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Living conditions and secular trend
Charles Susanne
Living Conditions and Secular Trend
Laboratorium Antropogenetica,Free University Brussels, Pleinlaan, 2 1050 Brussels, Belgium
Growth and maturation status is generally accepted as a suitable indicator of the health and nutritional status of a studied population. T h e secular trend has been observed in all industrialized countries: general health and better nutrition seem to be major factors. An analysis of the secular trend in Belgium of height, weight and weight-for-height relationship shows that the effects of environment are different for these three measurements. Differences in secular trends in the various age groups show that in proportion to the attained height the secular trend does not differ from childhood until adulthood. An analysis of nutrition in Belgium during recent years shows some effects of the current economic crisis.
Keywords: secular trend, nutrition, weight, height.
Journal of Human Evolution (1985) 14, 357-370
1. Introduction The phenomenon known as secular trend corresponds to the physical maturation of children occurring more rapidly, and to the children becoming taller for their age. This secular trend has been observed in all industrialized countries: an increased degree of outbreeding, correlated to the process of urbanization has been postulated as its cause (Hulse, 1957). However, general health, better nutrition, and better living standards seem to be more important factors (for a review, see Van Wieringen, 1978). Since it has been postulated that physical growth is influenced by nutrition and social conditions, we are expecting differences in relation to the socio-economie status: such differences have already been reported by Villermfi (1829) and Qu~telet (1830) and by numerous authors afterwards (for a review, see Susanne, 1980a). 2. Secular Trend in Sexual Maturation Sexual maturation is of course characterized by the growth of genitalia and testes and the development of pubic hair in boys and by the development of pubic hair and breasts and the onset of menstruation in girls. However, most available data concern the age at menarche. For age at menarche, a differentiation according to socio-economic status has been often reported (for a review, see Tanner & Eveleth, 1975; Susanne, 1980a). It is well-known that a secular trend is observed concerning the period of puberty, with decreasing age at menarche (for a review, see Tanner, 1962). This has also been observed in Belgium (Figure 1). But, secular trend also means an earlier age at peak height velocity and peak weight velocity. Lindgren (1979) showed that children not only reach puberty earlier but also pass it more quickly. She also showed that the rate of the trend for the age at menarche has been greater than for PWV and PHV ages. As a higher genetical control exists for height than for weight (Susanne, 1971, 1975, 1977), it is reasonable to suggest that PHV age could be also more genetically controlled than PWV age. It seems then possible to suggest that PWV age has been more influenced by the evolution toward better nutrition and health than PHV age.
Perhaps age at menarche has also been greatly influenced by the improving standard of living: it is indeed obvious that in most countries an earlier age at menarche is related to 0047-24-84/85/040357 + 14 $03.00/0
(~) 1985 Academic Press Inc. (London) Limited
358
c.
SUSANNE
Figure 1. Secular trend of menarche, x: Sweden; 9 Belgium. 16 ~15
•
E "6 14
g
9
•
•
12
, 1900 , , , , 1880 1890 19' 10 19'2 0 1930 1940 19; 0 1960 1970 1980
better occupational status of the parents, to a higher quantity or quality of the food, to better medical care and to a tower number of siblings. This secular trend is observed to be very regular except in some countries where bad environmental conditions, related to times of war, are causing fluctuations or absence of this trend (Vlatovsky, 1966). We may thus propose that the onset of maturation is a multifactorial characteristic controlled by different factors. The environmental factors are numerous: delay of maturation is observed to be influenced by some diseases (Hauspie, 1980; Hauspie et al., 1979), socio-economic differences and nutritional status. Family size, degree of urbanization, and season (Jeurissen, 1970) are other possible factors. The process of maturation may differ according to the age at menarche: more anovulatory cycles would occur after an early menarche than after a later one. Genetic causes for differences in potential (or minimum) age at menarche are also predictable (Susanne, 1980b). As Johnston (1974) has concluded already research in the future has to be conducted following a multifactorial design. Physical growth and sexual maturation are relatively independent. O f course, the association of pubertal maturation with fatness, body build and height has been observed. But studies ofheritability have shown that puberty is relatively independently determined by genetical factors (Bock & Thissen, 1978). Environmental factors can influence growth and the onset of maturation also in a somewhat independent way or, as already suggested in the analysis by Lindgren (1979), this influence could be more strong on menarche than on weight, more strong on weight than on height. Bruntland et al. (1973, 1975) mentioned for Oslo children no change in age at menarche between 1950 and 1970, but an increase in height. Dann & Roberts (1969) and Roberts et al. (1971), in an analysis of the influence of socio-economic status, found no differentiation in menarchal age but clear differences in height and weight.
3. Height Children are taller today than in the 1940s or at the end of the 19th century. These results have been often reported and reviewed (Tanner, 1966; van Wieringen, 1978): this secular increase is observed at all ages of the growth period up to adulthood and started when industrialization began to improve our living conditions. This trend is also observed in Belgium (Figures 2 and 3). Let us remember that, although Belgium is composed of two different linguistic populations, Flemish and Walloons, no systematic differences are found between the two populations at the biometrical and the auxological level (Sporcq, 1969; Hebbelinck & Borms, 1975;
359
LIVING CONDITIONS AND SECULAR TREND Figure 2. Secular trend of height and weight of Belgian boys. 180 1 8 3 5 ~ .......... 9 1929~- . . . . . . 1 9 4 8 o - - - - - - - * Twiesselmann ( 1 9 4 9 ) 1960.--4 Twiesselmann ( 1 9 6 9 ) 1977z z Ooossens etctl.(1977)
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Twiesselmann, 1980). Belgium is a country with a high population density, where the differences eventually observed between subcountries are lower than those due to sociobiological factors. The secular evolution of height in Belgium seems to follow another trend between 1880 and 1930 than after 1930, when the trend is more rapid than ever before. Taking into account only the adults, the trend is equal to 0"4 cm per decade between 1845 and 1930, 1"2 cm per decade between 1930 and 1950, and 2-0 cm per decade from 1950 onwards. This is very similar to results from the Netherlands where an increase of 1"0 cm per decade is observed between 1865 and 1953 and of2"l cm per decade between 1953 and 1970. The trend seems lower in the last decade (Vercauteren et al., 1984). In Belgium, at age seven the increase is about 2"3 cm each ten years between 1948 and 1977 for boys and 2"0 cm/10 years for girls. At 14 years for boys the secular trend is about 7'0 cm/10 years between 1948 and 1960, but 1.7 c m / t 0 years between 1960 and 1977. For girls of 12 years, the secular trend is about 3"8 cm/10 years between 1948 and 1960 a~d
360
SUSANNE
c.
