a leading cause of morbidity and premature mortality worldwide. It increases the risk for diabetes, cardiovascular diseases, hypertension, stroke, and a range of cancers and has become the fifth leading risk for global deaths. [1][2][3] Over 340 million children and adolescents aged 5 to 19 years had overweight or obesity in 2016, and the prevalence is increasing in both developed and developing countries. 1 Combating childhood obesity has become a significant public health challenge and has received substantial public attention. 4 The neighbourhood environment may shape children's lifestyles and interact with individual characteristics to affect their weight status. 5 Access to public transport in the neighbourhood is one such environmental factor and a form of active transport, also termed as public transit, mass transit, or urban transit. Public transport includes various transport services, such as metros, trams, trains, light rail, ferries, and buses. 6 The increased access to public transport provides additional opportunities for commuters to meet recommended physical activity (PA) levels while en route to transport stations. 7 To this end, we hypothesize that the lack of access to public transport would induce lower PA and higher levels of sedentary behaviours, eventually leading to weight gain among children and adolescents.
Several previous studies have confirmed this hypothesis. For example, residential proximity to subway stations was inversely associated with overweight and obesity among Massachusetts children. 8 It was also reported that the density of public transits had a positive effect on moderate to vigorous PA (MVPA) among pre-school and school children, although this association was not consistent across different statisticalmodels. 9,10 However, other studies reported conflicting health effects of public transport. For example, one study found that the density of bus stops was positively associated with the body mass index (BMI) z-score among white adolescents. 11 Another study reported that the density of public transport was negatively associated with the medium-intensity PA (MIPA) among boys. 12 To date, there has been no review study specifically focusing on the association between public transport access and childhood obesity. To fill this gap, we conducted this review to test our hypothesis that access to public transport was associated with higher PA and lower risk for childhood overweight and obesity. This review contributes to the literature by examining a full range of measurements of public transport access (eg, the number of public transport stops/lines, the density of public transport stops, and the proximity to the nearest public transport stop) around multiple sites (eg, home, school, and workplace) and their associations with both body-weight status and weight-related behaviours (eg, PA, sedentary behaviour, and active commuting).

| METHODS
A systematic review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. 13

| Study selection criteria
Studies that met all of the following criteria were included in the review: (a) study designs: longitudinal or cross-sectional studies (excluding letters, editorials, study/review protocols, or review articles); (b) study subjects: children and adolescents aged lees than 18 years; (c) exposures of interest: measures of access to public transport; (d) study outcomes: weight-related behaviours (eg, PA, sedentary behaviour, and diet) and/or weight outcomes (eg, BMI, overweight/ obesity, waist circumference, waist-to-hip ratio, and body fat); (e) time of publication: from the inception of an electronic bibliographic database to December 31, 2018; and (f) language: written in English.

| Data extraction and preparation
A standardized data extraction form was used to collect methodological and outcome variables from each selected study, including authors, year of publication, country, sampling strategy, sample size, age at baseline, follow-up years, number of repeated measures, sample characteristics, statistical model, attrition rate, measures of access to public transport, measures of weight-related behaviours, measures of body-weight status, and key findings on the association between public transport and weight-related behaviours and/or outcomes. L.J. and J.H. independently extracted data from each study included in the review, and discrepancies were resolved by F.X.

| Study quality assessment
We used the National Institutes of Health's Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies to assess the quality of each included study. This assessment tool rates each study based on 14 criteria. For each criterion, a score of "1" was assigned if "yes" was the response, whereas a score of "zero" was assigned otherwise (ie, an answer of "no," "not applicable," "not reported," or "cannot determine"). A study-specific global score ranging from zero to 14 was calculated by summing up scores across all criteria. The study quality assessment helped to measure the strength of scientific evidence but was not used to determine the inclusion of studies. Figure 1 shows the flow chart of study selection. We identified a total of 2316 studies through the keyword search. They underwent title and abstract screening, and 2229 of them were excluded. The full texts of the remaining 87 studies were reviewed against the study selection criteria, and 60 studies were further excluded. The remaining 27 studies were included in this review that examined the 89 associations (27 significant and 62 null findings) between access to public transport and children's weight-related behaviours and/or outcomes.

| Measures of access to public transport
Among 89 associations examined, both continuous (n = 60) and categorical variables (n = 29) were used to depict access to public transport (Table S1)  In addition, network buffer (n = 28) and sausage buffer (n = 17) were mostly used to measure the area range. For network buffers, 0.32 km was the most common buffer distance (n = 6), followed by

| Associations between public transport access and weight status
Nine studies examined a total of 25 associations between public transport access and obesity-related outcomes, among which BMI zscore (n = 8) was the most utilized measure of body weight status, followed by overweight (n = 6), obesity (n = 6), sum of skinfolds (n = 2), BMI (n = 1), BMI standard deviation score (n = 1), and overweight or obesity (n = 1) ( Table S2).
Seven of eight studies assessing BMI z-score reported a null association between public transport access and BMI z-score, while only one study reported a negative association in whites. Other continuous weight status such as sum of skin-folds (n = 2) reported a positive and a null association, while BMI (n = 1) and BMI standard deviation scores (n = 1) reported a null and a negative association, respectively.
The categorical measures such as overweight and obesity both reported five null associations and one positive association.

