Introduction
The occurrence of secular trends is one of the best documented phenomena of human variation, although it is not fully explained from the point of view of its determinants. A tendency for inter-generational changes is especially noticeable in the processes of human growth and development, and morphological evolution. In consecutive decades people grew taller and taller, and entered puberty earlier. This positive trend was mainly associated with a broadly understood progress of civilisation, to which – among others – the following can be included: improvement in socioeconomic status, change in diet and nutrition and considerable medical developments, which translate into better medical care and better hygienic and sanitary conditions. Observations of the way and pace of inter-generational transformations are, in this context, of special significance for the assessment of the effect of the socioeconomic factors described above. To put it simply, the acceleration of development and growth in children can be treated as an indicator of good general development of society. These issues are still relevant today and require further study, because in many populations such trends are still noticeable.
A further positive secular trend has been observed in many countries (Zellner et al., Reference Zellner, Ulbricht and Kromeyer-Hauschild2007; Papadimitriou et al., Reference Papadimitriou, Konstantinidou, Christopanou, Xepapadaki, Giannouli and Nicolaidou2009; Cardoso & Caninas, Reference Cardoso and Caninas2010). The tendency for maximum growth spurt in body height has been, for some time now, slowed in the countries of northern Europe, in Italy (Danubio & Sanna, Reference Danubio and Sanna2008), Japan (Kurokawa et al., Reference Kurokawa, Nakai, Suzuki, Sakurai, Shimada, Kameo, Nakatsuka and Satoh2008) and also in some regions of Poland (Krawczyński et al., Reference Krawczyński, Walkowiak and Krzyżaniak2003; Kaczanowski et al., Reference Kaczanowski, Głąb, Haduch, Mayer, Niedźwiecka, Schmager and Kaczanowski2005; Sitek et al., Reference Sitek, Szkudlarek and Antoszewski2007). Apart from the trends in body dimensions, some trends in pubertal development have also been noticed, which – as in the case of body size – are not uniform; acceleration is seen in some population groups, while in others deceleration can be observed. Krakow in Poland can boast one of the best studies on the determination of secular trends. The latest survey of children and adolescents in the Krakow population is a continuation of observations that have been conducted in that area for almost a hundred years, and their analysis aims at determining whether in the last decade Krakow still witnessed that secular trend.
Methods
Data collected during a survey conducted by a team of academic researchers of the Department of Anthropology at the University School of Physical Education in Krakow, Poland, between 2007 and 2010, were used for the analysis. The above survey was part of a project financed by the Polish Council for Science and Technology, and was carried out with the permission of the Bioethics Council (26/KBL/OIL/2007) and with the consent of the children's parents or legal guardians.
The schools and kindergartens in the area of Krakow, where the measurements were taken, were selected at random. This paper presents the analysis of the body height and weight and BMI of 1862 boys aged between 3.5 and 18.5 years, against the background of a survey series from the years 1938 (N=1801; Jasicki, Reference Jasicki1938), 1971 (N=2045; Gołąb, Reference Gołąb1979), 1983 (N=3124; Chrzanowska et al., Reference Chrzanowska, Gołąb, Bocheńska and Panek1988) and 2000 (N=2328; Chrzanowska et al. Reference Chrzanowska, Gołąb, Żarów, Sobiecki and Brudecki2002). The age brackets were made taking into consideration the year of birth, e.g. the group of 3.5-year-olds included boys aged 3.00–3.99. The mean values of analysed characteristics are shown in Table 1. The mean age of Tanner Stage 2 for pubic hair was calculated by probit analysis, for all series of study (Tanner, Reference Tanner1963). Student's t-test was employed to assess the significance of differences in the selected traits in the last decade (2000–2010).
