The Effect of Rural-Urban Migration on BMI in Thailand

As countries undergo the “nutrition transition”, reduced physical activity and increased consumption of saturated fats and sugars are growing concerns. Recent research suggests that migration to an urban area can play a role in these behaviors: rural to urban migration is associated with increased inactivity, consumption of fats and sugars, and risk of obesity and diabetes. Limitations exist, however, as most studies examining urbanization, migration and obesity have been limited to associations via cross-sectional data. We address this limitation by using data from Thailand to examine (1) potential selection of individuals with relatively higher or lower BMI into migration streams, and (2) the effect of rural to urban migration on BMI. The longitudinal nature of the data enables us to compare rural-urban migrants with non-migrant rural and urban residents; and to use fixed effects regression, thereby controlling for important unobserved time-invariant characteristics. We find a significant relationship between BMI and migration, which is explained by the selection of those with lower BMI into migration streams. We do not find any effect of migration on BMI for Thai young adults. 1 Tulane School of Public Health and Tropical Medicine, Department of Global Health Systems and Development. Introduction The rapid urbanization of many developing countries has important implications for the effect of migration on individual health. Research suggests that rural-to-urban migration is associated with changes in health-related behaviors, as migrants eventually adopt behaviors that are common among residents of their destination. As developing countries go through the “nutrition transition”, risk behaviors of particular concern are reduced physical activity and increased consumption of saturated fats and sugars (Popkin & Gordon-Larsen 2004; Mayosi et al. 2009). Such behaviors are typically more common in urban areas: recent research has found that ruralto-urban migration is associated with increased inactivity and consumption of fats and sugars (Torun et al. 2002), which likely explains increased risk among migrants of obesity and diabetes (Ebrahim et al. 2010; Sobngwi et al. 2004), high blood pressure (Sobngwi et al. 2004), and general cardiovascular risk (Sobngwi et al. 2004; Torun et al. 2002). Two important caveats on the relationship between migration and obesity are evident in the literature. First, increased health risks are not universal for all migrants and urban areas in developing countries. In some cases urban residents have not shown greater risk of diabetes, obesity or elevated blood pressure (e.g. Ntandou et al. 2009; Unwin et al. 2006). Thus, urban areas do not always imply greater risk. Secondly, primarily due to the reliance on cross-sectional data, much of the research on the relationship between migration or urban residence and obesity has been limited to associations, rather than identifying effects (this limitation is described in Mbanya et al. 2010). Thus, in many cases it has not been possible to distinguish between selection and effect: does rural-to-urban migration cause changes in diet that lead to increased BMI, or are individuals with greater BMI more likely to migrate? We use longitudinal panel data that includes measures of height and weight in 2005 and 2007 for Thai young adults. This longitudinal design is an important advantage over the cross-sectional approaches of most previous studies, as it enables us to (1) measure change in BMI over time, and (2) examine pathways that explain differences in BMI between migrants and non-migrants, and (3) use analytic approaches that control for unobserved time-invariant characteristics that may otherwise produce biased estimates of relevant relationships (fixed effects regression). A key feature of these data is a sub-sample of internal migrants who moved to urban areas between 2005 and 2007. To examine the relationship between BMI and rural-urban migration in Thailand, we proceed in the following steps. First, we determine whether there are differences in BMI between migrants and non-migrants. Next, we evaluate two mechanisms that could produce differences in BMI between migrants and non-migrants. We begin with an examination of migrant selection. Using longitudinal data, we examine whether individuals with higher BMI are more likely to migrate from rural to urban Thailand. Then we move to the effect of migration on BMI, in which we use fixed effects regression to assess the impact of migration on BMI.