Environmental quality, developmental plasticity, epigenetics and ...

Environmental quality, developmental plasticity, epigenetics and physical ...

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CHAPTER 15

Environmental quality, developmental plasticity, epigenetics and physical growth in a riverine population of Northeast India Mithun Sikdar* Abstract Human growth as a matter of concern is expected to have a wide exposure from different field of studies including anthropology, economics, human biology, mathematics, pediatrics and sociology due to its practical values in terms of human development. The attainment of optimal growth status for a particular population group is always regarded as a marker for human development index. However the growth trajectories of different population groups are achieved on the basis of their mutual interaction with the genetic basis as well as the surrounding environment with which they interact in their day to day life. The present write-up discusses on the interaction of the environment with human growth outcome among a tribal group of Assam, Northeast India. The outcome of the study reflects the impact of the quality of life in their physical growth performance. Environmental hazard such as flood and its associated confounding factors in Northeast India may be viewed as a determining factor for the suboptimal physical growth among the children particularly at their infant stage. But migration from the flood plain areas to more convenient places gave better opportunity for livelihood, economic transition as well as medical care. However the transition in their lifestyle can be viewed in terms of developmental plasticity to which the people is going through and also warrant us about the future risk of metabolic disease in the coming years. Here I put forward *

DNA laboratory Unit, Anthropological Survey of India, Western Regional Center, Udaipur, India

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Human Growth: The Mirror of the Society

environmental shock hypothesis which may explain the recent trend of increasing metabolic diseases in several countries. Research query Physical growth as referred to ‘the mirror of the society’ always put some light on the environmental quality where we live. Infact measuring growth has become one of the important tool to evaluate the health as well as economic prosperity. This conjecture has been supported by well known economic historians like Kolmos (1994), Fogel (2004) who consider body size as a mirror of economic affluence or collapse (Hermanussen, 2013). Quality of environment is usually regarded as a paramount precursor for human health and particularly achieving the normal course of physical growth one is expected. The geographical area, the health and hygiene condition of the surroundings, the psychological stability, the family setup all constitute the environmental setup where we are bound to grow to a mature adult. Since birth we start maturing to keep pace with the standards of physical growth balancing our genetic makeup as well as environmental quality. Genetic makeup seldom shows any discrepancy but the environment always has a unique pressure to make our growth trajectories either positive or negative. While talking about a particular population, environment always have its distinctive essence of work. Bio-cultural variables are always in a queue to show their impact. Therefore working on environmental quality for its impact on physical growth always adds number of new dimensions to our present knowledge. The present piece of note is an addition to what we have gathered from our decades of research on human growth and what more can be expected in this particular domain. The study concentrated in the remote localities of Northeast India where a particular tribe called “Mising” used to reside. Mising is a scheduled plain tribe of Assam, a State that comes under the conglomeration of Northeast India as a whole. Existence of cultural, ethnic as well as linguistic diversity in Northeast India always remained a pivotal area to attract number of researchers to come in its close contact. The unique nature of every tribal population of Northeast India also attracted different academicians form time to time. Mising with its unique course of history also remained a tribe of particular interest for their unique adherence to genetic as well as linguistic preservation from time immemorial. Their course of habitation beside the river bank also makes them unique in Assam province. They are particularly dwelling in the riverine course of Assam using thatched