Table 1. Means and standard deviation of body height, body weight and BMI in the last series of study (2010)
Results
According to the latest survey in the Krakow area, the mean body height in boys, in almost all age categories, was greater than in the case of their peers measured in 1938, 1971, 1983 and 2000 (Fig. 1). The mean body height of boys aged 3.5–18.5 was greater by 0.79 cm, as compared with a group of boys in the same age bracket examined in 2000 (Tables 2 and 3). However, that difference did not maintain at an equal level in individual age categories. Nowadays, boys from the youngest groups are even slightly smaller than their peers from a previous decade. A clear difference can be seen in later years: the boys aged 8.5–9.5 years from the latest series of measurements were statistically significantly taller than their peers measured in 2000, and the boys aged 12.5–17.5 were also taller than their peers from a previous decade (at the age of 12.5–14.5 those differences were statistically significant). If it is assumed that body height at the age of 18.5 has reached its maximum value, then the boys measured in the last survey were almost identical in height as their peers from a previous decade. Secular changes in the body height of the subjects aged 7.5–18.5, counted per decade, presented themselves in the following way: between 1938 and 1971 body height increased on average by 1.94 cm/decade; from 1971 to 1983 it was smaller and equalled 0.70 cm/decade; whereas between 1983 and 2000 a sharp increase in the mean body height by 1.71 cm/decade was observed again, and it was still noticeable from 2000 to 2010, and equalled 1.05 cm/decade (Table 3). However, in the youngest age groups (3.5–6.5 years) the maximum growth in height was rather small in the last decade (Table 2).
Fig. 1. Secular changes of body height in boys.
Table 2. Secular changes per decade between analysed series of study in boys aged 3.5–6.5
Table 3. Secular changes per decade between analysed series of study in boys aged 7.5–18.5
On average, the body weight of boys aged 3.5–18.5 years, measured in 2010, was greater by 1.78 kg than that of their peers from a decade ago. Similarly, as in the case of body height in individual age categories, those differences looked different. Only the boys aged 4.5 and 5.5, examined during the latest survey (2010), were slightly heavier than those examined a decade ago. In all the remaining age categories that difference was always ‘in favour’ of the present day. The biggest differences were noticed in the subjects at the age of 8.5 (statistically significant difference) and 9.5. In the following years that difference was slightly smaller, and from the age of 13.5 it rose abruptly. The subjects aged 13.5–16.5 examined in 2010 were statistically significantly heavier than their peers measured in 2000. The greatest body weight difference was at the age of 14.5 and it reached as much as 6.67 kg. At an early age (3.5–6.5 years) the body weights of the boys examined in 2010, 2000 and 1983 were almost identical (Fig. 2, Table 3). Only at the age of 7.5 was a gradual increase in differences noticed, and it reached its maximum value after the 12th year of life. The problem of weight gain counted per decade was puzzling; in the case of the subjects aged 7.5–18.5 it equalled 1.14 kg/decade between 1938 and 1971, then from 1971 to 1983 it grew slightly to 1.54 kg/decade, and between 1983 and 2000 it dropped slightly to 1.31 kg/decade, whereas the difference in body weight between the latest examinations in 2010 and the measurements from 2000 went up to 2.35 kg/decade, the greatest increase since anthropological measurements began in the Krakow area.
Fig. 2. Secular changes of body weight in boys.
The mean BMI increased in boys (3.5–18.5 years) by 0.49 kg m−2 in the last decade. As in the case of body weight in the males aged 4.5–5.5 years examined in 2010, a smaller BMI was observed in comparison with that of their peers as measured in 2000. The boys measured in 2010 had a smaller BMI at the age of 7.5 and 12.5, but it resulted from a substantial body height increase in those years, and not from body weight loss. Statistically significant differences ‘in favour’ of the boys measured in 2010 were noticed at the age of 8.5 and then between 13.5 and 16.5, and at 18.5 years of age (in that case BMI was greater by 0.96 kg m−2). From the age of 12.5, the boys from the last measurements were the heaviest as compared with all previous series of surveys in Krakow (Fig. 3). In the younger age categories (3.5–6.5 years) BMI was similar to that observed in the boys in 2000 and 1983 (Table 3). A distinct increase in BMI per decade was seen between 1971 and 1983; from 1983 to 2000 it was small and equalled 0.14 kg m−2, whereas in the last decade its value went up sharply to 0.61 kg m−2.
Fig. 3. Secular changes of body mass index (BMI) in boys.
