Changes in the Growth and Development of Adolescents in a Country in Socio-Economic Transition 1993–2013

To achieve the aims of this study, basic anthropometric data was obtained from three samples of 14-year-old subjects in Slovenia: from a sample measured in 1993, from a second sample measured in 2003 and from a third sample measured in 2013.

The present study is a part of the longitudinal ACDSi project (Analysis of Children’s Development in Slovenia), led by the Faculty of Sport of the University of Ljubljana, Slovenia and conducted in cooperation with other faculties. The project has been under way since 1970 (14). Measurements at primary schools take place every ten years in the same 11 locations all over Slovenia. The locations are selected to reflect four types of settlement (rural, rural-industrial, industrial-rural and industrial) and spread across the regions in order to be representative of the population as a whole (14). The project examines selected physical and developmental characteristics of children between the ages of 6 and 15. The measurements are pre-arranged with the administrators of the participating primary schools, with parental consent for participation being obtained in advance. In order to fulfil our research goals, we planned to include 200 children in each age- and sex-group, with an estimated drop-out rate of 15% (14).

Body height and leg length were measured. Body height was measured using a portable anthropometer (GPM, Switzerland). During this measurement, the subjects were barefoot, their back was straight, their hands were relaxed alongside the body, their feet and knees were together, their heels touched the anthropometer, and their head was held in the Frankfurt horizontal (15).

Leg length was measured as the iliospinal height (i.e. the vertical distance between the iliac spine and the floor) (16). A shortened portable anthropometer (GPM, Switzerland) was used for this purpose. During the measurements, the subjects stood with their knees and feet together, and the examiner positioned the top of the anthropometer at the anterior superior iliac spine of the pelvis. Trunk length was calculated by subtracting leg length from body height. The leg-to-body height ratio (i.e. leg length as a proportion of the height of the whole body) was calculated by dividing leg length by body height. The leg-to-trunk ratio was calculated by dividing leg length by trunk length.

In addition, body height data for adolescents who were 14 years old in either 1993 or 2013 was obtained over four consecutive years from ages 10 to 14 from the national surveillance system of somatic and motor development (SLOfit). The SLOfit measurements take place every year in April and cover children and adolescents from all Slovenian primary and secondary schools from ages 6 to 18. Annual increments in body height (cm/year) were calculated for each individual as the differences in their body height between two consecutive years. The largest annual increment in body height was adopted as the PHV, while the lower margin of an annual interval in which the PHV was observed was adopted as the age at PHV.

Descriptive statistics (means and standard deviation (SD)) were obtained for subjects’ age, body height, leg and trunk length, as well as for both ratios. The values of the three generations were compared with a one-way analysis of variance (ANOVA) and Tukey’s HSD post-hoc test. A student’s t-test was used to compare PHV and age at PHV between the generations of 1993 and 2013. The level of statistical significance was set at p<0.05.

Anthropometric data was obtained from a total of 1,221 adolescents (674 males and 551 females) measured in 1993, 2003 and 2013. Response rate was approximately 82%, which represents around 2% of the entire population of children in primary schools in Slovenia (14). Their mean age, body height, leg and trunk length, leg-to-body height ratio, and leg-to-trunk ratio, as well as the corresponding p-values for the comparisons between the three generations are presented in Table 1.

No significant differences in body height were detected in 14-year-old females between 1993 and 2013, although there was a trend (p=0.089) towards increasing body height in 14-year-old males. However, in comparison to the previous generation, males from the contemporary generation were taller at the age of 10 to 12 years. There was a noticeable difference in the body height of males and females between 1993 and 2003, and between the years 2003 and 2013. In both 1993 and 2013, males were on average 2.5 and 3.9 cm taller respectively, than their peers in 2003. Females in 2013 were on average 2 cm taller than their peers in 2003.

In males, a significant difference in leg length and trunk length was observed between 1993 and 2013. Between 1993 and 2013, the leg length of males increased by an average of 2.5 cm, while their trunk length decreased by an average of 1.2 cm. In females, while leg length did not change significantly in the same 20-year period, trunk length did increase by an average of 1 cm between 1993 and 2013.

A comparison of leg-to-body height ratio and leg-to-trunk ratio between the generations of males from 1993, 2003 and 2013 demonstrates that relative leg length (i.e. in comparison with the rest of the body) increased significantly with each decade. The leg-to-body height ratio increased by 0.5 percentage points, i.e. from 57.5% to 58.5% in the period observed. In females, leg-to-body height ratio remained unchanged between 1993 and 2003 (57.1%), falling significantly to 56.6% in 2013. The leg-to-trunk ratio increased by approximately 0.03, i.e. from 1.36 to 1.42 in the period observed. In females, the legto-trunk ratio remained unchanged between 1993 and 2003 (at approx. 1.33) and fell significantly to 1.31 in 2013. This means that leg length accounted for a lower percentage of total body height and trunk with head for a larger percentage of total body height in females in 2013 in comparison to females from previous generations.