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house that is made up of stilt. Legendary history says that they were hill dwellers and lived along with the Adi tribal population of Arunachal Pradesh. They are reported to have migrated to the plain region of Assam way back in 13th Century A.D. There are different views regarding their preference in choosing riverine areas. According to Hamilton (1983), the alluvial land near the river for agricultural purpose, grazing spaces for their domestic animals, water facilities etc are some of the factors for settling in the river side. They still retain all of their mythological, linguistic and institutional affinity with the tribes like Adi and Nishi of Arunachal Pradesh. When they came down from the Northeastern Hill region to the plains they came in close contact with the Chutiya people who had a kingdom around Sadiya town, Assam. When the Chutiya kingdom was invaded and subsequently conquered by the Ahom population (1376 A.D.-1500 A.D.) it is said that few nobles with their families took shelter in the Mising villages and displayed feigned identity as Mising. After the end of Chutia kingdom, Sadia-Khowa-Gohain, a representative of Ahom Kingdom, persuaded the Mising community to come to an agreement and they were asked to help the Ahom kingdom to resist the attack from other tribal population groups. Twelve chiefs (Baro-Gam) were appointed for that from main core Mising villages and ten chiefs (Doh-gam) from other villages. Thus the Mising population has a distinct place in the history of Assam and since then they are living happily beside the course of the river Brahmaputra and its tributaries. They are fully endogamous group and hardly interbreed with neighbouring communities. Their house pattern supports the riverine life and they are adapted to it since long time. But due to some confounding anthropogenic factors different environmental hazards are coping up in Assam since the last century. Earth quake is one of the foremost hazards that took many death tolls in 1897 and 1950. Flood is another environmental hazard that is taking a heavy toll perennially in due course of time. The occurrence of perennial flood along the river course causes the Mising population suffering from several socioeconomic problems. As a matter of fact one group of Mising population started shifting from their traditional riverine setup to settle in the areas those are away from riversides. This process started since long and this new group of Mising population is mainly settled in different villages of Dibrugarh and Sibsagar Districts of Assam and is engaged in different form of livelihood. It has been observed that change of habitation sites brought about considerable changes in their lifestyle specially house pattern, source of water, food habits etc. They have

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also become close to the modern health care facilities as well as educational level. Altogether, different social surroundings are visible within the two setups. In the foregoing pages Mising children are hypothetically divided into riverine and non-riverine on the basis of their habitation sites. The main focal point to be discussed is the impact of environmental condition on health particularly on child growth and the biological adjustment that are taking place within these two different environmental setups as a result of developmental plasticity. The present study has been segmented into three parts. The first part is on ‘environmental condition and child growth’ to decipher the environmental quality in terms of growth variables; the second part is about ‘environmental quality and stunting’ to have a look into the growth deficit in these two environments. The third part is about ‘recent migration and developmental plasticity’ to see the impact of migration on physical traits.

Figure 15.1 Map of Assam depicting the study areas and geographical setup

Environmental condition and child growth To evaluate the environmental homogeneity of the two different setups i.e. the riverine and non-riverine I have used the variability in the rate of


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growth as a proxy measure. As per Harrison and Schmitt (1989), the quality of human environment can be better assessed through the variability rather than the mean size or mean velocity of growth. Various experimental studies on animals (Harrison, 1963) and children from different socioeconomic background (Bogin, 1991) supported this hypothesis. It is already stated that boys are more eco-sensitive than girls (Bernis, 2003). Therefore to test the hypothesis a cross-sectional study has been carried out among 492 Mising boys from the two environmental setups (256 Riverine, 236 Non-riverine). Children from 6 to 10 years of age are taken into consideration using standard technique cited elsewhere (Sikdar, 2010). Co-efficient of variation has been computed for the parameters following Bogin (1991). Children from older age groups are also available for analysis but were not included because of the within sample increase in growth variability that occurs at adolescence due to difference in growth timing (Bogin, 1991) and moreover the present datasets represents a cross-sectional study. The Co-efficient of variation of the NCHS samples is taken from CDC growth chart and plotted as a reference (Kuczmarski et al., 2000). The plotted curves for CV of height increments have been shown in Figure 15. 2. The pattern of age distribution in the CVs shows that the riverine children have higher CV than the non-riverine as well as NCHS data up to 8 years of age. Being a ratio CV is not suited for analysis by inferential statistics (Tanner,1951). But here a mean CV (weighted for age) is computed for the whole group of sample following Nei et al., (1975) and the standard errors for the weighted mean of CVs were calculated applying Sokol and Rohlf (1969) method. Using ANOVA statistics the mean CVs are compared which shows no significant result within the three groups (F=2.836, df=2, p=0.06). It clearly indicates that riverine environment has no impact on the growth characteristics in the early period of child growth. It is already reported that early childhood growth is a good measure of population health, because it is sensitive to variation in environmental quality (Waterlow et al., 1977, Frisancho, 1990). It is very difficult to interpret the present outcome. Data on child mortality trend might have something to do as a potential confounding factor. On the same population higher infant as well as child mortality in the riverine environmental setup as compared to nonriverine environmental setup might have compensated the phenomenon. The