Puberty acceleration in boys was assessed on the basis of the second stage of pubic hair growth. From 1971 that stage of the development of pubic hair systematically appeared in younger and younger boys. In a survey series for a given year the mean age at which that stage appeared in boys equalled 12.99 years. Nowadays, after 40 years, that age had clearly shifted to 11.80 years, i.e. by almost 14 months (Fig. 4). The last decade (2000–2010) saw a decrease of over 3 months in the age of initial growth of pubic hair in boys.
Fig. 4. The mean age of puberty onset (stage II of pubic hair) in boys.
Discussion
The results of this somatic traits analysis, i.e. of body height and weight, BMI and rate of pubertal development, in a group of male subjects examined in Krakow in 2010 show that the tendency towards inter-generational changes is still present. The analysis of changes in body height in boys revealed that for the last several dozen years that trait systematically increased in value for the whole period under research, showing a greater or smaller intensity in different decades. A positive trend in body height, most often related to developmental acceleration, has been observed in the last decade in many countries worldwide. Body height in boys increased, among others, in the Czech Republic (Vignerová et al., Reference Vignerová, Brabec and Blahá2006), Portugal (Cardoso & Caninas, Reference Cardoso and Caninas2010), Belgium (Roelants et al., Reference Roelants, Hauspie and Hoppenbrouwers2009), Greece (Papadimitriou et al., Reference Papadimitriou, Konstantinidou, Christopanou, Xepapadaki, Giannouli and Nicolaidou2009), Brazil (Bianka Caliman et al., Reference Bianka Caliman, Castro Franceschini Sdo and Priore2006), Australia (Ranjitkar et al., Reference Ranjitkar, Lin, Macdonald, Taylor and Townsend2006), the Republic of South Africa (Hawley et al., Reference Hawley, Rousham, Norris, Pettifor and Cameron2009), Mexico (Peña Reyes et al. Reference Peña Reyes, Cárdenas Barahona, Lamadrid, Del Olmo Calzada and Malina2009) and Hungary (Gyenis & Joubert, Reference Gyenis and Joubert2004). However, that trend was not constant everywhere and it did not always concern all age categories. For instance, in the case of the Chinese population the increase in body height was extremely big and no signs of its weakening have been observed up to now, which may suggest that it will be maintained in the future (Ji & Chen, Reference Ji and Chen2008). In Mexico the inter-generational changes in body height were noticed only in boys, exclusively in the younger age groups (6–11), and at a later age that trend disappeared (Peña Reyes et al., Reference Peña Reyes, Cárdenas Barahona, Lamadrid, Del Olmo Calzada and Malina2009). In Italy a positive trend was noticed already from the 1850s and lasted until the last decade of the 20th century (Arcaleni, Reference Arcaleni2006), but the results of the latest surveys show that it has already been hampered (Danubio & Sanna, Reference Danubio and Sanna2008). Secular changes in body height in northern Europe and in many other highly developed countries have shown a similar course, but they have halted there now too (Malina, Reference Malina2004). Surveys conducted in different regions of Poland have revealed a slightly different picture of inter-generational changes, but a positive trend was still observed in Warsaw (Palczewska et al., Reference Palczewska, Niedźwiecka, Szilagyi-Pągowska and Pawlik2000) and Rzeszow (Radochońska et al., Reference Radochońska, Dudzik and Perenc2005).
In the last decades, considerably greater changes have been observed in body weight and BMI in the population of Krakow. In the younger age categories the inter-generational changes in these traits were not significant, but in the older age categories a sharp increase in both body weight and BMI was observed. A similar phenomenon has been noticed in most countries worldwide, e.g. in Germany (Zellner et al., Reference Zellner, Ulbricht and Kromeyer-Hauschild2007; Meigen et al., Reference Meigen, Keller, Gausche, Kromeyer-Hauschild, Blüher, Kiess and Keller2008), Italy (Danubio & Sanna, Reference Danubio and Sanna2008), Portugal (Cardoso & Caninas, Reference Cardoso and Caninas2010), China (Zhang & Wang, Reference Zhang and Wang2010), Lithuania (Tutkuviene, Reference Tutkuviene2005), Hungary (Mészáros et al., Reference Mészáros, Mészáros, Völgyi, Sziva, Pampakas, Prókai and Szmodis2008), Brazil (Costa et al., Reference Costa, Pino and Friedman2011) and Cuba (Esquivel & González, Reference Esquivel and González2010). The inter-generational changes in BMI are usually not observed in developing countries, but it is related to the fact that body height in those countries is still increasing (in proportion to body weight), which in effect does not cause changes in BMI (Hosseini et al., Reference Hosseini, Taslimi, Dinarvand, Jones and Mohammad2010). Body mass index increase in the population of Lithuania presents a similar course as in Krakow: it is more often observed in teenage boys than in the younger ones (Tutkuviene, Reference Tutkuviene2005).
