The attainment of larger size and acceleration of maturation over several generations are collectively labeled as the secular trend. It includes several trends-increase in height and weight during childhood and adolescence, reduction in the age at menarche and reduction in ages of attaining other indicators of biological maturity. These are reversible and complex phenomena that reflect the remarkable sensitivity, or plasticity, of the growth and maturation processes to the environmental conditions under which children and adolescent are reared (Malina et al., 2004). The period of secular changes evident in different population varies in response to differential rates of improvement in environmental influences related to nutrition, physical activity, socio-economic status and health.
In a review of secular trends taking place in industrialized nations of the world, Hauspie et al. (1996) offer the following overview:
“Secular changes in body size and tempo of growth have occurred during the last century in almost all the industrialized countries and have been well documented almost all of the secular increase in adult height is established during childhood . The secular increase in body length is due predominantly to an increase in leg length …the legs are the fastest growing part of the body during early childhood when the impact of the environment is greatest …..Secular changes in body dimensions has occurred simultaneously with the secular change in tempo of growth, as shown during the last 100 years by an advancement of age at menarche and at peak height velocity of about 3-4 months per decade in most European countries ….The higher tempo of growth has resulted in adult height being reached at an earlier age for males adult height is now reached at about 18 years but in the 1910s was only reached at about 26 years of age … “
Secular trends can be: Positive , Negative and Absent
Positive Secular Trend
In the contemporary period, children on an average are taller, heavier and mature earlier than those of several generations ago indicating positive secular trend. Haupsie and colleagues (1996) reviewed the evidence for these secular trends in 17 nations including many European countries as well as Japan, Cuba, Brazil, North America and Taiwan. They found that, following World War Il, the Japanese experienced strongest secular trend so far recorded for an entire nation. In 1950 the mean height of Japanese young adult men was 160 cm, whereas in 1995 it was almost 172 cm. Over the period of 45 years height increased at an average rate of 2.67 cm per decade. The rate of change was much faster in the first decade (about 4 cm) than in the last decade (about 1 cm). In contrast, the rate of secular increase in height in Sweden and Norway, between 1952 and 1985, was only 0.3 cm per decade.
Positive secular changes in the height of children and adolescents are largely, but not entirely, related to earlier maturation, as evidence by adult trend in comparison with pre-pubertal children (Cole, 2000; Krawczynski et al., 2003). Concomitant to the positive secular trend in height and dramatic increase in childhood weight, increased prevalence of obesity has been widely reported in almost all developed and developing countries in the last two to three decades. Pacific have the highest prevalence of overweight among children (approximately 20-30%), whereas parts of South East Asia and much of Sub-Saharan Africa appear to have the lowest values (Wang and Lobstein, 2006).
Secular trend takes place not only within a region or country but also with migration of individuals from one place to another, which may be associated with the movement from low socio-economic status (SES) to higher SES or in situ socio-economic improvement (Bogin, 2001). The ‘migration effect’ on secular trend can be substantiated by the classic work of Boas (1940) with European children. These children experienced increase in height when they migrated to United States. Japanese immigrants to Hawaii were significantly taller than their parents residing in Japan (Shapiro, 1939). Greulich (1976) showed that Japanese children born and brought up in California (America) grew to be taller than Japanese children born and brought up in Japan. Follow up study of these same population shows that the growth in height of each generation of the children of migrants continues to increase until it converges on that of the host population (Roche, 1979).
Sometimes the rate of the secular trend is much more rapid than these classic cases. The ‘Maya in Disneyland’ showed a rapid change in amount and rate or growth. In less than one generation, the Guatemalan Maya refugee children living in Indiantown, Florida and Los Angeles became 5.5 cm taller, on an average, than their age mates at Guatemala. These evidences suggest that the plasticity of human phenotype changes at different rates for different trait (Bogin and Loucky, 1997). Maya children were compared with three different ethnic groups: the Whites, the Blacks and the Mexican-American . They were shorter than other groups and weighed less than the Whites and the Blacks but did not differ significantly from Mexican Americans. There was no ethnic difference in body composition measures such as arm fat area or for and muscle area. Thus, in terms of energy and protein Maya children appear to be generally healthy and well-nourished but why shorter (Bogin,2001)?
This evidence shows that the plasticity of human phenotypes changes at different rates for different traits. In most studies referring to positive secular trend, the increase in mean height fr m generation to generation lags behind increases in weight and body composition. This could be explained as height reflects health and nutritional history, weight and body composition reflect recent events (Taylor and Bogin, 1995). With reference to the ‘intergenerational effect hypothesis’ a child’s height is an historical record of both the individual and his or her parents. Incase of the Maya refugee, the effects of chronic undernutrition and disease suffered by the parents are still being expressed in the growth of their children. Conversely, children who are better nourished and healthier will give their own offspring a healthier prenatal start in life (Bogin, 200 I).