Figure 3. Secular trend of height and weight of Belgian girls. 180 1 8 3 5 , .......... 9 1929-- .... 4 t 9 4 8 o-------o Twiesselmann (1949) 1960o-------e Twiessetmann (1969) 1977, - Joossens et al. (1977)
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about 2"0 cm/10 years between 1960 and 1977. This difference is related to a retardation of the pubertal growth spurt in 1948 and results from the poor living conditions during the war. At 18-20 years, the secular trend is about 2"0 cm/10 years for boys and 2"2 cm/10 years for girls between 1960 and 1977. Very similar results have been published by Ljung et al. (1974) and van Wieringen (1978). If the global trend is rather linear, a detailed analysis of Dutch conscripts (van Wieringen, 1978) showed an arrest in the positive trend during different crises: a cholera epidemic in 1866--67, an agrarian crisis in 1888-96, World War I, the economic crisis of the 1930s, World War II, the unemployment in 1950-53. Van Wieringen postulated that all these unfavourable living conditions were present in a large part of the population and that their influence was especially marked during adolescence. Socio-economic differences influencing the secular trend are also observed: taking Belgian university students as representatives of a high social level, a trend of 1'0 cm each
LIVING
CONDITIONS
AND
SECULAR
TREND
361
12"5 years has been found for them while this trend for the whole Belgian population is equivalent to 1"0 cm each eight years (Susanne & Heyne, 1972). In Switzerland, the secular trend in the last two decades has been higher where boys were shortest (Molinari et al., 1984).
4. Weight A secular trend of weight has also often been reported and we would expect this trend to be proportional to the changes of height. The trend of the weight curves is however, relatively different from that of height in Belgium, at least from 1960 onward. In childhood and in adulthood, no differences are observed between the studies of 1960 and 1977, except for boys after 18 years. Moreover, the weight is higher in 1960 than in 1977 for girls between 11 and 16 years, the maximal difference being 2 kg around 12-5 years. Taking into account only the adults, the secular trend implies a regular increase of weight, although the trend is less strong for weight than for height. The trend of weight is clearly influenced by other factors than that for height in these last decades. Nutritional factors related to the fashion of weight control are probably involved: for instance, the mean weight of the Belgian female students at the university of Brussels has not changed since 1945, while an increase of stature has been observed.
5. Weight for Height Relationship The positive secular trend in height is not necessarily accompanied by a proportional weight increment. Figures 4 and 5 show indeed that Belgian children have increased relatively more in height than in weight. This phenomenon seems to be more pronounced in girls than in boys: it is only evident in boys with a height over 140 cm. A more detailed analysis of Figures 4 and 5 shows, however, that the samples from between t924 and 1960 do not differ concerning their height/weight relationship and that the tendency toward more linear children is evident only in the comparison between the samples of 1960 and 1977. However, at least for the lower ages the children of 1835 do not differ from the children of 1977 in their height/weight relationship: in 1835 the children also have a lower weight for height than in 1929 and 1960. Similar results have been observed by Ljung et al. (1974) by means of the same diagram method, and by means of indices such as W / H 3, W/H 2 and W / H by Broman et al. (1942), Wolanski (1967) and Vajda et al. ( 1974). A proportionally larger increase in height than in weight has been observed in Sweden between 1883 and 1971 resulting in thinner children (Ljung et al., 1974). From 1938 onward, this tendency seems to have been more pronounced in girls than in boys. Himes (1979) and Malina (1979) have summarized studies where, on the contrary, increases in weight for height have been observed such as in the United States, England, Canada and Australia.
6. Differences in Secular Trends in the Different Age Groups The secular trend observed during the growth period is of course a function of the age period which is taken into consideration. The largest shifts of stature and weight are observed during puberty, because they result also of a trend toward earlier maturation.
362
C. SUSANNE Figure 4. Secular trend of the weight/height relationship for Belgian boys.
70
60
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.....
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It can be asked whether the increase in height is proportional to the attained height in the different age groups. We tried to answer this question by calculating the ratio between the differences of the means of stature in two successive periods of time (tl and t2) and the mean of stature in the first period in different samples: 't2 -- X'tl Y--
Xtl
Most of these new values, the secular trend of height in proportion to the attained height, are relatively constant during infancy and puberty (Figure 6). During that period of life the observed increase due to the secular trend is proportional to the stature attained at the specific studied age. Sometimes, the secular trend relative to the attained height is somewhat higher during puberty (in the Polish, Swedish and Venezuelean samples). The trend observed in the Dutch samples of the years 1965 and 1980 is exceptional: the values of the secular trend divided by the height attained in 1965 increase from birth to puberty. These observations concerning the proportionality of the secular trend for stature could be interpreted as a result of the fact that the secular trend is manifest rather specifically
363
LIVING CONDITIONS AND SECULAR TREND
Figure 5. Secular trend of the weight/height relationship for Belgian girls. 60 ,4
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1 160
during the first years of age, probably from birth to three years, which corresponds of course to a period of great growth velocity. In a similar way Garn et al. (1984) give the magnitude of the socio-economic effect on size, expressed in scores for an American sample: the greater part of this social effect is already reached at the age of one year. T h e last results from the Netherlands' survey do not correspond completely to the other observations, and it will be interesting to follow the publications from our colleagues van Wieringen and Roede. In this recent sample of the Netherlands, on the one hand the effects are observed of the recent fashion to give less rich food to babies and young children resulting in a lower height in 1980 than in 1965 until two years and in a lower weight until four or five years. O n the other hand we probably observe the effect of a decrease of the secular trend in the Netherlands, not yet observed at adulthood, but manifest before puberty.
7. Nutrition Nutritional and hygienic factors have a quite obvious influence on growth, they wi,ll be specially important during the first three years of life. Protein-energy malnutrition of young children is of course due to ~the impossibility to
364
(3. SUSANNE Figure 6. Differenceswith age of the secular trends ofheight in proportion to the attained height in the different age groups. • boys; O: girls.
Age 0
1
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purchase sufficient food for the child or to an inadequate nutrition and health education. Growth not only depends on the intake of food, but also on its digestion, absorption, utilization and excretion: therefore, the general health and hygienic situation of the child are of considerable importance. The increase in height being greater from the 1950s onward than it was before, the relative increase in weight being lower from the 1960s onward, because of the changing patterns in the fashion of weight control, it seemed interesting to analyse the evolution of food intake in Belgium during this period. From t h e O C D E data, Figure 7 illustrates the changes occurring in Belgian food
365
LIVING CONDITIONSAND SECULAR TREND Figure 7. Changes in Belgian food consumption between 1955 and 1978.
200
1955 Bose 100
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~
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~Vegetables
0 I I I I I 'I I I I I I I P I I I 55-59 6 4 6 5 66 67 68 69 70 71 72 T3 74 75 76 T'F ?'8
consumption between 1955 and 1978: in the first place, a considerable increase in the consumption of milk products, meat and fruits is observed; simultaneously the consumption of potatoes and cereals is decreased. For vegetables, oils and fats a limited increase is observed (Susanne et al., 1985). An analysis of nutrition in Belgium during recent years (1975 to 1982) shows some effects of the economic crisis (Susanne et al., 1985). For instance, the consumption of meat did not change very much in absolute quantities, a slight decrease is observed from 1980 onward (Figure 8). However, various patterns are observed in the consumption of the different kinds of meat: a decrease in consumption of pork and beef, but an increase of mixed meats (minced meats). " The amount of money dedicated to food increased during the last decades but the percentage of a person's income spent on food decreased in time during .the last decades (Table 1): from 35"8% in 1961 to 20"9% in 1978-79 for workers. This variation in time is a sign of an environmental improvement and could be important enough to explain a secular trend. However, the social variation in the expenditure on food is not great: in 1978-79, it varied between 105,095 Belgian francs yearly for clerks and 125,879 Belgian francs for liberal professions. Such little social variation may result in a decrease in the socio-economic differentiation of height and weight. In other samples social differences do, however, exist: poorer boys and: girls appear to have lower serum and urinary vitamin levels, their globulin levels are higher (more exposure to infection), and their exposure to pollutants is greater (Garnet al., 1984). The research of the influence of life style on nutritional status and of the association of both with growth should be promoted. 8. Discussion We must realize that comparisons of cross-sectional studies across decades, and between populations have limitations at the level of sampling and methods of measurement.