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present author has developed a measure to infer the selection intensity at different stages of human life from infant to adulthood (Sikdar, 2012 a) and on applying the same on the Mising population form the two environmental setup it has been found that selection intensity at infant stage is almost double (0.070) in the riverine setup than the non-riverine one (0.041). If the mortality is selective then mortality prior to the age of five years could result in reduced variability in growth performance for the population group at later ages (Bogin, 1991). If the smallest and the slowest growing children are washed out by selective mortality just like one of Jamaican study (Garrow and Pike, 1967) there would be an attenuation of total within population variation in growth performance. It should be kept in mind that CV is more a measure of the homogeneity of the environment rather than the criteria for adversity or optimality. Whatever may be the case it can be inferred that till the attainment of initial childhood period, for the Mising boys, both the environmental setup seems to be a heterogeneous mass which turns into a homogeneous mass showing homogeneity in the CV after the end of initial childhood period. Let us have a look whether this homogeneity has anything to do with adaptation in terms of growth performance. The frequency of stunting in the two environmental setups can be a better measure to infer the situation.

Figure 15.2 CV in height increments among the Mising boys in two different setups (R=Riverine, NR= Non-riverine)


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Environmental quality and stunting Stunting is a well-established child health indicator for chronic malnutrition related to environmental and socio-economic circumstances (WHO, 1995; WHO, 1996). World health Ogranisation is in a constant move to standardize the method and collect the data on the prevalence of stunting at national level and is readily available in their datasets. The systematic standardization carried out by World Health Organisation allows the derivation of trends in stunting around the world (ACC/SCN, 2000; de Onis, 2000) as well as regional and cross-country comparisons of malnutrition (WHO,1997). The WHO definition, cut-off and reference population used to calculate the indicator has been widely accepted since the 1980s. Now prevalence of stunting is in use as a proxy measure of the standard of living condition. Several recent studies are available to decipher the probable determinants of stunting in Indian children (Fenske et al., 2013) and how stunting is related to several environmental factors. It is better to have a look into the prevalence of stunting among the Mising children in the two environmental setups. WHO 2007 (de Onis et al., 2007) data set has been taken as a standard reference for assessing stunting among the children. Individual Z score (Zind) values are calculated for each of the indices following LMS method (Cole and Green, 1992). For all the indicators the tabulated values of Box-Cox power, median and co-efficient of variation corresponding to age (t) are denoted by L (t), M (t) and S (t) respectively. Following WHO 2007, the Z scores for a measurement y at age t are computed as

Zind

[ y / m(t )] =

L (t )

S (t ) L(t )

-1

=

y - M (t ) StDev(t )

Individual Z scores were again categorised to represent normal, moderate and severe cases of stunting following WHO 2007 criterion. The prevalence of stunting among the Mising boys in the two environmental setups has been depicted in Table 15.1.

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Human Growth: The Mirror of the Society Table 15.1 Prevalence of stunting among the Mising boys in two environment setup (As per WHO, 2007)

Age Riverine in Total years boys

Moderately stunt

Severely stunt

Non-riverine Total Modeboys rately stunt

Total stunt

N

%

N

%

N

%

Severely stunt

Chi-square p-values

Total stunt

N

%

N

%

N

%

Severe Total stun- stunting ting

6

51

11

21.6

12

23.5

23

45.1

45

10

22.2

8

17.8

18

40.0 0.574 0.749

7

52

12

23.1

9

17.3

21

40.4

41

9

22.0

4

9.8

13

31.7 0.362 0.554

8

50

11

22.0

2

4.0

13

26.0

50

9

18.0

2

4.0

11

22.0 1.000 0.714

9

51

19

37.3

2

3.9

21

41.2

45

14

31.1

6

13.3

20

44.4 0.126 0.837

10

52

5

9.6

0

0.0

5

9.6

55

7

12.7

0

0.0

7

12.7

(6-10)

