Trends in urban/rural inequalities in cardiovascular risk bio-markers among Chinese adolescents in two decades of urbanisation: 1991–2011

Urban living affects children’s nutrition and growth, which determine their survival, cognitive development and lifelong health. It has been well documented that urban children tend to fare better than their rural counterparts [1‐3]. Urban environments have been linked to a range of human health issues. Three approaches have been proposed regarding research on urban health [4]. Urban health penalty and urban sprawl focus on the health problems of cities [5]. The urban penalty approach posits that cities concentrate poor people and expose them to unhealthy physical and social environments. Urban sprawl focuses on the adverse health and environmental effects of urban growth into outlying area. The urban penalty and urban sprawl approaches concentrate on negative aspects of health, and fail to consider the strengths within metropolitan areas. The third approach, ‘urban health advantage’, considers the special resources and protective effect of cities and only focuses on the positive aspects of urban living. All three perspectives are relatively static, describing health within a particular stage of urban development rather than exploring the changing processes between and within cities and their surrounding areas [5].

Urbanisation is a complex phenomenon, associated with a large array of changes in environmental and lifestyle factors that may affect individual health and wellbeing. As the pace of urbanisation accelerates, new challenges arise to characterise these environments, and to understand their positive and negative implications for health [6]. However, attributing diseases to specific environmental influences has proved elusive [7]. Factors beyond including the social and physical environment (not just individual risk factors) are the primary determinants of health and wellbeing of people [4]. The influence of urbanisation on health can be mixed. On the one hand, there are the benefits of ready access to healthcare, sanitation, and secure nutrition, whilst on the other there are the evils of overcrowding, pollution, social deprivation, crime, and stress-related illness.

China is a good study case to examine the influence of urbanisation on population health given the size of its population, the special conditions of market socialism and its institutional capacity to manage the rural-urban transformation. China will reach the same levels of urbanisation much quicker and the number of people involved in this shift will be much larger compared to other countries [8]. Unlike Western and former socialist countries, China’s rapid urbanisation is not merely driven by spontaneous rural-urban migration, but also partly planned by the government that has maintained an urban bias in favour of urban residents to preserve regime stability [9, 10]. The process of China’s rapid urbanisation may cause the unequal distributions of socio-economic resources and life chances among its population, thereby contributing to rural-urban health inequalities [11].

Alongside its rapid socio-economic change, China has also experienced an epidemiological transition shifting from infectious to chronic diseases in a much shorter time frame than many other countries [12]. Lessons learned from epidemiological studies mostly carried out in Europe and the US may not be directly transferable to China [7]. Children and adolescents in China are becoming increasingly overweight and obese over the past decades [13‐16], particularly among higher socioeconomic children [17]. Liang et al. [14] analysed the CHNS data and have found that blood pressure levels and prevalence of hypertension increased dramatically among Chinese children and adolescents aged 6–17 years from 1991 to 2004. Obesity itself has been shown to be strongly associated with the onset of hypertension [18, 19]. Children with higher blood pressure are more likely to become hypertensive adults [20‐22] and further increase the risk of cardiovascular diseases [23]. It has been suggested that targeting at childhood or early years of life has the potential to reduce health inequalities within one generation [24]. This study focused on adolescents because they are more likely to be affected by neighbourhoods directly than younger children for whom neighbourhood effects may be mediated through family processes [25‐27], since most of their time is spent at home and their interactions with larger social and physical contexts are determined by and largely experienced through their parents/caregivers.

Most studies that examined the health consequences of urbanisation employ a dichotomous urban-rural classification of individuals living in either urban or rural environments. This approach ignores the heterogeneity of experiences within and across urban and rural situations, and fails to consider the specific aspects of local environments that are closely related to health outcomes [6]. This is particularly relevant given substantial regional variations in economic development and public resources across China. Another pitfall of employing urban-rural dichotomy is its inability to detect changes in urbanisation over time [28]. This makes it impossible to examine temporal trends and patterns of urbanisation on health outcomes among adolescents in China undergoing rapid urbanisation in the last few decades.

Despite well documented rural-urban inequalities in child health in China, there are few attempts to examine how these relationships have changed during the process of China’s urbanisation. Previous research tends to rely on a cross-sectional study design and failed to examine the dynamics in child health in a rapidly urbanizing society. This study will ask three questions:

Tables 1 summarizes characteristics of adolescents aged 12–18 by gender and survey years, the CHNS 1991–2011. The age distributions of study participants did not show significant differences across survey years. Family socioeconomic circumstances measured by maternal education and per capita household income appear to improve over 20 years. With regards to anthropometric measures (height, weight, BMI and WC), adolescents became taller and heavier over time. For systolic and diastolic blood pressure measurements, trends do not show a consistent pattern. Boys tend to have higher blood pressure than girls over last two decades.