366
C. SUSANNE
Table 1
Amount of money dedicated yearly to food consumption, absolute and as a percentage of the total salary
Years 1961 1973-74 1978-79
Workers Amount 37 244 BF 75513BF 107 023 BF
%
Clerks Amount
35"8 24'7 20"9
40 764 BF 70 975 BF 105 097 BF
Figure 8. Changes in Belgian consumption of meat between 1975 and 1982.
%
Farmers Amount
26"9 16-8 15'8
-81 809 BF 109 890 BF
%
Liberal professions Amount
%
30"1 20'6
83 363 BF 125 879 BF
19-7 14'3
1975
/"Mixeci" meat (minced)
140
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120
110
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Therefore the fact that we observe in our study similar results for different samples and populations gives some significance to these results. It is clear that growtl~ and development are determined by genetic factors (Susanne, 1980b) and that they are also strongly influenced by environmental factors. Growth and maturation status is generally accepted as a suitable indicator of the health and nutritional status of a studied population. The most important environmental factors in this respect are disease and nutrition, but many other factors may have a direct or indirect influence such as a guaranteed minimum income, social security, transportation, pure drinking water, sewerage, proper food preparation, the disappearance or diminution of epidemics such as bovine tuberculosis, good sanitary conditions, health care, national health assurance, immunization against childhood diseases, medical care and education, absence of heavy work by children, the limitation of the number of children per family, and maternal efficiency. It is quite clear that at the level of one of the primary factors, nutrition, even in Belgium where in 1960 the essential foods were available in quantity and quality, the situation improved between 1960 and 1982: the over-all consumption of meat, milk products and fruit increased. However, the predominant influence of environmental factors in a
LIVING CONDITIONS AND SECULAR TREND
367
multifactorial and polygenic system does not exclude that some differences between populations or between individuals from the same population may be of genetic origin. In the explanation of the secular trend the same controversy exists: genetic and environmental theories have been postulated. The genetic theory supposes the existence in human populations of a kind of heterosis, observed in reality only in crossings between heavily inbred plants or animals. The slow disappearance of some marriage barriers and endogamy, and the correlated increase of the number of heterozygotes is supposed to contribute to the positive secular evolution. However the hypothesis is still ambiguous: the differences between exogamous (outbred) and endogamous (inbred) populations have to be controlled for the socio-economic conditions and for all related environmental factors: only a research with a multifactorial design can bring us clear results. Different authors have observed an increase in stature in countries where exogamy increased (Hulse, 1957), where consanguinity diminished (Schreider, 1967) or where the distances between partners increased (Wolanski et al., 1970; Billy, 1971, 1979). However, in the interpretation the socio-economic factors have seldom been kept under control. Hulse (1957) in his classical study of Swiss villagers, indicated the existence of differences in family size and cultural differences between the inbred and outbred groups. However, he did not take these differences into account. Other authors using marriage distances do not take into account that these distances are increasing with the socio-economic level and that the endogamy is also varying in the different professional groups (Marreyt, 1975; Claeson & Egero, 1972; Coleman, 1973; Kfichemann et al., 1974; Harrison et al. 1971) even in homogeneous countries, e.g. essentially rural populations. On the other hand, various authors did not confirm the effects of heterosis (Barret & Brown, 1971; Crognier, 1973; Marquer, 1979). Even when the influence of consanguinity on multifactorial characters such as height and weight is studied, the influence of socio-economic factors could still be relatively more important than that of consanguinity itself. The results of the influence of consanguineous marriages are in any way not very evident: a slight negative influence is observed by Morton, (1958), Schull (1962) and Schreider (1967) but not by Schork (1964). Although not contradictory but essentially complementary to the genetic hypothesis, the environmental hypothesis has a stronger basis. In the industrializing countries a manifest improvement in living conditions has been observed: the positive secular trend started at the moment when this improvement began, with the inception of industrialization. The environmental factors seem to affect more groups, living in unfavourable conditions or at a lower socio-economic level: on the one hand a diminution of the secular trend for the lowest statures has been observed in periods of crisis (van Wieringen, 1978; Brodar, 1978) and on the other hand a lower secular trend has been observed in higher socio-economic groups, such as university students, than in the general population (Susanne & Heyne, 1972; Aubenque, 1957). The secular trend in weight is less intense than that in height and it is sometimes even absent in girls: the weight of Swedish girls in the age 15-16 is lower in the period 1965-71 than in the period 1938-39 (Ljung et al., 1974); the weight of Belgian girls studying at university has not increased since 1945 (Susanne & Heyne, 1972). Such a discrepancy between the sexes may best be explained from a cultural point of view. Homologous results have been observed by Lindgren (1976) who found that girls from the lowest socio-economic classes have significantly more weight for height than girls from the higher classes. Boys show the same tendency, but weaker and not significant.
368
c. SUSANNE
Similar multifactorial processes are at the origin of the evolution of the proportion of weight for height in the last decades toward more linear children. The trend of the increase in height is proportionally stronger than that in weight. The analysis of the ratio mean height/mean weight showed us that this tendency to become thinner is true for both sexes, although tess marked for boys. The origin of this trend could be a better nutrition or better balanced diets, but perhaps also a change of the fashion to more slim and well-trained bodies for women and men. In some countries, positive secular changes are coming to an end (Brundtland & Wallo~, 1973; Dann & Roberts, 1973; Tanner, 1975; Walter et al., 1975). It can be questioned whether this observation is in favour of the genetic or the environmental hypothesis. In the genetic hypothesis we can postulate that the positive secular changes would have reached a biological and genetic maximum, but it seems more probable that the slower trend would result from a stabilization of the living conditions at the socio-hygienic level. In some populations, the differences expected in relation to the socio-economic status are also absent: typical of this situation is the study of Lindgren (1979) who did not find differences between socio-economic classes defined by the father's occupation and family income in a sample of Swedish children collected during the period 1965-71. This absence of socio-economic differences is an argument in favour of the hypothesis of a stabilization of the living conditions in some populations and thus favourable to the environmental hypothesis. Indeed, if this hypothesis is correct, a trend toward equality of the standards of living in the different socio-economic classes may result in a diminishing disparity in the growth rates of different socio-economic strata. The results of Walter et al. (1975) on German children aged six to 11 years and Waiter (1977) on German children aged 9"5 to 10"5 years also show this absence ofsocio-economic differences for height and weight. For menarchal age too, the absence of social effects in modern industrial countries has been observed (Brundtland & Wallofi, 1973; Degnbol, 1964; Furu, 1976; Richter, 1964; Jenicek & Demirjian, 1974; Kantero & Widholm, 1971; Roberts et al., 1971; Wachholder & Cantraine, 1974), as well as for skeletal maturity (Roche et al., 1978). In these countries, nutritional and hygienic factors, responsible for an optimal biological development are nowadays generally present. These nutritional and hygienic factors would be of primordial importance during the first three years of life, also for the establishment of a secular trend. A specific increase of the secular trend in proportion to the attained stature is observed during these first three years. The relative changes are less important afterwards.