256

58

22.6

25

9.8

83

32.4

236

49

20.8

20

8.5

69

29.2 0.650 0.578

—-

0.648

In both the environmental setups the frequency of total stunting is found to be higher in the initial stages of childhood period. Although no significant difference has been noticed in terms of stunting in the two setups but a high rate of stunting (30.89%) among the Mising children warrants attention. Infact it is found that the economic condition of the tribe is one of the confounding factors for the vulnerability of malnutrition (Sikdar, 2010; Sikdar, 2012b). Riverine children with poor economic condition are the worst sufferer among them. If these children pass through socioeconomic transition in latter stages of life whether it may create load of metabolic diseases among them? There are evidence that population that experienced food scarcity in some generations back and have passing though socio-economic transition may experience increasing trend of metabolic diseases (Sikdar, 2014). There are evidence that Northeast India including most of the developing countries are going through nutrition transition with the improvement of socio-economic condition (Sikdar, 2008; Ahmed Das and Sikdar, 2010; Sikdar, 2013) and increasing prevalence of overweight cases among the Mising children (Sikdar, 2012 c) may reflect the same condition among them. Recent evidences are available to show a new trend of stunted-overweight cases in some population because of the prevalence of nutrition transition (Said-Mohamed et al., 2012, Varela-Silva et al., 2012) and it is also not uncommon among the Mising children. Table 15.2 shows that out of 27 overweight cases found among the studied Mising children, a total of 14 children (51.8% of total overweight)

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are stunted-overweight. Perhanps intergeneration effect hypothesis might be a complementary way to understand the coexistence of overweight as well as stunting in the same subjects (Emanuel, 1993; Varela-Silva et al., 2007). It is suggested that early metarnal biological changes could affect the child’s foetal and neonatal development (even if the mothers reside in preferential environment) that may predispose the child to develop overweight in later stages (Said-Mohamed et al., 2012). Bouchad (2007) suggested that predisposition to weight gain can be associated with five genotypes (that are not mutually exclusive). The first one is a thrifty genotype (low metabolic rate and insufficient thermogenesis. The second one is hyperphagic genotype (poor regulation of appetite and satiety and propensity to overfeed). The third one is sedens genotype (propensity to be couch potato and physically inactive). The fourth one is a low lipid oxidation genotype (propensity to be a low lipid oxidizer) and the fifth one is adipogenesis genotype (ability to expand complement of adipocytes and high lipid storage capacity. Thus the susceptibility for becoming overweight-stunting may differ from population to population on the basis on their genetic background. Table 15.2 Overweight and stunted-overweight cases among the studied Mising children Age in years

Riverine

Non-Riverine

Overweight (%)

Stunted Overweight (%)

Overweight (%)

Stunted Overweight (%)

1(2.0)

0(0.0)

1 (2.2)

0 (0.0)

7

1(1.9)

1(1.9)

3(7.3)

2(4.9)

8

5(10.0)

2(4.0)

4(8.0)

2(4.0)

9

3(5.9)

2(3.9)

3(6.7)

2(4.4)

6

10

3(5.8)

1(1.9)

3(5.5)

2(3.6)

6-10

13(5.08)

6(2.34)

14(5.93)

8(3.39)

Now the question arises whether these socioeconomic transitions have anything to do with the health outcome of the adult Mising population. A comparative study on the Mising adults form variable environments would shade some light on this issue. Recent migration and developmental plasticity Whether migration has anything to do with development plasticity is a big question to answer. Whether migration always facilitates improved