| Associations between public transport access and PA
Twenty-one studies examined 64 associations between public transport access and weight-related behaviours, primarily PA (n = 52) and travel modes (n = 12) (Table S2)

| DISCUSSION
This review summarizes 89 associations between public transport access and weight-related behaviours and/or outcomes from 27 original studies. Because of a wide variety of measures of variables, analysis methods, scales, and sampling population, we elected not to conduct a meta-analysis. If these discrepancies in the meta data were not accounted for, access of public transport was found to be healthpromoting in 18 studies and health-damaging in nine studies.
The increase in public transport accessibility tended to reduce childhood obesity. This observation has also been evidenced in several environmental health reviews examining PA and childhood obe- influence childhood and adolescent obesity in a review study. 38 Only one study in their review had found no significant effect of public transport access/availability on the level of obesity. Other reviews evaluated previous literature that studied the association between the built environment and children's PA. [39][40][41] The built environmental variables in these reviews extend beyond the public transport system, such as recreational facilities.
In this review, we had mixed findings concerning the association of public transport accessibility with childhood obesity.
This inconsistency may be induced by mode choice during every-day trips. Active travel, particularly walking and cycling, has confirmed health-promoting benefits in preventing and mitigating obesity and obesity-related commodities. [42][43][44] Public transport, referred to as the semi-active travel mode, 17 also has contingent health effects as it increases walking time en route to and from a public transit station. 42 However, easy access to public transport facilities does not indicate the use of public transport services, as commuters' mode choices vary.
Additionally, other nonspatial factors may play a role in dictating the mode choice, such as time allowed for travel, travel cost, availability of bike paths and sidewalks, parking spaces, and weather conditions. 45,46 Some reviews summarized a full range of factors in built environments affecting PA or sedentary behaviour but showed few consistent findings. 37 Furthermore, different results might be produced after controlling different variables even in the same study. Sjolie and Thuen (2009) showed that the distance of walked/cycled to school/bus stop (km) was associated with walked/cycled to weekly activities (km). 30 A study conducted by Nelson and Woods (2010) also reported that ease of walking to public transit was associated with active commuting to school in males and females after controlling for age and socioeconomic status, but this association only remained in females after adjustment for density (size of settlement), socio-demographics, density, or distance travelled to school. 28 An inverse association reported by Zhu and Lee (2013) was that the presence of bus stops en route reduced the likelihood of walking to or from school in 19 elementary school students, 36 but this scenario differed in elementary school students with low SES and of whom a high percentage were Hispanic. 35 This interesting phenomenon may suggest that different sample characteristics might account for the diverse travel mode and lead to the inconsistent association between public transport accessibility and obesity. 11,45,46 We screened the studies that met the inclusion/exclusion criteria after systematically searching a variety of databases (eg, Cochrane Library, PubMed, and Web of Science). However, limitations remain as there exists a limited scope of studies having reported the association between public transport accessibility and childhood obesity whereas public transport accessibility has been considered a control variable rather than the independent variable in some of the multivariate analyses. Additionally, most of these studies adopt a crosssectional design, which does not allow for an exploration of the causal relation between the two subjects. Finally, as demonstrated in the results of the quality assessment (Table 4), most included studies are of a moderate quality.
There are also some limitations of our review. First, we only summarize the major findings rather than adopting the meta-analysis, but pants differed in household and regional characteristics, such as family income, educational attainment, race, and living conditions. This variety suggests directions for future exploration that (a) designs a universally acceptable standard for accessibility measurements, (b) reaches a consensus on obesity-related physical environmental features, and (c) consolidates a large number of studies and extracts consistent sample characteristics. Also, public transport accessibility, especially its utilization which is more associated with obesity, can be affected by climatic and weather factors. Therefore, the effect on obesity of public transport accessibility, or the underlying climatic and weather factors (through public transport utilization), may vary spatially and temporally. 49 This has not been considered in most, if not all, of the previous studies. Fourth, we failed to conduct subgroup analyses (ie, by gender, race, and age) because of the small number of studies included. Given that the health effects of public transport accessibility on subgroups may differ, 11,45 it is necessary to target different subgroups in future reviews once a sufficient number of studies have been accumulated. In addition, we grouped similar indicators together to describe results, but this may not be completely appropriate because of the differences between studies involving those indicators.
For example, MVPA differs in measure (in minutes 15,16,21,22 or hours per day/week 12 ), definition (using age-specific cut-points, 15 non-agespecific cut-offs, 21,22 or self-perception 16,23 ) and monitoring time (3 days, 22 7 days, 15 or other). Last, the majority of the included studies in this study are cross-sectional designs, which mainly show some correlation rather than revealing causal relationships between the public transport accessibility and obesity-related behaviours. Thus, additional longitudinal studies should be conducted in future efforts.

| CONCLUSIONS
In this study, we find more positive than negative findings when excluding the null findings regarding the association between access to public transport and childhood obesity. However, there are a great variety of measures, analyses, and samples among the included studies. Future research should reach a consensus on the definition of variables, analysis methods, and sample characteristics in order to better justify the health effects of public transport accessibility and convince multiple stakeholders to work together for improving the access to public transport.