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Physique and family environment in girls attending a Welsh college. British Journal of Preventive and Social Medicine 23, 65-71. Dann, T. C. & Roberts, D. F. (1973). End of the trend? A 12 year study of age at Menarche. British Medical Journal 3, 265-267. Degnbo[, B. (1964). On asagerna til menarkens tidsmaessige variation (On the causes of age variations at menarche). Ugeskriflfor Laeger 126, 1195-1198. Furu, M. (1976). Menarcheal age in Stockholm girls. Annals of Human Biology 3, 587-590. Garn, S. M., Pesik, S. D. & Pilkington, J.J. (1984). The interaction between prenatal and socio-economic effects on growth and development in childhood. In (J. Borms, R. Hauspie, A. Sand, C. Susanne & M. Hebbelinck, Eds) Human Growth and Deoelopment, pp. 59-70. New York: Plenum Press. Graffar, M., Asiel, M. & Emery-Hauzeur, C1. (1962). La croissance de ['enfant normaljusqu'~ trois ans. Analyse statistique des donn6es relatives aux poids et ~ la taille. Acta Paediatrica Belgica 16, 6-23. Harrison, G. A., Hiorns, R. W. & Kuchemann, C. F. (1971). Social class and marriage patterns in some Oxfordshire populations. Journal of Biosocial Science 3, 1-12. Hauspie, R., Gyenis, G., Alexander, F., Simon, G., Susanne, C. & Madach, A. (1979). Heights and weights of Hungarian and Belgian asthmatic boys. Human Biology 51,507-521. Hauspie, R. (1980). Abnormal growth. In (F. Johnston, A. Roche & C. Susanne, Eds) Human Physical Growth and Maturation. Methodologies and Factors, pp. 193-201. New York: Plenum Press. Hebbelinck, M. & Borms, J. (1975). Biometrische Studie van een Reeks Lichaamsbouwkenmerken en Lichamelijke Prestatietests van Belgische Kinderen uit bet Lager Onderwijs. Brussel: Vrije Universiteit Brussel. Himes, J. H. (1979). Secular changes in body proportions and composition. In (A. F. Roche, Ed.) Secular Trends in Human Growth. Monographs of the Society for Research in Child Development, Serial no. 179, pp. 28-58. Hulse, F. S. (1957). Exogamie et h6t~rosis. Archives Suisses d'Anthropologie Ggnerale 22, 103-125. Jenicek, M. & Demirjian, A. (1974). Age at menarche in French/Canadian urban girls. Annals of Human Biology 1, 339-346. Jeurissen, A. (1970). Mois de naissance et pubert6 chez les filles. Biomltrie Humaine 5, 1-16. Johnston, F. E. (1974). Control of age at menarche. Human Biology 46, 159-171. Kantero, R. L. & Widholm, O. (1971). The age of m enarche in Finnish girls in 1969. Acta Obstetrica et Gynecologica Scandinavica 50, Suppl. 14, 7-18. K/ichemann, C. F., Harrison, G. A., Hiorns, R. W. & Carrivick, P.J. (1974). Social class and marital distance in Oxford city. Annals of Human Biology 1, 13-27~ Lindgren, G. (1976). Height, weight and menarche in Swedish urban school children in relation to socio-economic and regional factors. Annals of Human Biology 3, 501-528. Lindgren, G. (1979). Physical and Mental Development in Swedish Urban Schoolchildren. Stockholm Institute of Education, Studies in Education and Psychology, Vol. 5, pp. 1-69. Ljung, B. O., Bergstein-Brucefors, A. & Lindgren, G. (1974). The secular trend in physical growth in Sweden. Annals of Human Biology 1, 245-256. Marquer, P. (1979). Endogamie, exogamie et variations de la stature et de l'indice cephalique dans la population B6arnaise (Pyr6n6es-Atlantiques). Bulletins et Mgmoires de la Soci~tg d'Anthropologie de Paris 6, 333-342. Marreyt, J. (1975). De Huwelijksafstand. "Barometer" voor Migraties en Socio-Culturele Invloedsferen. Thesis, Vrije Universiteit Brussel. Molinari, L., Largo, R. H. & Prader, A. (1984). Target height and secular trend in the Swiss population. In (J. Borms, A. Hauspie, A. Sand, C. Susanne & M. Hebbelinck, Eds) Human Growth and Development, pp. 193-200. New York: Plenum Press. Morton, N. E. (1958). Empirical risks in consanguineous marriages: birth weight, gestation time and measurements of infants. American Journal of Human Genetics 10, 294--343. OCDE. (1983). Statistiques de la Consommation des Denr~es AIimentaires. Paris: Organisation de Goop6ration Economique et de D6veloppement. Qu~telet, A. (1830). Sur la taille moyenne de l'homme dans les villes et dans les campagnes, et sur l'~tge off la croissance est compl~tement achev6e. Annales Hygiene Publique 3, 24.