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socioeconomic condition? Whether socioeconomic transition always account for betterment of human body? Another study stated that socioeconomic transition may increase genetic load where there is a preference of heterozygote selection (Sikdar, 2008). It is well accepted that migration is always for the search for better opportunity for food, medical care and other facilities. But it is not clear that whether migration always leads to improved physical condition or not. Sometimes heterosis may result because of migration as a consequence of increased exogamous marriages with concomitant decrease in consanguinity that may results in the increase in heretozygotes. People migrating in childhood acquire the prevalence of characteristic diseases of the country to which they immigrate (Susane et al., 2000). There are some instances where migrants are found to be shorter and lighter than sedentes (Piplai, 1995). The socio-economic origin of the sample is of great importance while dealing with the consequences of migration. Hulse (1969) found that the significant increase in stature of migrants from Ireland to be due to an over-representation of farmer’s sons in the sample of migrants and of migrants who were at the same time civic guards. The migrants who were not civic guards did not differ significantly in their average stature from that of sedentes (cited in Susane et al., 2000). A recent study (Sikdar, 2012 d) also confirms rural-urban variation in growth characteristics as well as blood pressure level among the Mising adults. But in the present study I have tried to control socioeconomic factor and further verified the impact of migration among the Mising adults. Here I choose Mising adults from three comparative environmental categories. The studied population has been divided into three income groups following All India Whole Price Index (Agarwal, 2008). Multiplying the AIWPI at the time of study (Rs 196.35) with the hypothetical value of 0.53 helped us to derive the multiplication factor of 104.06 (US 1 Dollar= 60.08 Indian Rupees as on 11/ 04/2014). It forms the basis of classifying our population into three economic gradations. The two cutoff values are derived by multiplying the multiplication factor (104.06) by 5 and 15 respectively. As per the division, families with per capita monthly income (PCMI) of Rs 520 and below are classified as low income groups, those with Rs. 520 to 1560 are graded as middle income group and families with per capita monthly income of Rs. 1560 and above


285

are classified as high income group (Sikdar, 2012 a). The difficulty to evaluate the per capita monthly income in rural setup was overcome by taking into consideration the housing condition, type of house, monthly expenditure, occupational status, possession of different household materials, livestock etc. It is found that almost 61% of the population so far studied belongs to middle income group (Sikdar, 2012 a). Therefore to control the economic variability of the sample here I select the adults form middle income group and try to interpret the difference of growth variables in the three environmental conditions. A total of 364 adult individuals between the ages of 40 to 50 years are studied in terms of their residential status. Selecting individuals below 50 years will definitely minimize the impact of age on physiological variables. Of them 123 are reared as well as settled in Riverine environmental setup (Rrs), 126 of them are reared and settled in Non-Riverine environment (NRrs) and 115 are reared in Riverine environment and settled in Non-Riverine environment (RrNRs). The individuals those who are reared in the riverine environment and settled in non-riverine environment have migrated from their parental habitation setup after completing minimum 15 years of age. The comparative results are incorporated in Table 15.3. All the variables are showing significantly varying results in terms of migration to the new environment. A considerable increase in stature is discernible among the Mising adults reared and settled in Non-Riverine setup (NRrs) as compared to the adults reared and settled in Riverine setup (Rrs). It can also be noted that the newly migrant adolescents achieved new height within a very short time span i.e. after the age of 15 years. From this output we can think that new height doesn’t reflect living condition. If adult height reflects living conditions throughout all childhood and adolescents, a few adolescent years would not be enough to significantly change height in migrants. Though an impact of economic condition on child height has been found to be important (Sikdar, 2010) but here the role of economic background or living condition on final height is found to be obscure. It is also found that adolescents quickly re-adjust their height according to their immediate surroundings. Recent evidence suggests a community-based factor on growth (Aßmann and Hermanussen, 2013 a, b). The process of generating tall people by the tall communities in adverse condition has obviously got health implications as being short is not necessary portray poor health condition. In the absence of

