Height inequalities and their change trends in China during 1985–2010: results from 6 cross-sectional surveys on children and adolescents aged 7–18 years

Height is an important indicator for growth and development because it not only reflects the nutrition and health condition in people’s childhood [1],but also influences their later life qualities and well-being. It is reported that taller people tend to have more superior outcomes by a variety of measures, such as productivity, income, longevity, life satisfaction and self-report health [2‐6]. Height has been used as a proxy of the biological standard of living [7] and it can mirror the fluctuations of economic and epidemiological conditions of a country. It is a goal of every country to create a better environment for everyone to obtain his or her maximum genetic potentials in growth.

In the last few decades, China has great improvement in economy and public health [8] accompanied by a positive secular trend in the physical growth of children and adolescents [9‐12]. However, these achievements were not distributed equally. In other words, increased income inequalities have emerged since the market reforms in 1978 [13] along with increased health inequalities [14‐16]. Height of children and adolescents among different places also exerted strong heterogeneity in this period. A study on world height inequalities indicated that China existed greater inequalities in height compared with some other developing countries [17], suggesting that enough attention should be paid to this issue. Height inequalities highlight inequalities in health, nutrition and social well-being within populations and can provide important biological evidence for policy makers to develop some strategies in improving public welfare and equalities.

There was limited literature about height inequalities in Chinese children and adolescents. Chen TJ et al. [10] mentioned that height growth rate of children and adolescents aged 7–18 years varied among eastern, central and western rural regions during 1985–2010. In addition, Li H et al. [18] and Zong XN et al. [12] found that there was obvious height difference between urban and rural children aged 0–7 years. However, the fact that how height inequalities changed over time in the past few years remains unclear and the extent of difference in urban and rural areas as well as in regions with different socioeconomic levels needs to be further studied. We will address these issues in this study based on the national height data of Chinese students aged 7–18 years.

The purpose of this article is to examine height inequalities and their change trends from 1985 to 2010 with the measure of absolute difference. We will discuss in detail: (1) Whether there was urban-rural height difference and how it changed over the past 25 years. Whether height inequality in urban was greater than that in rural? (2) How height difference between eastern and western regions changed and how height difference between the richest (Shanghai) and poorest (Guizhou) provinces changed over time?

Those above results showed height inequalities in China from three dimensions and depicted how they changed over time by height data of children and adolescents among different age and sex groups.

As we all know, approximately 80% of individuals’ stature change is genetically or biologically regulated [29] and the remaining 20% is determined by environmental factors such as nutrition, diseases, living conditions and psychological stress, while genetic deviations cancel each other out from the average at the population level [30‐33]. Therefore, it is plausible to speculate that not genetics but environment conditions related to socioeconomic factors are important determinants in height inequality.

Height is an appropriate indicator for measuring health inequality and health inequality is a priority for the post-2015 sustainable development goals (SDGs) [34]. The most obvious health inequality indicators such as mortality, morbidity, and life expectancy usually have measurement errors or reporting bias, but height does not suffer from these problems [35] and can be easily comparable across time and location with an objective scale. A large body of literature has investigated health conditions of children aged 0–17 based on the indicator of height-for-age-z-score (HAZ) [36‐39]. In this study, we use mean stature data of children and adolescents aged from 7 to 18 years whose height were more sensitive to environment conditions to show the welfare and health inequalities in different areas and their change trends during 1985–2010.

The average urban-rural difference in height decreased from 4.24 cm to 2.85 cm for boys and 3.72 cm to 2.4 cm for girls during 1985–2010. From all the 24 sex-age groups, we found there was still an obvious height difference between urban and rural subjects although the difference decreased year by year. Huge disparity has existed for a long time between urban and rural areas not only in socioeconomic development but also in physical growth levels [18, 40]. It is generally recognized that the economic status of an area affects kids’ physical development by local infrastructure, sanitary conditions, nutrition quality and health-care access. Although income gap in urban and rural areas has deepened in the past few decades, our study indicates that height difference in these two areas has narrowed since 1985 and has a trend of decrease in the future. Our result was consistent with Chen’s study, which reported that rural children aged 7–18 years had a higher growth rate in stature than their urban counterparts [10] and it meant rural boys and girls were under their way to gain genetic potentials with improved rural living environment. On the other hand, urban children have been gradually moving toward the genetically determined upper limit of an individual’s potential for growth [9]. Thereby, children of older age groups in rural areas are catching up and shrinking the height difference with urban children by adequate supply of energy and nutrition. By age 18 years, the height gap between urban and rural kids narrowed to the least of all age groups in both 1985 and 2010.