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Richter, J. (1964). Das gegenw~irtige Menarchealter in G6rlits. A rtztliche Jugendkunde 55, 393-398. Roberts, D. F~, Rozner, L. M. & Swan, A. V. (1971). Age at menarche, physique and environment in industrial north east England. Acta Paediatrica Scandinavica 60, 158-164. Roberts, D. F., Danskin, M. J. & Chinn, S. (1975). Menarcheal age in Northumberland. Acta Paediatrica Scandinavica 64, 844-852. Roche, A. F., Roberts, J. & Hamill, P. V. (1978). Skeletal maturity of youth of 12-17 years: racial, geographical area, and socio-economic differentials. National Centerfor Health Statistics Series no. 11, no. 167, t-120. Schork, M. A. (1964). The effects of inbreeding on growth. American Journal of Human Genetics 16, 292-300. Schreider, E. (1967). Body-height and inbreeding in France. American Journal of Physical Anthropology 26, 1-4. Schull, W.J. (1962). Inbreeding and maternal effects in the Japanese. Eugenics Quarterly 9, 14-22. Sporcq, J. (1969). L'anthropologie des miliciens belges en 1963. Bulletin de l'Institut Royal des Sciences Naturelles Belgique 45, 1-115. Susanne, Ch. (1971). H6r~dit~ des caract~res anthropologiques mesurables. Bulletins et M~moires de la Soci[t( d'Anthropologie de Paris 7, 169-224. Susanne, Ch. (1975). Genetic and environmental influences on morphological characteristics. Annals of Human Biology 2, 279-287. Susanne, Ch. (1977). Heritability of anthropological characters. Human Biology 49, 573-580. Susanne, Ch. (1980a). Socio-economic differences in growth patterns. In (F. Johnston, A. Roche & Ch. Susanne, Eds) Human Physical Growth and Maturation. Methodologies and Factors, pp. 329-338. New York: Plenum Press. Susanne, Ch. (1980b). Developmental genetics of man. In (F. Johnston, A. Roche & Ch. Susanne, Eds) Human Physical Growth and Maturation. Methodologies and Factors, pp. 221-242. New York: Plenum Press. Susanne, Ch. & Heyne, D. (1972). Stature, poids et eapacit~ vitale des 6tudiants et ~tudiantes de l'universit~ libre de Bruxelles. Bulletin de la Soci~t~Royale Belge d'Anthropologie et de Prghistoire 83, 101-105. Susanne, Ch. & Lepage, Y. (1985). Social factors and nutrition in secular trends in Belgium. In (G. Pelto, Ed.) Dietary Change. Mexico: International Union of Nutritional Sciences. Tanner, J. M. (1962). Growth at Adolescence. Oxford: Blackwell Scientific Publications. Tanner, J. M. (1966). The secular trend towards earlier physical maturation. Tijdschrij2 voorSociale Geneeskunde 44, 524-539. Tanner, J. M. (1975). Trend towards earlier menarehe in London, Oslo, Copenhagen, The Netherlands and Hungary. Nature 243, 95-96. Tanner, J. M. & Eveleth, P. B. (1975). Variability between populations in growth and development at puberty. In (S. R. Berenberg, Ed.) Puberty Biologic and Psychosocial Components, p. 256-273. Leiden: Stenfert Kroese. Twiesselmann, F. (1980). Populations prfihistoriques, historiques et actuelles de la Belgique et du Grand-Duch~ de Luxembourg. In (I. Schwidetzky, Ed.) Rassengeschichte der Menschheit, pp. 103-146. Stuttgart: Oldenburg Verlag. Vajda, A., Hebbelinck, M. & Ross, W. D. (1974). Size and proportional weight characteristic of Belgian children since 1840. Proceedings 6th International Symposium on Paediatric Work Physiology. Vercauteren, M., Susanne, C. & Orban, R. (1984). Secular evolution in Brussels between 1960 and 1980. In (J. Borms, R. Hauspie, A. Sand, C. Susanne & M. Hebbelinck, Eds) Human Growth and Development, pp. 215-224. New York: Plenum Press. Villerm~, L. R. (1829). Mfimoire sur la tailte de l'homme en France. Annales Hygiene Publique. 1, 351. Vlastovsky, V. G. (1966). The secular trend in the growth and development of children and young persons in the Soviet Union. Human Biology 38, 219-230. Wachholder, A. & Cantraine, F. (1974). Etude de la Pubertg. Param~tres Somatiques (Taille-Poids) et Sociaux des 556 Filles de L'Echantillon Belge. Centre International de l'Enfance, Compte-Rendu 64. Walter, H. (1977). Socio-economic factors and human growth. Observations on schoolchildren from Bremen. In (O. Eiben, Ed.) Growth and Development; Physique, pp. 49-62. Budapest: Akad~miai Kiad6. Walter, H., Fritz, M. & Welker, A. (1975). Untersuchungen zur sozialen Verteilung von K/Srperh6he und K6rpergewieht. Zeitschrifl fir Morphologie und Anthropologie 67, 6-18. Wieringen,J. C. van (1978). Secular growth changes. In (F. Falkner &J. M. Tanner, Eds) Human Growth, Vol. 2, pp. 445-473. New York: Plenum. Wolanski, N. (1967). The secular trend: microevolution, physiological adaptation and migration, and their causative factors. In (J. Kiihman, Ed.) Proceedings of the Seventh International Congress of Nutrition 4: Problems of World Nutrition, pp. 96-108. Wolanski, N., Jarosz. E. & Poyruk, M. (1970). Heterosis in man: Growth in offspring and distance between parents birthplaces. Social Biology 17, 1-16.
Living Conditions and Secular Trend
Laboratorium Antropogenetica,Free University Brussels, Pleinlaan, 2 1050 Brussels, Belgium
Growth and maturation status is generally accepted as a suitable indicator of the health and nutritional status of a studied population. T h e secular trend has been observed in all industrialized countries: general health and better nutrition seem to be major factors. An analysis of the secular trend in Belgium of height, weight and weight-for-height relationship shows that the effects of environment are different for these three measurements. Differences in secular trends in the various age groups show that in proportion to the attained height the secular trend does not differ from childhood until adulthood. An analysis of nutrition in Belgium during recent years shows some effects of the current economic crisis.
Keywords: secular trend, nutrition, weight, height.
Journal of Human Evolution (1985) 14, 357-370
1. Introduction The phenomenon known as secular trend corresponds to the physical maturation of children occurring more rapidly, and to the children becoming taller for their age. This secular trend has been observed in all industrialized countries: an increased degree of outbreeding, correlated to the process of urbanization has been postulated as its cause (Hulse, 1957). However, general health, better nutrition, and better living standards seem to be more important factors (for a review, see Van Wieringen, 1978). Since it has been postulated that physical growth is influenced by nutrition and social conditions, we are expecting differences in relation to the socio-economie status: such differences have already been reported by Villermfi (1829) and Qu~telet (1830) and by numerous authors afterwards (for a review, see Susanne, 1980a). 2. Secular Trend in Sexual Maturation Sexual maturation is of course characterized by the growth of genitalia and testes and the development of pubic hair in boys and by the development of pubic hair and breasts and the onset of menstruation in girls. However, most available data concern the age at menarche. For age at menarche, a differentiation according to socio-economic status has been often reported (for a review, see Tanner & Eveleth, 1975; Susanne, 1980a). It is well-known that a secular trend is observed concerning the period of puberty, with decreasing age at menarche (for a review, see Tanner, 1962). This has also been observed in Belgium (Figure 1). But, secular trend also means an earlier age at peak height velocity and peak weight velocity. Lindgren (1979) showed that children not only reach puberty earlier but also pass it more quickly. She also showed that the rate of the trend for the age at menarche has been greater than for PWV and PHV ages. As a higher genetical control exists for height than for weight (Susanne, 1971, 1975, 1977), it is reasonable to suggest that PHV age could be also more genetically controlled than PWV age. It seems then possible to suggest that PWV age has been more influenced by the evolution toward better nutrition and health than PHV age.
Perhaps age at menarche has also been greatly influenced by the improving standard of living: it is indeed obvious that in most countries an earlier age at menarche is related to 0047-24-84/85/040357 + 14 $03.00/0
(~) 1985 Academic Press Inc. (London) Limited
358
c.