A review of secular trend among the adult Indian population has presented a comprehensive, empirical description of mean height differences and the underlying variation among adult in India across diverse socioeconomic, demographic, and geographically oriented groups as well as birth cohorts. It has indicated an increase of 4.5 cm per decade in men’s height, which is similar, albeit modest, compared to changes shown in nations experiencing economic transitions whereas adult stature increase over the past decades varied between 0.3 and 3.0 cm/decade across Western countries. This is in contrast to women in India who have experienced little growth in height with increases in year of birth (0.1 cm per year). Although there has been increased economic growth in India, there is considerable evidence of little improvement in nutrient intake, especially in rural areas (Perkins et al., 2011).
Negative Secular Trend
Children shorter, lighter and maturing later than previous generations are indicative of negative secular trend. A discriminant incidence of the negative trend comes from Guatemala during the period from 1974 to 1983, a time of intense civil war and political repression. The cross-sectional samples of 10- and 11- year old boys and girls from families of high, moderate and very low socio-economic status revealed discernible decline in the mean stature (Bogin and Keep, 1998). A general deterioration of the quality of life in Guatemala, especially the quality of nutrition and health of the population seems to be the major cause of negative secular trend.
Puberty is an important milestone in reproductive life and secular changes in the timing of puberty may perhaps reflect the general reproductive health of a population. Recently, a large cohort of Danish school children has demonstrated secular trends in the age at onset of pubertal growth spurt (OGS) and the age at peak height velocity (PHV) during puberty in four decades (1930-1969). Age at OGS declined significantly by 0.2 and 0.4 years in girls and boys, respectively, whereas age at PHV declined by 0.5 and 0.3 years in girls and boys, respectively (Aksglaede et al., 2008). A longitudinal study on Indian children and adolescents enrolled in Sri Aurobindo International Centre of Education (SAICE), has also revealed decline in the age at OGS and at PHV in girls over four decades (1950-89) whereas the same parameters were constant in boys (Virani, 2005).
Age at menarche is closely linked to the general process of development and is rigidly constrained by biological limit. In Poland, age at menarche declined from 1955 to 1978 by about 4.15 months per decade for girls living in villages and towns. For city girls the decline was 3.0 months per decade. Despite the higher rate of decline for village and town girls, the city girls have always had the earliest mean age at menarche. In 1955, the mean ages were: village 14.3 years, town 13.9 year and city 13.4 years and in 1978 these mean ages were 13.5, 13.1 and 12.9 years, respectively. The differences were attributed to the lower quality of nutrition, health care and greater physical labors in town and villages compared with cities (Hulanika and Waliszko, 1991). In India, an increase in stature and decrease in age at menarche was reported when daughters were compared with their mothers thereby indicating the presence of secular trend in the Punjabi Arora females (Khanna and Kapoor, 2004). Studies on mean menarcheal age of Maharashtrian girls, from 1960s onwards to 2000, have shown a consistent decline of age at menarche on an average, by about 6 months per decade as compared to 3-4 months in some countries of Europe, North America, and several parts of the world. It reflects the improved socio-economic, nutritional and general health conditions in India as compared to these countries where similar standards were achieved much earlier (Bagga and Kulkarni, 2000). The Saharia, a primitive tribal group of Madhya Pradesh, depicts lower mean age at menarche among daughters (13.3 years) than their mothers (13.5 years). This may be attributed to the improved socio-cultural life as a consequence of shift from traditional practice of cultivation, hunting, gathering, pastroalism to daily wages(Biswas and Kapoor, 2004).
Absence of Secular Trend
A population may have attained or is near its genetic potential for height and timing of maturity, so further changes may not be possible. On the other hand, a population may be living under environmental conditions that have not sufficiently improved or impoverished over time to induce positive or negative trend, respectively (Malina et al., 2004). For example, the persistence of traditional agricultural practices, relatively poor farmland and limited economic resources for the improvement of agriculture is characteristic of many rural areas in developing countries. Lifestyles are also changing in some rural areas, resulting in a shift from subsistence farming to cash crops in parts of Latin America or to a ‘depastoralized’ lifestyle in some parts of Africa (Malina, 1990). Crowded living conditions, especially in rapidly growing urban slums, inadequate or marginal nutrition, disease and associated social stresses persist in many developing countries. These conditions have similarities to those that were persistent in Europe in the 18th and 19th centuries, during which no secular trend was observed (Malina et al., 2004).
The interrelated aspects contributing to secular trend may include:
- Elimination of growth inhibiting factors such as epidemic and endemic disease or reduction in their incidence rate, a condition conducive to the transmission of infectious microbes. With reduced infectious disease load, energy and nutrients that would have diverted to ward off infection would now be available to support the cellular processes of growth and maturation.
- Favourable economic circumstances as indexed by a blend of family income, occupation, education level of parents, deciphered as improved living conditions and nutrition for growing and maturing infants, children and adolescents. Thus, improved nutrition and public health work synergistically with reduced infectious disease load.
- Genetic changes such as population admixture or increased outbreeding have occurred with increased migration and interclass mobility, but changes associated with heterozygosity are rather small in the context of the major changes that characterize secular trends.