Body mass index and height in 11- to 16-year-old Austrian students attending two different school types with divergent socioeconomic backgrounds

Overweight and obesity have emerged in developed countries, particularly among people with low education and associated low socioeconomic status (SES) [1‐3]. The obesity pandemic has already impinged on rapidly developing countries such as China as unhealthy eating habits and less active life styles spread out from high SES urban regions. Complex regional differences in overweight/obesity rates between rural and urban communities within developed and developing countries have been observed, with rates closely connected to SES or educational level [4‐7].

Various studies have investigated correlations of height with intelligence quotient (IQ), education and SES. High body mass index (BMI) has been found to be associated with worse grades at school, independent of IQ, leading to a downward spiral encompassing lower education levels, poor future employment prospects, lower income and a continuance of low SES in overweight/obese families [8‐10]. On the other hand, taller height has been associated with higher education levels dependent on IQ, leading to higher career achievements [11]. Moreover, taller students have been shown to be regarded as more competent than the shorter counterparts and taller women as better managers [12‐14]. Former studies suggested a genetic linkage through assortative mating [15] or interactions between genetic and environmental factors [16, 17] explaining the association between tallness and IQ. A more recent study showed that most of the covariation between height and IQ was genetic in nature, with both pleiotropy and assortative mating contributing equally to this genetic correlation [18]. Regarding anthropometry, larger gray matter volumes have been observed in taller people [19].

Austria is one of 8 Organisation for Economic Co-operation and Development (OECD) countries where students are separated into groups based on occupational interests early, at the age of 11 years or below [20]. Approximately 70% of school choice is considered to be associated with a child’s social origins and only 30% with educational performance. Of the students who attend schools beyond the compulsory 9 years, 69% have parents with higher education. Only 8% of students whose parents have no education beyond the compulsory years have the prospect of higher education [21]. Since no formal entrance examination is required, parents may influence the choice of school, thus higher parental SES channels students into higher achieving schools. Consequently, the education system in Austria provides the prospect to study the correlation of BMI/height closely related to educational status and familial SES.

In this study, auxological data from adolescents sampled within an Austrian nationwide project (Austrian Working Group on Pediatric Endocrinology and Diabetology, APEDÖ) were studied and the academic level of the school compared with the BMI and height of the students measured. Furthermore, it was investigated whether BMI and height correlated with the population size of the region in which they live.

The findings of this study suggest that the Austrian “education inheritance” corresponding to socioeconomic indicators is reflected in differences in BMI and height between students according to the academic level of the schools they attend.

Although height (~80%), and to a lesser extent weight, is determined by genetic factors [29, 30], environmental factors, especially those associated with SES, are also important determiners of height and weight. In developed countries, low SES/low maternal education is associated with short stature, overweight and obesity, starting at preschool age [1‐3]. At school age, overweight/obese students seem to get worse marks for identical performance, suggesting a discrimination that hampers their access to and graduation from university [8, 10]. This might in turn impact on attainment of maximum income and SES, resulting in an association between lower IQ and heavier weight that could be modifiable if measures were taken early enough [9].

In this study, the mean difference in BMI between VSS and AHS was +0.87 kg/m², with statistically significant differences at each age from 11 to 16 years (Fig. 3). Overweight and obesity were 1.8-fold and 2.5-fold more prevalent, respectively, among students at VSS (overweight in 14.8–15.5%; obesity in 6.2–6.5%) than among those at AHS. These results are similar to those for Germany between 2003 and 2006 (overweight in 17–18.9%; obesity in 7–8.9% between 11 and 17 years) [31]; however, precise comparison is not possible due to differences between cohort composition and reference levels [24].

As to height, upward social mobility has led to a levelling out of pronounced historical differences between manual and non-manual workers during the last decades (from 3–6 cm down to 1–2 cm) [32‐34]. A mean difference of +0.93 cm in students with a higher SES background between the ages of 11 and 16 years was found (Fig. 3). This difference may be smaller than in the earlier studies because final height is rarely reached by the age of 16 years, particularly in male adolescents. More strikingly, a significantly higher proportion (2.2-fold) of students of both sexes at VSS had short stature (3% in f, 4% in m) compared with those at AHS who lay below expected rates. Overall, there was a greater spread of outliers ±2 SDS among students at VSS, which might in part be assigned to the higher frequency of students from immigrant backgrounds with a lower genetic height potential [35] and/or different age at pubertal onset. In fact, height SDS was similar in VSS and AHS between 11 and 13 years of age in both sexes, before height SDS generally dropped in adolescents attending VSS compared with AHS (Fig. 2c and d). This suggests that students at VSS had an earlier pubertal growth spurt, possibly because of higher BMIs as obesity is known to predispose to earlier puberty [36, 37]. During childhood, increase in BMI correlates with an increase in height gain [38], associated with advanced bone age probably driven by increased dehydroepiandrosterone sulfate, an adrenal androgen [39]. This leads to an earlier pubertal onset, which is followed by a decreased height gain during adolescence, explaining the lack of correlation between childhood overnutrition and final height [38]. Notably, in this cohort taller height contributed by around 25% to the lower BMI of students at AHS, whereas the remaining difference was attributable to heavier weight in students at VSS.

Initially, urbanization was associated with higher overweight/obesity rates in developed and developing countries, particularly in large and megacities [4, 5, 7]. Such an effect was found in Vienna, the largest city in Austria (1.74 million inhabitants) where students had a significantly higher BMI than students in the four cities above 100,000 (p < 0.001) and in communities with 20,000–100,000 inhabitants (p = 0.045). Interestingly, no statistical difference in BMI between Vienna and small communities (<20,000) could be found (Fig. 4a). This might in part be caused by nutrition transition and, due to less entertainment facilities in rural regions, an increase in sedentary activities driven by the pervasiveness of consumer electronics. Similarly, a Swedish study of 7‑ to 9‑year-old school children even found an increasing overweight/obesity gradient between metropolitan and rural areas depending on the level of education provided [6]. Thus, an urban-rural dichotomy can no longer be generalized because urbanized life styles are permeating into less densely populated areas [40], requiring detailed analyses of urbanicity-associated effects that consider complex regional factors and interactions. No association between height and urbanicity could be ascertained.

This study has several strengths. Firstly, data were obtained from a representative large number of schoolchildren and adolescents in Austria (around 1%). Secondly, 98% were measured by the same investigator. Thirdly, all the students were from state schools with a widespread distribution of SES level, covering all the geographic regions in Austria and taking account of different degrees of urbanization; however, there were limitations. Firstly, since the age range of the students lay between 11 and 16 years, definite data on final height differences cannot be provided, as final height, especially in boys, is often reached after this age; however, sufficient data beyond 16 years of age in VSS leavers were not available. Secondly, information on ethnicity which is correlated with individual target height could not be collected. Thirdly, urbanicity could not be reliably assigned for older VSS students because many of them commuted from another district (BMS 43%) [26], which could have confounded differences between rural and urban areas.

In summary, the present study revealed statistically significant correlations between BMI and height of students and the school type attended, reflecting their parent’s SES and educational level. This study pinpoints the auxological effects of an education inheritance in Austria and possibly other countries with similar school systems, perpetuating socioeconomic inequalities with long-term consequences on morbidity in later life. These results support the need to work on equality of opportunity by overcoming students’ early SES-based grouping between school types. Meanwhile, obesity prevention and intervention measures should be reinforced, not only in large cities but also in rural communities.