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proper explanation it is speculated that growth hormone (GH) and insulinlike growth factor 1 (IGF1) may be involved in such process (Aßmann and Hermanussen, 2013a). On the other hand an exponential increase of Weight, Body Mass Index, Blood pressure measures as well as hypertension level in the recent migrants than the older migrants to non-riverine setup is a matter to look into. Whether sudden exposure to new environment has anything to do with the increase of metabolic diseases? It is found that immigrants from developing countries take two to three generations to reach an upper plateau in average stature when they have migrated to a more favorable environment (Hall, 2010). Even Bogin (1989) found that Mexican immigrants to United States have become taller on average with each generation since 1930s. But why the impact of new environmental setup is so profound in other parameters like Weight, BMI, Blood pressure as well as the frequency of hypertension among the new migrants? Here, should we think that along with plasticity of environment there exist plasticity in traits also? In one hand Boas (1912) found continued increase in stature as well as weight of American born progeny of immigrants from several European countries from one generation to next generation. Even Italians born in Belgium or individuals who migrated before the age of 25 years are taller and heavier than those who migrated after the age of 25 years after controlling for differences in age, level of study and professional status (cited in Susanne et al., 2000). Table 15.3 Physical parameters of the adult Misings in different environmental condition Mising Adults (Males)

Height Weight

Rrs (N=123) X±SD

NRrs (N=126) X±SD

RrNRs (N=115) X±SD

ANOVA P value

F value

162.5±4.7

165.2±4.3

163.9±6.2

0.000

8.725 33.267

54.8±5.9

57.2±5.2

61.3±7.4

0.000

Body Mass Index

20.75±4.3

20.95±5.7

22.81±5.1

0.003

5.918

Waist circumference

70.96±5.1

72.01±4.3

72.32±5.9

0.098

2.342

Bicep Skinfold thickness

6.26±2.9

7.12±3.2

7.87±3.9

0.001

6.908

Tricep Skinfold thickness

11.27±3.1

12.13±2.7

13.34±3.7

0.000

12.693

Systolic Blood Pressure

116.53±8.4

121.97±9.3

133.62±11.2

0.000

96.420

Diastolic Blood Pressure

79.76±7.2

80.58±5.4

82.21±7.3

0.017

08(6.5%)

13(10.3%)

21(18.2%)

Hypertension*

*Chi-square test: ++P value, +chi-square value.

++

0.015

4.136 +

8.330


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But the present result depicts a different picture. As per Bogin and Loucky (1997), in most of the studies which demonstrate secular trend of increment in mean height from generation to generation generally lags behind increment in the body weight as well as body composition. Whether community effect on growth is only perceptible in some particular traits ? Whether the recent events like developmental transition among the Mising population can affect some traits so effectively is a matter to contemplate. Whether the present result can be discussed in the light of epigenetics theory? Whether human microbiome analysis can give any clear picture for that or there may be other possible explanation for the same. Early environmental adjustment, plasticity, epigenetics and future implications There are various biological adjustments or developmental plasticity that has to be carried out by the fetus in response to the prenatal stress condition. Infantile as well as childhood growth always predict adult health outcomes. There are evidences that prenatally undernourished individuals tend to be smaller and have fewer nephrons which increases the risk of hypertension and renal failure in adulthood (Lampl et al., 2002). The Dutch famine in 1944 is a unique counterpart for animal models that study the effects of restricted maternal nutrition during different stages of gestation. Exposure to famine during any stage of gestation was associated with glucose intolerance, more coronary heart disease, a more atherogenic lipid profile, disturbed blood coagulation, increased stress responsiveness and more obesity (Roseboom et al., 2006). We can expect different phenotypic expressions of a given genotype on the basis of the early environment where it develops. Several signals from the environment work as predictor of the phenotypic expression. If the environment keeps intact then the organism’s phenotype adapts itself to that providing close match as represented in the figure (Figure 15.3).

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Figure 15.3 Match and mismatch of genotype, phenotype and environment [taken from Bateson, 2007]

If the environment changes between the elicitation of the particular pattern and development then the phenotype mismatches with the environmental conditions in adult life. The recent trend of such studies comes under epigenetic changes that describe the chemical modifications that change the pattern of gene expression in a specific tissue or organ without changing the nucleotide sequence of the DNA. The term epigenetics was used by Conrad Waddington way back in 1957 to define the interplay between individual’s genetic constituent and environment. But modern definition of epigenetics has become narrower to understand the molecular processes by which traits defined by a given profile of gene expression can persist across mitotic cell division but which do not involve changes in the nucleotide sequence of the DNA (Felsenfeld, 2007). The term often used to define the molecular mechanism by which dynamic and stable changes in gene expression are achieved and ultimately how variations in environmental experiences can modify this regulation of DNA. But question arises whether these changes are fixed or they keep on changing with the changes in environment. As a given genotype may give rise to different phenotypes depending on the state of the environment in early developmental stage. Cues from the environment may be used as predictors, determining which of a set of alternative developmental pathways is elicited. As per epigenetics theory if the environment does not change, then the organism’s phenotype will be well