SUSANNE
Figure 1. Secular trend of menarche, x: Sweden; 9 Belgium. 16 ~15
•
E "6 14
g
9
•
•
12
, 1900 , , , , 1880 1890 19' 10 19'2 0 1930 1940 19; 0 1960 1970 1980
better occupational status of the parents, to a higher quantity or quality of the food, to better medical care and to a tower number of siblings. This secular trend is observed to be very regular except in some countries where bad environmental conditions, related to times of war, are causing fluctuations or absence of this trend (Vlatovsky, 1966). We may thus propose that the onset of maturation is a multifactorial characteristic controlled by different factors. The environmental factors are numerous: delay of maturation is observed to be influenced by some diseases (Hauspie, 1980; Hauspie et al., 1979), socio-economic differences and nutritional status. Family size, degree of urbanization, and season (Jeurissen, 1970) are other possible factors. The process of maturation may differ according to the age at menarche: more anovulatory cycles would occur after an early menarche than after a later one. Genetic causes for differences in potential (or minimum) age at menarche are also predictable (Susanne, 1980b). As Johnston (1974) has concluded already research in the future has to be conducted following a multifactorial design. Physical growth and sexual maturation are relatively independent. O f course, the association of pubertal maturation with fatness, body build and height has been observed. But studies ofheritability have shown that puberty is relatively independently determined by genetical factors (Bock & Thissen, 1978). Environmental factors can influence growth and the onset of maturation also in a somewhat independent way or, as already suggested in the analysis by Lindgren (1979), this influence could be more strong on menarche than on weight, more strong on weight than on height. Bruntland et al. (1973, 1975) mentioned for Oslo children no change in age at menarche between 1950 and 1970, but an increase in height. Dann & Roberts (1969) and Roberts et al. (1971), in an analysis of the influence of socio-economic status, found no differentiation in menarchal age but clear differences in height and weight.
3. Height Children are taller today than in the 1940s or at the end of the 19th century. These results have been often reported and reviewed (Tanner, 1966; van Wieringen, 1978): this secular increase is observed at all ages of the growth period up to adulthood and started when industrialization began to improve our living conditions. This trend is also observed in Belgium (Figures 2 and 3). Let us remember that, although Belgium is composed of two different linguistic populations, Flemish and Walloons, no systematic differences are found between the two populations at the biometrical and the auxological level (Sporcq, 1969; Hebbelinck & Borms, 1975;
359
LIVING CONDITIONS AND SECULAR TREND Figure 2. Secular trend of height and weight of Belgian boys. 180 1 8 3 5 ~ .......... 9 1929~- . . . . . . 1 9 4 8 o - - - - - - - * Twiesselmann ( 1 9 4 9 ) 1960.--4 Twiesselmann ( 1 9 6 9 ) 1977z z Ooossens etctl.(1977)
170
160 // / P
150
60
140
_//
./-
.~':'"
,6,;', I I/'.,' J,'... /./,,# :.:
-~ 13o
A/i;:'
/
~,
J " -f '9-
50
,.
/,'/7
120
_ ~
I
i4o
9"'"
" v
3o
110 !
20
100
' ,..
i
..r Z ' - ' . ......
9C
8C :5
I 4
I 5
I 6
I 7
ilo
I 8
I 9
I t I I I 10 11 12 15 14 15
I I I I I 16 17 18 19 2 0
I 21
Age
Twiesselmann, 1980). Belgium is a country with a high population density, where the differences eventually observed between subcountries are lower than those due to sociobiological factors. The secular evolution of height in Belgium seems to follow another trend between 1880 and 1930 than after 1930, when the trend is more rapid than ever before. Taking into account only the adults, the trend is equal to 0"4 cm per decade between 1845 and 1930, 1"2 cm per decade between 1930 and 1950, and 2-0 cm per decade from 1950 onwards. This is very similar to results from the Netherlands where an increase of 1"0 cm per decade is observed between 1865 and 1953 and of2"l cm per decade between 1953 and 1970. The trend seems lower in the last decade (Vercauteren et al., 1984). In Belgium, at age seven the increase is about 2"3 cm each ten years between 1948 and 1977 for boys and 2"0 cm/10 years for girls. At 14 years for boys the secular trend is about 7'0 cm/10 years between 1948 and 1960, but 1.7 c m / t 0 years between 1960 and 1977. For girls of 12 years, the secular trend is about 3"8 cm/10 years between 1948 and 1960 a~d
360
SUSANNE
c.
Figure 3. Secular trend of height and weight of Belgian girls. 180 1 8 3 5 , .......... 9 1929-- .... 4 t 9 4 8 o-------o Twiesselmann (1949) 1960o-------e Twiessetmann (1969) 1977, - Joossens et al. (1977)
170
~.---_...'~_.~--o,
ap-'*
--
: : ~_-o -
160
/,",. -
150
140
50
.~130 -1-
120
40
110
30
100
20
~
q
lO
9O
80 5
I
I
I
F
I
I
4
5
6
7
8
9
I
I
I
I
I
I
F
I
I
I
I
10 11 12 13 14 15 16 17 18 19 20 21 Age
about 2"0 cm/10 years between 1960 and 1977. This difference is related to a retardation of the pubertal growth spurt in 1948 and results from the poor living conditions during the war. At 18-20 years, the secular trend is about 2"0 cm/10 years for boys and 2"2 cm/10 years for girls between 1960 and 1977. Very similar results have been published by Ljung et al. (1974) and van Wieringen (1978). If the global trend is rather linear, a detailed analysis of Dutch conscripts (van Wieringen, 1978) showed an arrest in the positive trend during different crises: a cholera epidemic in 1866--67, an agrarian crisis in 1888-96, World War I, the economic crisis of the 1930s, World War II, the unemployment in 1950-53. Van Wieringen postulated that all these unfavourable living conditions were present in a large part of the population and that their influence was especially marked during adolescence. Socio-economic differences influencing the secular trend are also observed: taking Belgian university students as representatives of a high social level, a trend of 1'0 cm each
LIVING
CONDITIONS
AND
SECULAR
TREND
361
12"5 years has been found for them while this trend for the whole Belgian population is equivalent to 1"0 cm each eight years (Susanne & Heyne, 1972). In Switzerland, the secular trend in the last two decades has been higher where boys were shortest (Molinari et al., 1984).
4. Weight A secular trend of weight has also often been reported and we would expect this trend to be proportional to the changes of height. The trend of the weight curves is however, relatively different from that of height in Belgium, at least from 1960 onward. In childhood and in adulthood, no differences are observed between the studies of 1960 and 1977, except for boys after 18 years. Moreover, the weight is higher in 1960 than in 1977 for girls between 11 and 16 years, the maximal difference being 2 kg around 12-5 years. Taking into account only the adults, the secular trend implies a regular increase of weight, although the trend is less strong for weight than for height. The trend of weight is clearly influenced by other factors than that for height in these last decades. Nutritional factors related to the fashion of weight control are probably involved: for instance, the mean weight of the Belgian female students at the university of Brussels has not changed since 1945, while an increase of stature has been observed.