The weight gain and BMI increase in the last decade are alarming phenomena and have become research projects for scientists from many research centres. An excessive increase in these traits could cause serious health problems in a substantial part of society in the future. As some authors suggest (Mucha & Kryst, Reference Mucha, Kryst, Łuczak and Bronowicki2010; Szczepanowska et al., Reference Szczepanowska, Merski, Górnik, Pławińska, Drozdowski, Łuczak and Bronowicki2010), insufficient physical activity, the wrong diet and a passive way of spending spare time are, to a great extent, responsible for such a state. The level of physical activity fell drastically in the last 20–30 years in the countries of post-economic transformations (Maffeis, Reference Maffeis2000; Roberts, Reference Roberts2000). Despite the fact that a majority of adolescents from Krakow declares active forms of spending their spare time, it seems that the energy expenditure during that time is too small to balance the energy supplied to the body by food intake (Libuda et al., Reference Libuda, Alexy, Sichert-Hellert, Stehle, Karaolis-Danckert, Buyken and Kersting2008). Also, children's and adolescent's diets are reported to be bad and inadequate for their physical activity. Surveys show that the consumption of carbohydrates and fats is far too high. In the last dozen or so years in the countries that recently went through significant economic transformations, access to sweet soft drinks, chocolate, chips and fast food has increased sharply (Kromeyer & Jaeger, Reference Kromeyer and Jaeger1995; Ulbricht, Reference Ulbricht and Edwards1996). Surveys in many countries reveal that in recent years children and adolescents spend more and more of their leisure time in a passive way watching TV, playing computer games or surfing the Internet. Younger and younger children watch television regularly. In the USA, children become active television viewers already at about their 9th month of life, and at the age of 12 months they watch TV for 1 hour a day on average, and at the age of two they already spend 1.5 hours watching it (Zimmerman et al., Reference Zimmerman, Christakis and Meltzoff2007). Surveys in Turkey also show that children spend more and more time watching television; on average the youngest ones spend 1.9 hours per day and that systematically increases in older children and adolescents (Kayiran et al., Reference Kayiran, Soyak and Gürakan2010). In Poland, according to the latest questionnaires conducted by OBOP (Centre for Public Opinion Research in Poland; 30th April 2011), children older than 11 years spend 2.4 hours per day watching television and that quantity constantly increases up to 4 hours a day in the case of adults. For this reason children may not have enough physical exercise and their bodies may even lack of oxygen, which in turn may cause developmental disorders of their muscles, bones and respiratory and circulatory systems. A sedentary way of life, and the bad habits connected with it, also strongly contribute to a significant increase in children's BMI. It is well known that the time spent sitting and watching television or using the computer increases considerably the risk of being overweight and obese (Fulton et al., Reference Fulton, Wang, Yore, Carlson, Galuska and Caspersen2009; De Gouw et al., Reference de Gouw, Klepp, Vignerová, Lien, Steenhuis and Wind2010). Spare time spent in a passive way clearly depends on the child's sex, age and parental socioeconomic status (Vicente-Rodríguez et al., Reference Vicente-Rodríguez, Rey-López, Martín-Matillas, Moreno, Wärnberg and Redondo2008). Bad nutritional habits and a sedentary way of life pose another problem. It has been proved that watching television for longer than 2 hours a day is connected with consuming energy-rich snacks and drinks (Manios et al., Reference Manios, Kondaki, Kourlaba, Grammatikaki, Birbilis and Ioannou2009). It has also been shown that the amount of high-calorie snacks consumed is affected by mother's education: children of poorly educated mothers were found to eat three times more unhealthy products than those whose mothers had higher education (Rey-López et al., Reference Rey-López, Vicente-Rodríguez, Répásy, Mesana and Ruiz2011).