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adapted to that environment, providing a close match, as is represented in the diagram by the pattern and shape of the phenotype and the pattern and shape of the environment. However, if the environment does change between the elicitation of the particular pattern and development, then the phenotype may be mismatched to the conditions of adult life. Epigenetic inheritance may serve to protect the better-adapted phenotypes within the population until spontaneous mutation leading to phenotypic fixation. But it seems that the present outcome cannot be justified under the domain of epigenetics theory as all the adult population thus selected were from middle income group and they hardly passed through any famine in their lifetime for such variation in outcome within such a short span of two generations. Here in this study only hypertension and BMI have been selected as a risk of metabolic disorder and more parameters as suggested by World Health Organisation (1999) like raised plasma triglycerides, microalbuminurea will shade more light in this issue. We can think of another hypothesis based on the outcome of the present study. Here I have termed it environmental shock hypothesis which is more or less similar to thermal shock theory of thermodynamics. Thermal shock occurs when a thermal gradient causes different parts of an object to expand in different direction with different amount of force ultimately leading to fail in its structure. More simply it is the cracking that is caused by rapid changes in temperature and mostly happens on objects made up of ceramic or glass. This is said to be due to the low level of heat conductivity of the material as well as high coefficient of thermal expansion. We can think the increase in BMI, weight, systolic, diastolic blood pressure as well as frequency of hypertension in the new environment which may lead to high metabolic diseases among the population is a result of environmental shock to the new comers. If we tag the environmental change with a rate then this rate of change will be high for the new comers than the old migrants. There are three criteria required for thermal shock. First, there is an expansion rate for every material and the materials that expand and contract quickly due to temperature change are more likely to thermal shock than the materials with low expansion rates. This is like the case of the traits as well as related diseases in man. The traits as well as related diseases which trigger very quickly are definitely having high expansion rate with respect to environmental change.

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Secondly, materials those are brittle (do not stretch or bend far without breaking) are also more likely to thermal shock than flexible materials. The rise of metabolic disorders with the sudden change in the environment in many countries may be due to such brittleness of such disorders. Thirdly, materials that conduct heat well are less likely to thermal shock because, by transferring the heat quickly throughout the material, they increase the likelihood that the material will expand or contract evenly. Here the genetic composition of the individual may be thought as the power of conductivity of environmental shock. If the thermal expansion is slow then the probability of thermal shock is minimized. If the process of changes in the environment (transition in lifestyle) is slow then we can expect reduced rate of metabolic diseases in a population. Cross population comparison can shade more information to this issue. If this environmental shock hypothesis is accepted then we are not also far away when we can think of a borosilicate phenotype with reduced expansion co-efficient and greater strength to overcome the environmental shock or rapid change in the environment and reduce the metabolic diseases in near future. However the rate of change that an area or country will follow in terms of food transition as well as transition towards lifestyle activities will also add some information to mitigate such global problem. Acknowledgement I am thankful to Prof. Michele Hermanussen for thoroughly going through the previous version of the article and suggesting corrections for necessary improvement. I am also thankful to Indian Council of Social Science Research, New Delhi for defraying the expenses of this project. Reference Aßmann, C., and Hermanussen, M., 2013 a, The community effect on Growth, In Auxology: studying human growth and development, Hermanussen, M. (Ed.), pp. 68-71 Stuttgart, Germany: Schweizerbart Science Publishers. Aßmann, C., and Hermanussen, M., 2013 b, Modeling determinants of growth: evidence for a community-based target in height? Pediatr Res, 74, 1:88-95. ACC/SCN, 2000, The fourth report on the world nutrition situation: nutrition throughout the life cycle, Geneva, Administrative Committee on Coordination, Subcommittee on Nutrition.


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