The urban-rural differences varied in different provinces, for example, the average difference in Guizhou 7 years old boys was nearly 6 times larger than that in Tianjin. From the trend line in Figs. 3 and 4, we could find that the height differences between rural and urban children aged 7 years were relatively large in provinces where boys and girls had lower mean heights. This phenomenon was also seen in boys aged 2 to 3 months [41] and it might be explained by less access for nutrition and health resources with an unequal distribution of them in these provinces. It had been reported that there was a systematic negative and concave relationship between height inequality and average height [31]. However, some provinces such as Guizhou, Sichuan and Guangxi have great growth potentials for the high marginal returns, thus relevant polices should be formed to improve the living standard of these provinces so that children lived in these provinces can gain their genetic potentials.

It is evident that coefficient of variation (CV) is a widely used and robust index for measuring inequality [28] as it does not increase with average height like standard deviation (SD). Figures 5 and 6 show that CVs of rural subjects are higher than that of urban subjects in most age groups except 17–18 years old boys and 15–18 years old girls where rural CVs are similar to urban CVs. The corresponding explanation may be that rural children have a more unequal opportunity for necessities that are needful for growth such as food and medicine in contrast to their urban peers. Several studies have reported that urban areas exert lower extent of income inequality than rural areas [42] which may be related to the greater height inequality in rural area. Uneven distribution of height within rural areas was also found during 1975–1992 [41].Boys and girls at older age groups have a slower growth rate and relatively stable height with similarly little variations in urban and rural areas. From 1985 to 2010, height inequalities measured by CVs increased at most age groups with the exceptions of 13 to 15 years old boys and 12 to 14 years old girls. The figures show that maximal variation of height (CV) is around puberty period and it is very low when this period ends. This phenomenon can be related with an earlier age of growth peak over 25 years period. It was reported that the maximum height difference with the former age occurred in 13 years old boys and girls in 1985,while it changed to 12 and 10 years old for boys and girls in 2010, respectively [9]. Menarche age has been used for measuring tempo and adolescent maturation in a population and a marked secular trend in menarche has occurred in China since 1985. The average age at menarche for urban girls was 13.08 years in 1985, and it had fallen to 12.35 years with a decreasing rate of 3.5 months/decade in 2010. The decreasing rate of menarche age was even larger in rural girls with a rate of 5.8 months/decade (from 13.79 years to 12.59 years during 25 years period) [19 22]. The faster a population matures, the earlier the kids start their growth spurt and stop earlier [43].Height variation also decreases earlier in puberty period with the earlier end of growth peak. Therefore, it is natural that the CVs of height are decreasing during puberty period over study period. It is said that China has experienced high speed of economic development as well as increasing inequalities in the last few decades. The Gini coefficient which reflects economic inequality is 0.53–0.55 in 2010 (nearly twice the value of 1980s) [13], so China has a higher level of inequality than United States [44]. Previous research on British children found that there was a social gradient in height whereby children from poor groups had a lower average height than their more affluent peers [45]. Nevertheless, how income inequalities affect children and adolescents’ height in China is still unclear. We compared the height between western and eastern students as well as Shanghai and Guizhou students aged from7 to 18 years. It is well documented that for a long time eastern regions have higher level and speed of economic development than western regions with a wider income gap between these two regions [46]and significant differences are found between them by health outcomes such as infant mortality rate (IMR), maternal mortality rate (MMR) and under 5 mortality rate (U5MR) [47]. Shanghai is a modern metropolis with the highest per capita GDP every year in China since 1985, but Guizhou is a poor place where socioeconomic level lags behind all other provinces. Just as income difference in eastern-western regions and Shanghai-Guizhou provinces, significant height differences were found between these places in all sub-groups from 1985 to 2010. The largest eastern-western difference and Shanghai-Guizhou difference was found in puberty period and the age of maximum difference was earlier in 2010 compared with 1985, which can be explained by the earlier onset of peak growth in China as we mentioned before. We also found that there was an obvious increasing trend of the eastern-western height differences in rural subjects over the 25 years. While urban subjects in these areas did not have similar trends as the difference did not increase obviously and even decreased for some age groups in the past few years and it might indicate that the economic growth speed and physical growth rate in western urban region are similar to those in eastern urban region. This phenomenon was also seen in Shanghai and Guizhou urban regions. So rural students in western region and Guizhou province should be paid more attention in making polices to narrow height inequalities in China.

In summary, our study found great height difference existed between urban and rural, eastern and western, Shanghai and Guizhou children and adolescents aged 7–18 years. Although the urban-rural height difference narrowed year by year, rural areas had greater inequality with uneven distribution of resources for children. The differences in eastern-western and Shanghai-Guizhou students’ height increased obviously from 1985 to 2010, especially in their rural subjects. Therefore, we should put rural areas in the priority place when making some policies and public health interventions. In order to shrink height inequalities and attain equal well-being for everyone in China, more equal opportunities for economic development as well as nutrition and health services should be given to different places especially in rural western regions and deprived provinces like Guizhou.