Children also spend a large amount of time using computers. Indeed, the energy expenditure while playing computer games is greater than while watching television or performing other activities in a sitting position, but it does not balance the amount of energy supplied with food (Rey-López et al., Reference Rey-López, Vicente-Rodríguez, Biosca and Moreno2008; White et al., Reference White, Schofield and Kilding2011). There is also evidence that even movement connected with getting to school may affect BMI. Children driven to school are in greater danger of becoming overweight or obese than their peers who walk to school (Wen et al., Reference Wen, Merom, Rissel and Simpson2010). It can be assumed that physical activity of children and adolescents is insufficient, despite their declarations. The more time a child spends in a passive way, e.g. watching television or sitting in front of a computer, the heavier he or she is (Zellner et al., 2001), and the greater their risk of having abdominal obesity and cardiovascular diseases in the future (Martinez-Gomez et al., Reference Martinez-Gomez, Rey-López, Chillón, Gómez-Martínez, Vicente-Rodríguez and Martín-Matillas2010). It is known that lack of exercise affects inter-generational changes in body weight and BMI in highly developed countries (Nagel et al., Reference Nagel, Wabitsch, Galm, Berg, Brandstetter and Fritz2009). It seems that without firm action to increase physical activity and decrease the quantity, and increase the quality, of food consumed the growing trends in weight gain and BMI increase will continue.
The group of male subjects from Krakow also showed increasing acceleration of puberty. Some countries have witnessed similar trends (Ji et al., Reference Ji, Hu and He2007), whereas in other places they have ceased (Bundak et al., Reference Bundak, Darendeliler, Gunoz, Bas, Saka and Neyzi2007; Garcia Cuartero et al., Reference García Cuartero, Gónzalez Vergaz, Frías García, Arana Cañete, Díaz Martínez and Tolmo2010). The improvement in socioeconomic status clearly affected puberty acceleration in Poland in earlier decades. In the last decade the level of socioeconomic status did not change significantly and despite that the rate of pubertal development still accelerated. The results of the latest surveys suggest that it might not necessarily be a positive phenomenon. It has been proved that different types of psychological stress, e.g. growing in a one-parent family or frequent family conflicts, may accelerate the pace of pubertal development in girls (Wierson et al., Reference Wierson, Long and Forehand1993; Kim & Smith, Reference Kim and Smith1998). In the last decade the number of divorces has increased considerably in Krakow (Cichocka & Żarów, Reference Cichocka and Żarów2002). Also, the content of some food products (e.g. phtalans or phytoestrogens), which belong to the so-called endocrine-disrupting chemicals (EDC), can modify child development (Walvoord, Reference Walvoord2010). Yet, the influence of these substances on the human body has not been fully explained, because according to some studies they affect puberty acceleration (Wolff et al., Reference Wolff, Britton, Boguski, Hochman, Maloney and Serra2008), and according to others they slow it down (Gladen et al., Reference Gladen, Ragan and Rogan2000). However, it has also been suggested that these substances may affect the quantity of fat tissue and disturb the energy balance of the body, which in turn may cause overweight and obesity (Bourguignon et al., Reference Bourguignon, Rasier, Lebrethon, Gérard, Naveau and Parent2010). A similar effect may also be caused by different types of environmental pollution which, depending on the type of substance, may modify the rate of pubertal development (Toppari & Juul, Reference Toppari and Juul2010).
In conclusion, the last decade saw the cessation of body height increase in Krakow, the maximum body height stabilizing at approximately 179 cm, but weight and BMI gains were still observed in boys; also distinct puberty acceleration, assessed on the basis of the age of pubic hair growth, was noticed. Cessation of height increase at the same time as weight gain, accompanied by puberty acceleration, show a progressing developmental disharmony.
Acknowledgments
This survey was a part of a project financed by the Polish Council for Science and Technology (NN404 177 035) and by the University School of Physical Education in Krakow (204/KA/206). The authors declare that they have no conflict of interest.