For the calculation of PHV and age at PHV, data was successfully obtained from 236 14-year-old adolescents from 1993 and 311 14-year-old adolescents from 2013. The mean (SD) age at PHV determined for males of the 1993 and 2013 generations was 12.9 (1.0) and 12.7 (1.0) years respectively. The observed difference was close to the level of statistical significance (p=0.077). Females of the 1993 and 2013 generations were, on average, 11.4 (1.1) and 11.2 (1.0) years old at PHV, i.e. a trend similar to that in males was observed (p=0.071). Table 2 presents the number of adolescents of the 1993 and 2013 generations, who reached PHV at a particular age, values of their mean PHV for a particular age, as well as the corresponding p-values for the comparisons between the two generations.

The results of the present study demonstrate that contemporary generations experience PHV at a younger age. This is true for both genders, even in adolescents born no more than two decades (1993, 2013) apart. As expected, females in both generations experienced PHV sooner than their male peers. The comparison between the 1993, 2003 and 2013 generations demonstrates that males have longer legs and females longer trunks in the latest generation compared to the two previous generations. At the same time, it appears that contemporary females and males experience somewhat different pubertal growth patterns in terms of intensity. While both experience earlier PHV, females experience a growth intensity that is similar to that of previous generations (of females), resulting in earlier conclusion of pubertal growth, while males experience earlier PHV than in previous generations, but also lower intensity. This results in their slower (although longer) pubertal growth. Related to the above, leg-to-body height ratio and leg-to-trunk ratio increased significantly in males and decreased significantly in females over the time period observed.

It has been reported that adolescence in females starts at least two years earlier than in males: females enter adolescence at the age of around 11 years on average and males at the age of 13 years on average (17, 18). Females also reach PHV earlier than males, as was confirmed in the study by Malina et al. (8) performed in North America and Europe between 1988 and 1998. In this study, females reached PHV between the ages of 11.3 (11 years and 4 months) and 12.2 years (12 years and 2 months), and males between the ages of 13.3 (13 years and 4 months) and 14.4 (14 years and 5 months). The study by Malina et al. (8) therefore observed a difference in age at PHV between the genders of approx. two years. This observation can be confirmed by the results of the present study, with females in the 1993 and 2013 generations reaching PHV approximately 1.5 years sooner than their male peers.

The comparison of PHV attainment between the 1993 and 2013 generations demonstrates that PHV was experienced earlier in the 2013 generation, suggesting an earlier onset of puberty in the most recent generation. In both genders, the age at PHV decreased for 3 months over a twenty-year period on average. This is consistent with findings from other studies from Denmark (18), Sweden (19), Japan (20), and Portugal (21), covering a longer period of 50 years or more, during which time these countries underwent an economic transition similar to Slovenia’s, albeit at a slower pace. There is a lack of data on age at PHV across Eastern and Southern Europe, as most of the studies have focused on age at menarche (e.g. Croatia (22), Poland (23)).

In terms of the growth of various parts of the body in human beings, legs are a part of the body, that grow faster than other segments from birth to puberty (5). The leg length of males and females from our study was quite similar to the situation reported for a wider region, e.g. in Croatia in 1997, where leg length at the age of 14 years was 99.0 cm in males (N=225) and 93.8 cm in females (N=209) (24). In most other studies of Eastern Europe (e.g. 25) leg length was obtained by subtracting sitting height from body height, which prevents a direct comparison of results. In our study, the leg-to-body height ratio and leg-to-trunk ratio – the proportion of leg length to trunk length – increased significantly in males over the three generations observed. By contrast, both ratios diminished in females over the course of the same 20-year period. According to the results of the present study, we can reasonably assume that females from the 2013 generation entered puberty sooner than females of previous generations. Moreover, it is most likely that girls from the 2013 generation had already been experiencing the trunk growth stage at the time the measurements were taken, in contrast to previous generations. This is supported by the leg-to-trunk ratio data, which indicates that in the most recent generation of females leg length accounted for a lower percentage of total body height then was the case with females from previous generations. In contrast, the leg length of males from the 2013 generation accounted for a larger percentage of total body height in comparison with males from previous generations. This indicates that, despite their earlier PHV timing, males from the 2013 generation were at an earlier phase of trunk growth than their peers from previous generations, which indicates that pubertal growth intensity was lower than in previous generations.