5. Weight for Height Relationship The positive secular trend in height is not necessarily accompanied by a proportional weight increment. Figures 4 and 5 show indeed that Belgian children have increased relatively more in height than in weight. This phenomenon seems to be more pronounced in girls than in boys: it is only evident in boys with a height over 140 cm. A more detailed analysis of Figures 4 and 5 shows, however, that the samples from between t924 and 1960 do not differ concerning their height/weight relationship and that the tendency toward more linear children is evident only in the comparison between the samples of 1960 and 1977. However, at least for the lower ages the children of 1835 do not differ from the children of 1977 in their height/weight relationship: in 1835 the children also have a lower weight for height than in 1929 and 1960. Similar results have been observed by Ljung et al. (1974) by means of the same diagram method, and by means of indices such as W / H 3, W/H 2 and W / H by Broman et al. (1942), Wolanski (1967) and Vajda et al. ( 1974). A proportionally larger increase in height than in weight has been observed in Sweden between 1883 and 1971 resulting in thinner children (Ljung et al., 1974). From 1938 onward, this tendency seems to have been more pronounced in girls than in boys. Himes (1979) and Malina (1979) have summarized studies where, on the contrary, increases in weight for height have been observed such as in the United States, England, Canada and Australia.
6. Differences in Secular Trends in the Different Age Groups The secular trend observed during the growth period is of course a function of the age period which is taken into consideration. The largest shifts of stature and weight are observed during puberty, because they result also of a trend toward earlier maturation.
362
C. SUSANNE Figure 4. Secular trend of the weight/height relationship for Belgian boys.
70
60
1835-
..........
9
1929=-
.....
4
1948 ~--n---o
Twiesselmann ( 1 9 4 9 )
-- - .-o Twiesselmenn (1969)
1960--
1977:
: doossens et o/.(1977)
50 /
J
/
/
#
./
4o
30
20
10 80
I 90
,
I 100
I
I I I 120 140 Height ( c m )
160
180
It can be asked whether the increase in height is proportional to the attained height in the different age groups. We tried to answer this question by calculating the ratio between the differences of the means of stature in two successive periods of time (tl and t2) and the mean of stature in the first period in different samples: 't2 -- X'tl Y--
Xtl
Most of these new values, the secular trend of height in proportion to the attained height, are relatively constant during infancy and puberty (Figure 6). During that period of life the observed increase due to the secular trend is proportional to the stature attained at the specific studied age. Sometimes, the secular trend relative to the attained height is somewhat higher during puberty (in the Polish, Swedish and Venezuelean samples). The trend observed in the Dutch samples of the years 1965 and 1980 is exceptional: the values of the secular trend divided by the height attained in 1965 increase from birth to puberty. These observations concerning the proportionality of the secular trend for stature could be interpreted as a result of the fact that the secular trend is manifest rather specifically
363
LIVING CONDITIONS AND SECULAR TREND
Figure 5. Secular trend of the weight/height relationship for Belgian girls. 60 ,4
1835,," .......... 9 1929"
50
.....
1948.-------e 1960e-
4
/" Twiesselmann (1949)
-- -- -~ Twiesselmann (1969)
1977z
: Joossens
etal,(1977)
40 v
.E
30
20
d 10 80
I 90
I 100
I
I , 120 Height (cm)
I 140
I
1 160
during the first years of age, probably from birth to three years, which corresponds of course to a period of great growth velocity. In a similar way Garn et al. (1984) give the magnitude of the socio-economic effect on size, expressed in scores for an American sample: the greater part of this social effect is already reached at the age of one year. T h e last results from the Netherlands' survey do not correspond completely to the other observations, and it will be interesting to follow the publications from our colleagues van Wieringen and Roede. In this recent sample of the Netherlands, on the one hand the effects are observed of the recent fashion to give less rich food to babies and young children resulting in a lower height in 1980 than in 1965 until two years and in a lower weight until four or five years. O n the other hand we probably observe the effect of a decrease of the secular trend in the Netherlands, not yet observed at adulthood, but manifest before puberty.
7. Nutrition Nutritional and hygienic factors have a quite obvious influence on growth, they wi,ll be specially important during the first three years of life. Protein-energy malnutrition of young children is of course due to ~the impossibility to
364
(3. SUSANNE Figure 6. Differenceswith age of the secular trends ofheight in proportion to the attained height in the different age groups. • boys; O: girls.
Age 0
1
2
3 4
6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
2 1i
oL z#
•
9Z
~/
x 9
•
X
Belgium 1960-80 Poland 1955-78
1OF-21-
9
9
X
X
~
Sweden 1883-1939
9
0--
1
Sweden 1939 - 7 1
0 2 1-
9
X
0
Venezuela 1948-76
x"
2
D.D.R. 1921-75
1 04
0 2 1
D.D.R. 1944-78
X
0 2 1 0 -1
v
9 X
).
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)4 X
X
Netherlands 1965 -80
X
t~ li
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il
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0 2' 1
Netherlands 1955-65
0
purchase sufficient food for the child or to an inadequate nutrition and health education. Growth not only depends on the intake of food, but also on its digestion, absorption, utilization and excretion: therefore, the general health and hygienic situation of the child are of considerable importance. The increase in height being greater from the 1950s onward than it was before, the relative increase in weight being lower from the 1960s onward, because of the changing patterns in the fashion of weight control, it seemed interesting to analyse the evolution of food intake in Belgium during this period. From t h e O C D E data, Figure 7 illustrates the changes occurring in Belgian food
365
LIVING CONDITIONSAND SECULAR TREND Figure 7. Changes in Belgian food consumption between 1955 and 1978.
200
1955 Bose 100
1.%0
~
Cheese
~
~ ' ~ ' ~ - ~
~Vegetables
0 I I I I I 'I I I I I I I P I I I 55-59 6 4 6 5 66 67 68 69 70 71 72 T3 74 75 76 T'F ?'8
consumption between 1955 and 1978: in the first place, a considerable increase in the consumption of milk products, meat and fruits is observed; simultaneously the consumption of potatoes and cereals is decreased. For vegetables, oils and fats a limited increase is observed (Susanne et al., 1985). An analysis of nutrition in Belgium during recent years (1975 to 1982) shows some effects of the economic crisis (Susanne et al., 1985). For instance, the consumption of meat did not change very much in absolute quantities, a slight decrease is observed from 1980 onward (Figure 8). However, various patterns are observed in the consumption of the different kinds of meat: a decrease in consumption of pork and beef, but an increase of mixed meats (minced meats). " The amount of money dedicated to food increased during the last decades but the percentage of a person's income spent on food decreased in time during .the last decades (Table 1): from 35"8% in 1961 to 20"9% in 1978-79 for workers. This variation in time is a sign of an environmental improvement and could be important enough to explain a secular trend. However, the social variation in the expenditure on food is not great: in 1978-79, it varied between 105,095 Belgian francs yearly for clerks and 125,879 Belgian francs for liberal professions. Such little social variation may result in a decrease in the socio-economic differentiation of height and weight. In other samples social differences do, however, exist: poorer boys and: girls appear to have lower serum and urinary vitamin levels, their globulin levels are higher (more exposure to infection), and their exposure to pollutants is greater (Garnet al., 1984). The research of the influence of life style on nutritional status and of the association of both with growth should be promoted. 8. Discussion We must realize that comparisons of cross-sectional studies across decades, and between populations have limitations at the level of sampling and methods of measurement.
