20 Secular Trends

Ms. Shumayla and Dr. Rashmi Sinha

Content:

1.      Introduction

2.      Positive Secular Trend

3.      Negative Secular Trend

4.      Absence of Secular Trend

5.      Possible causes of Secular Trend

a.  Interrelated aspects contributing to secular trend

6.      Summary

Objective: To understand the term “secular trend” and its type.

1. Introduction

Changes in body size, fatness, rate of growth, and timing of maturation have been occurring over the past 150 years. We also are witnessing a decline in mortality, an increase in life expectancy and a large increase in the proportion of the population that is elderly in both industrialized and developing countries.

The attainment of large size and acceleration of maturation over several generations are collectively labeled as the secular trend.

The term “secular trends” refers to differences among groups within a population that are explained totally or largely by differences in birth dates. If the range of birth dates is less than about ten years, any observed growth differences that are related to birth dates are more likely to be due to sampling variations than to secular trends. Exceptions to this statement may occur, however, when the environmental influences on growth change very rapidly, as can occur during wars and famines. It includes several trends-increases 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 phenomenon 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.

Over the last two centuries, secular trends in child growth have generally taken the form of increases; that is, children at a particular age are larger and more mature now than in the past. Such secular trends are commonly referred to as “positive”. Similarly secular trends can be negative and absent as well.

2. Positive Secular Trend

Nowadays, 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 in Japan, Cuba, Brazil, North America and Taiwan. They found that, following World War II, 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 (approx. 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 trends 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 to Higher socioeconomic 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 trends is much more rapid than these classic cases. The ‘Maya in Disneyland’ showed a rapid change in amount and rate of growth. In less than one generation, the Guatemalan Maya refugee children living in Indian town, 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 traits (Bogin and Loucky, 1997). Maya children were compared with three different ethnic groups: the Whites, the Blacks Mexican –  Americans. 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 different in body composition measure such as arm fat area muscle area. Thus, in terms of energy and protein Maya children appear to be generally healthy and well-nourished but why shorter (Bogin, 2001)

The plasticity of human phenotypes changes at different rates for different traits. In most studies referring to positive secular trends, the increase in mean height from 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. In case 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, 2001).

A secular trend among Indian adult population has presented a comprehensive, empirical descriptive of mean height differences and the underlying variation among adults 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, albiet modest, compared to changes shown in nations experiencing economic transitions whereas adult stature increase over the past decades varied between 0.3 and 3 cm per 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).

3. Negative Secular Trend

The direction of secular trend when get reversed is termed as Negative secular trend. In this phenomenon children are shorter and lighter, mature later than previous generation. 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.

Secular trends during 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 of onset of pubertal growth spurt and the age at peak height velocity during puberty in four decades (1930-1969). The age at onset of pubertal growth spurt declined significantly by 0.2 and 0.4 years in girls and boys, respectively, whereas age at peak height velocity declined by 0.5 and 0.3 years in girls and boys respectively (Aksglaede et al., 2008). A longitudinal study on Indian children and adolescent enrolled in Sri Aurobindo International Centre of Education (SAICE), has also revealed decline in the age at onset of pubertal growth spurt and at peak height velocity in girls over four decades (1950-89) whereas the same parameters were constant in boys (Virani, 2005).

The first strike of menstruation that is age at menarche is closely related to the general process of development and is rigidly constrained by biological limits. In Poland, age at menarche declined from 1955 to 1978 by about 4.15 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 years and city 13.4 years and in 1978 these mean changes to 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 to cities (Hulanika and Waliszko, 1991). Studies from India on menarcheal age in 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-economi, nutritional and general health conditions in India as compared to these countries where similar standards were achieved much earlier (Bagga and Kulkari, 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, pastoralism to daily wages laborers (Biswas and Kapoor, 2004).

4.    Absence of Secular Trend

As the word ‘absence’ depict, it is the nonexistence of any kind of secular trend. For instance, 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 such environmental conditions that have not significantly or sufficiently improved or impoverished over time to induce positive or negative trend respectively (Malina et al., 2004). 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 cah crops in part 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).

5.    Possible causes of Secular Trend

Secular trend has occurred worldwide. The phenomenon is of great interest for the researchers and scientists. What causes these secular trends? Some researchers believe that a general improvement in nutritional, better sanitation, better health services and less tedious lifestyle may be the possible causes. These factors have permitted individuals to come closer to the genetically determined potential weight and stature. Today a leveling off the secular trend appears to be occurring among the higher socio-economic urban population. It must be pointed out here that all positive secular trends are not desirable. For example, increased level of adiposity without corresponding increase in height noticed in some parts of the world specially in developed parts are associated with increased prevalence of obesity, particularly among children and adolescents, which is known to be a health risk.

5.1 Interrelated aspects contributing to secular trend

Elimination of growth inhibiting factors such as epidemic and endemic disease or reduction in their incidence rate, a condition conductive 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.

Favorable economic conditions as indexed by a blend of family income, occupation, education level of parents, deciphered as improved living conditions and nutrition for growth 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 increases migration and interclass mobility, but changes associated with heterozygosity are rather small in the context of the major changes that characterized secular trends.

6.    Summary

Secular trend is not an overnight process it takes years to occur. We assume that the secular increase in size and earlier maturation is a result of improved environmental conditions within a population. These have led to improved health status of the population as a whole. Similarly, secular decrease in size or delayed maturation may come about because of negative conditions. But the reasons remain only partially uncovered. Quite assuredly, many factors help children to more closely approach their full genetic potential. Among these are improved nutrition, more widespread health and medical care, higher education level of mother (maybe the father, too), better housing and living conditions, better transportation, clean water, improved sanitation, immunization programs and population mobility, both geographically to urban areas and socially upward.