Our results regarding gender-specific changes in trunk and leg growth between adolescents of the same age between 1993 and 2013 (i.e. longer legs in males and longer trunks in females than in previous generations) are consistent with the study conducted by Bowles (26), where sitting height and leg length were compared at the same age between sons and fathers, as well as daughters and mothers, enrolled at Harvard University between 1840 and 1930 (all subjects had a high social status). In this study, sons were taller and heavier, and had a greater legto-body height ratio than their fathers, suggesting that, on a relative basis, the greatest change in the length of body segments between the generations appeared in the legs. Daughters were taller and heavier than their mothers, but had a greater trunk-to-body height ratio, suggesting that they had already entered the trunk growth stage. No data on PHV timing was reported for the participants (27).

Keyfitz (28) and Meredith and Meredith (29) reported that the mean height of ten-year-old children in 1892, 1923, 1930 and 1939 increased by 1.5 cm per decade. The observed secular trend in body height was caused by several factors. Although healthier diet was a strong factor, it could not be the sole responsible factor, as the same trend has been observed in various societies irrespective of the economic or social status of their individuals (30). In the present study, we did not detect significant differences in body height between the 14-year-old adolescents of the 1993 and 2013 generations. This was true for both genders. Interestingly, the body height of 14-year-old adolescents of the 2003 generation was less than that of the 1993 or 2013 generations. It is possible that the increase in body height in 1993 was superficial and influenced by the large number of child refugees from Bosnia and Herzegovina (31) who attended Slovenian schools in 1993, but who left Slovenia after 1995 (existing evidence shows that the population of Bosnia and Herzegovina is among the tallest in Europe (32)). The overall increase in mean body height between 2003 and 2013 may partly be explained by the process of economic transition, with Slovenia being one of a group of European countries in which significant social and economic changes have taken place in recent decades (33). The transition began in Slovenia in 1991 with the disintegration of the former Yugoslavia, the introduction of consumerism, increased nutritional intake from increased sale of highly processed food, and an increase in sedentary lifestyles resulting from increased use of screen technology. Entry into the European Union in 2004 and the subsequent closing of the gap with wealthier countries resulted in a significant change in GDP: from EUR 11 billion in 1993 to EUR 26 billion in 2003 to EUR 36 billion in 2013 (34). Significant (albeit less drastic) socioeconomic changes were also observable in the decades prior to Slovenia gaining independence (34).

In many societies, improved environmental conditions have enabled children to reach their genetic maximum growth potential (7), which is the ability of an individual to reach their theoretical maximum height with proper development, training and nutrition. It has also been reported that puberty begins earlier in children who live in developed countries (35) and whose parents have a good social position (which is determined by good economic status, social power and reputation) (2). Despite the lack of evidence regarding the influence of socio-economic transition on pubertal growth in Eastern Europe, there is evidence that, with economic growth and consequently improved living conditions, the share of children from the region who are short for their age has decreased (36) and their BMI increased (37). Once the conditions for achieving maximum growth potential are established, full body height is probably attained first and earlier development established later. In our study on a sample from Slovenia, a country that is still undergoing economic transition, no significant differences in body height were observed between 14-year-old adolescents born 20 years apart. However, we did observe a trend towards PHV timing in the most recent generation.

Limitations of our study were that we examined just as much particiapants to satisfy the criteria for the recommended sample size, furthermore, we did not ask the participants for ethnicity, also we did not determine the Tanner stages for pubertal development, as this is an invasive approach, which would risk higher refusal rates, and self-report approach by the children or their parents would result in 30-40% error. We did not use the same measuring device over 20 years, but we used device of identical standard, and body height was obtained only once a year and therefore also the observed difference in age at PHV. However, on the other hand our study has important strengths, as it is a longitudinal monitoring of children with a large number of different measurements and highly trained measuring team.

According to the above, we can conclude that our study is important for public health, as it demonstrates that the socio-economic changes in Slovenia, that included a three-time increase in national GDP over the period of 20 years, were profound enough to result in earlier attainment of PHV in adolescents and, through earlier entrance into the period of the trunk growth, a change in body proportions over the observed two-decade period.

In future research it can be reasonably assumed that the difference in age at PHV between generations would be even more pronounced, if the age at PHV could be determined more accurately for each individual, and measuring of sitting height would give us possibility to calculate leg length as substracting sitting height from body height, which would enable comparison of our results with similar studies in other countries.