366
C. SUSANNE
Table 1
Amount of money dedicated yearly to food consumption, absolute and as a percentage of the total salary
Years 1961 1973-74 1978-79
Workers Amount 37 244 BF 75513BF 107 023 BF
%
Clerks Amount
35"8 24'7 20"9
40 764 BF 70 975 BF 105 097 BF
Figure 8. Changes in Belgian consumption of meat between 1975 and 1982.
%
Farmers Amount
26"9 16-8 15'8
-81 809 BF 109 890 BF
%
Liberal professions Amount
%
30"1 20'6
83 363 BF 125 879 BF
19-7 14'3
1975
/"Mixeci" meat (minced)
140
150
120
110
\ ./
X
,\
~
~
Pork-butcher's
v ~ m e o t
Pork
100
9C
~" Beef I
I
I
Therefore the fact that we observe in our study similar results for different samples and populations gives some significance to these results. It is clear that growtl~ and development are determined by genetic factors (Susanne, 1980b) and that they are also strongly influenced by environmental factors. Growth and maturation status is generally accepted as a suitable indicator of the health and nutritional status of a studied population. The most important environmental factors in this respect are disease and nutrition, but many other factors may have a direct or indirect influence such as a guaranteed minimum income, social security, transportation, pure drinking water, sewerage, proper food preparation, the disappearance or diminution of epidemics such as bovine tuberculosis, good sanitary conditions, health care, national health assurance, immunization against childhood diseases, medical care and education, absence of heavy work by children, the limitation of the number of children per family, and maternal efficiency. It is quite clear that at the level of one of the primary factors, nutrition, even in Belgium where in 1960 the essential foods were available in quantity and quality, the situation improved between 1960 and 1982: the over-all consumption of meat, milk products and fruit increased. However, the predominant influence of environmental factors in a
LIVING CONDITIONS AND SECULAR TREND
367
multifactorial and polygenic system does not exclude that some differences between populations or between individuals from the same population may be of genetic origin. In the explanation of the secular trend the same controversy exists: genetic and environmental theories have been postulated. The genetic theory supposes the existence in human populations of a kind of heterosis, observed in reality only in crossings between heavily inbred plants or animals. The slow disappearance of some marriage barriers and endogamy, and the correlated increase of the number of heterozygotes is supposed to contribute to the positive secular evolution. However the hypothesis is still ambiguous: the differences between exogamous (outbred) and endogamous (inbred) populations have to be controlled for the socio-economic conditions and for all related environmental factors: only a research with a multifactorial design can bring us clear results. Different authors have observed an increase in stature in countries where exogamy increased (Hulse, 1957), where consanguinity diminished (Schreider, 1967) or where the distances between partners increased (Wolanski et al., 1970; Billy, 1971, 1979). However, in the interpretation the socio-economic factors have seldom been kept under control. Hulse (1957) in his classical study of Swiss villagers, indicated the existence of differences in family size and cultural differences between the inbred and outbred groups. However, he did not take these differences into account. Other authors using marriage distances do not take into account that these distances are increasing with the socio-economic level and that the endogamy is also varying in the different professional groups (Marreyt, 1975; Claeson & Egero, 1972; Coleman, 1973; Kfichemann et al., 1974; Harrison et al. 1971) even in homogeneous countries, e.g. essentially rural populations. On the other hand, various authors did not confirm the effects of heterosis (Barret & Brown, 1971; Crognier, 1973; Marquer, 1979). Even when the influence of consanguinity on multifactorial characters such as height and weight is studied, the influence of socio-economic factors could still be relatively more important than that of consanguinity itself. The results of the influence of consanguineous marriages are in any way not very evident: a slight negative influence is observed by Morton, (1958), Schull (1962) and Schreider (1967) but not by Schork (1964). Although not contradictory but essentially complementary to the genetic hypothesis, the environmental hypothesis has a stronger basis. In the industrializing countries a manifest improvement in living conditions has been observed: the positive secular trend started at the moment when this improvement began, with the inception of industrialization. The environmental factors seem to affect more groups, living in unfavourable conditions or at a lower socio-economic level: on the one hand a diminution of the secular trend for the lowest statures has been observed in periods of crisis (van Wieringen, 1978; Brodar, 1978) and on the other hand a lower secular trend has been observed in higher socio-economic groups, such as university students, than in the general population (Susanne & Heyne, 1972; Aubenque, 1957). The secular trend in weight is less intense than that in height and it is sometimes even absent in girls: the weight of Swedish girls in the age 15-16 is lower in the period 1965-71 than in the period 1938-39 (Ljung et al., 1974); the weight of Belgian girls studying at university has not increased since 1945 (Susanne & Heyne, 1972). Such a discrepancy between the sexes may best be explained from a cultural point of view. Homologous results have been observed by Lindgren (1976) who found that girls from the lowest socio-economic classes have significantly more weight for height than girls from the higher classes. Boys show the same tendency, but weaker and not significant.
368
c. SUSANNE
Similar multifactorial processes are at the origin of the evolution of the proportion of weight for height in the last decades toward more linear children. The trend of the increase in height is proportionally stronger than that in weight. The analysis of the ratio mean height/mean weight showed us that this tendency to become thinner is true for both sexes, although tess marked for boys. The origin of this trend could be a better nutrition or better balanced diets, but perhaps also a change of the fashion to more slim and well-trained bodies for women and men. In some countries, positive secular changes are coming to an end (Brundtland & Wallo~, 1973; Dann & Roberts, 1973; Tanner, 1975; Walter et al., 1975). It can be questioned whether this observation is in favour of the genetic or the environmental hypothesis. In the genetic hypothesis we can postulate that the positive secular changes would have reached a biological and genetic maximum, but it seems more probable that the slower trend would result from a stabilization of the living conditions at the socio-hygienic level. In some populations, the differences expected in relation to the socio-economic status are also absent: typical of this situation is the study of Lindgren (1979) who did not find differences between socio-economic classes defined by the father's occupation and family income in a sample of Swedish children collected during the period 1965-71. This absence of socio-economic differences is an argument in favour of the hypothesis of a stabilization of the living conditions in some populations and thus favourable to the environmental hypothesis. Indeed, if this hypothesis is correct, a trend toward equality of the standards of living in the different socio-economic classes may result in a diminishing disparity in the growth rates of different socio-economic strata. The results of Walter et al. (1975) on German children aged six to 11 years and Waiter (1977) on German children aged 9"5 to 10"5 years also show this absence ofsocio-economic differences for height and weight. For menarchal age too, the absence of social effects in modern industrial countries has been observed (Brundtland & Wallofi, 1973; Degnbol, 1964; Furu, 1976; Richter, 1964; Jenicek & Demirjian, 1974; Kantero & Widholm, 1971; Roberts et al., 1971; Wachholder & Cantraine, 1974), as well as for skeletal maturity (Roche et al., 1978). In these countries, nutritional and hygienic factors, responsible for an optimal biological development are nowadays generally present. These nutritional and hygienic factors would be of primordial importance during the first three years of life, also for the establishment of a secular trend. A specific increase of the secular trend in proportion to the attained stature is observed during these first three years. The relative changes are less important afterwards.
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