Tables 2 and 3 present trends in anthropometric measures including BMI and waist circumference among adolescents aged 12–18 years in China. Positive trends were observed in BMI for boys and girls (Model 1, Table 2). Positive trends persisted after adjusting for urbanisation index (Model 2, Table 2) for both genders. There was no significant interaction effect between urbanisation index and survey years on BMI. With regards to waist circumference, a positive trend was observed for both genders before and after controlling for urbanisation index (Model 1 & 2, Table 3). A significant interaction (between urbanisation index and survey years) was observed for girls: the positive trend became greater when urbanisation index increased (Model 3, Table 3).

Figure 1 plots the time trends of waist circumference for the most urbanised and the least urbanised areas for boys and girls. We found significant and positive interaction effects between survey years and urbanisation index among girls, but not for boys. The difference between the most urbanized and the least urbanized areas remained unchanged for boys over two decades (interaction not significant). This may indicate the absence of urban-rural inequality in waist circumference among Chinese adolescent boys during China’s rapid urbanisation. The pattern appears more complex for girls. The most urbanized areas tend to have lower waist circumference than the least urbanized by 2004 and the gap continues to narrow. From 2004 onwards, a reversed pattern was observed among girls whereas girls from the most urbanized areas tend to be affected more in the association between waist circumference and survey years than those from the least urbanized areas, and the disparity further widened.

Tables 4 and 5 present trends in blood pressure among adolescents aged between 12 and 18 years in China. Positive trends were observed for both boys and girls (Model 1) and persisted after taking into account of urbanisation index (Model 2). The positive trends appear to decline when areas became more urbanized (Model 3). This was only observed for boys but not for girls.

Figures 2 and 3 illustrate the time trends of systolic and diastolic blood pressure for both the most urbanized and the least urbanized areas for boys and girls separately. An upward trend was observed for both genders regardless of levels of urbanisation. For boys, those from the most urbanized areas appear to have higher systolic blood pressure than those from the least urbanized. However the gap continued to converge till 2004; and after that the gap widened with the least urbanized areas overriding or surpassing the most urbanized. For girls, those from the least urbanized areas appear to have higher systolic blood pressure than those from the most urbanized areas. This pattern persisted and remained constant over two decades (interaction not significant). A similar pattern for diastolic blood pressure was observed for boys (Fig. 3).

Our study provides an overall picture of time trends of biomarkers among Chinese adolescents and how it changes with urbanisation over last two decades. It has drawn on a nationwide longitudinal dataset–the CHNS to examine trends in related biomarkers among adolescents aged 12–18 years in China by considering an important wider contextual factor–urbanisation. Chinese adolescents are experiencing an upward trend of cardiovascular risks in the last two decades. Its rapid urbanisation appears to further increase the risk, especially for boys from less urbanised areas and girls from more urbanised areas, which may contribute to the development of cardiovascular disease in adulthood. There are at least two strengths of this study compared to previous evidence. First, the use of multilevel modeling techniques in this study allows for disentangling effects due to individual, family and neighborhood characteristics, which is methodologically more robust. Therefore it can provide an opportunity for deeper insights into how features of urban living may impact health [4]. Another advantage of this study is, unlike previous studies mostly drawing on the crude classification of rural/urban dichotomy that is prone to misclassification error, our urbanisation measure has encompassed dimensions beyond population size and density. It has captured some important dimensions of social, economic and physical characteristics of urban living over time and space [36].

Chinese adolescents are experiencing an upward trend of cardiovascular risks in last two decades. Its rapid urbanisation appears to further increase urban/rural inequalities in cardiovascular risks, especially for boys from less urbanised areas and girls from more urbanised areas, which may contribute to the development of cardiovascular disease in adulthood. Findings may provide a sound evidence basis to inform decision-making in public health and social welfare in relation to tackling health and welfare inequalities among children in China and potentially other developing countries that are undergoing similar socio-economic change and associated nutritional and epidemiological transitions. Unlike the detrimental influence of urbanisation and migration in other developing countries which often resulted in shantytowns and slums causing public health and other subsequent social problems, China’s ‘planned’ urbanisation may be beneficial to its population by providing opportunities for better health infrastructure, access to healthcare services, and essential services. There is a possibility for policy intervention to reduce inequality during the process of China’s planned urbanisation.