Paper No. : 06 Human Growth Development and Nutrition ...

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Anthropology Human Growth Development and Nutrition

Adaptation of Growth Rates to Heat Stress

Paper No. : 06 Human Growth Development and Nutrition

Module : 07 Adaption of Growth Rates to Heat Stress

Prof. Anup Kumar Kapoor Department of Anthropology, University of Delhi

Development Team

Principal Investigator

Paper Coordinator

Content Writer

Content Reviewer

Dr. Meenal Dhall Department of Anthropology, University of Delhi

Dr. Monika Bhuker Research Associate EHI International Pvt Ltd, Delhi

Department of Anthropology, University of Delhi Prof. Satwanti Kapoor Department of Anthropology, University of Delhi

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Description of Module

Subject Name Anthropology

Paper Name 06 Human Growth Development and Nutrition

Module Name/Title Adaption of Growth Rates to Heat Stress

Module Id 07

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Contents of this unit

1. Introduction

1.1. Adaptation

1.2. Growth rate

1.3. Thermoregulation

1.4. Temperature and growth rate

1.5. Allen’s rule

1.6. Bergmann’s rule

1.7 Why is heat adaptation necessary?

2. Summary

3. Self-assessment

4. Suggested reading

Learning Objectives

How human body regulate its body temperature

Why people living in different climates have different body shape and size

Relationship between heat and growth rate

To what extent Allen’s rule apply on different populations

INTRODUCTION

The environment can have major influences on human physiology. Environmental effects on human

physiology are numerous; one of the most carefully studied effects is the alterations in

thermoregulation in the body due to outside stresses. This is necessary because in order for enzymes to

function, blood to flow, and for various body organs to operate, temperature must remain at consistent,

balanced levels. (http://en.wikipedia.org/wiki/Ecophysiology)

Humans and many other mammals have unusually efficient internal temperature regulating systems

that automatically maintain stable core body temperatures in cold winters and warm summers. In

addition, people have developed cultural patterns and technologies that help them adjust to extremes of

temperature and humidity (http://anthro.palomar.edu).

Two main turning points in evolution are considered as the landmarks of progress in adaption to heat

stress. One was the transition from ectothermic to endothermic control of body temperature and the

other, morphological changes that occur in response to physical stress placed on bone (Bipedalism)

(Drinkwater).

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1.1 ADAPTATION

Adaptation is used as a generic term encompassing acclimatization, acclimation and habituation (Folk,

1974). In biology, an adaptation, also called an adaptive trait, is a trait with a current functional role in

the life history of an organism that is maintained and evolved by means of natural selection.

Adaptation refers to both the current state of being adapted and to the dynamic evolutionary process

that leads to the adaptation. Adaptations contribute to the fitness and survival of individuals.

Organisms face a succession of environmental challenges as they grow and develop and are equipped

with an adaptive plasticity as the phenotype of traits develops in response to the imposed conditions.

The developmental norm of reaction for any given trait is essential to the correction of adaptation as it

affords a kind of biological insurance or resilience to varying environments. The capacity of any stress

to induce adaptation is a function of its application intensity, duration, frequency and variability

(Adolf, 1964), and of the genetic, phenotypic and situational status of the individual. In a modification

of the training impulse model (Banister, 1980), these variables may be used to quantify the cumulative

adaptation impulse (stress volume).

Figure 1: Adaptation theory: A hypothetical illustration of physiological changes following the

application and withdrawal of an adaptation stimulus. (Source: Taylor and Cotter, 2011)

1.2 GROWTH RATE

The changes in height of the developing child can be thought of in two different ways: the height

attained at successive ages and the increments in height from one age to the next, expressed as rate of

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growth per year. If growth is thought of as a form of motion, the height attained at successive ages can

be considered the distance travelled, and the rate of growth, the velocity. The velocity or rate of growth

reflects the child’s state at any particular time better than does the height attained, which depends

largely on how much the child has grown in all preceding years. The blood and tissue concentrations of

those substances whose amounts change with age are thus more likely to run parallel to the velocity

rather than to the distance curve. In some circumstances, indeed, it is the acceleration rather than the

velocity curve that best reflects physiological events. (Britanica.com/human-development)

Figure 2a. Height chart for girls and boys (Encyclopedia Britanica, 2010)

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‘

Figure 2b. Weight charts for girls and boys (Encyclopedias Britanica, 2010)

1.3 THERMOREGULATION

Thermoregulation is performed by a physiological control system consisted of central and peripheral

thermo receptors, an afferent conduction system, a central control for integration of thermal impulses,

and an efferent responses system leading to compensatory responses (Romanovsky, 2007). This system

regulates the balance between production (thermo genesis) and dissipation (thermolysis) of heat, in

order to maintain the body temperature near a constant level of 36.5°C (Gomes et al, 2013).

Humans lose thermal energy to the environment by convection, radiation and sweating. Dubois (1937)

demonstrated that the role of sweating becomes increasingly important in humans as ET rises, as it is

impossible to lose heat by other methods when the environment is hotter than the body.

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Figure3. Process of thermoregulation (Source: adapted from Wikipedia)

1.3.1 Heat stress

It is defined as an environment that acts to drive body temperature above set-point temperature

(Hansen, 2009). Heat stress is composed of two components: 1) heat load which rises from

metabolism, heat exchange, radiation, and convection with the environment; and 2) heat dissipation

which is release of the heat load through sweat evaporation (Vernacchia, 1998). According to

Brotherhood (1987), heat stress is any factor (e.g., high temperatures) or any combination of factors

(e.g., high temperature and high humidity) that overloads the thermoregulatory system thereby raising

the body temperature.

Heat stress can lead to disruptions in reproductive processes through two general mechanisms. First,

the homeokinetic changes to regulate body temperature can compromise reproductive function. One

example is redistribution of blood flow from the body core to the periphery to increase sensible heat

loss. Another homeokinetic control mechanism for body temperature is reduced feed intake during heat

stress. Reducing feed intake reduces metabolic heat production but also can lead to changes in energy

balance and nutrient availability that can have large effects on cyclicity, establishment of pregnancy

and foetal development. A second mechanism for disruption of reproduction during heat stress is the

failure of homeo-kinetic systems to regulate reproduction. (Hansen, 2009)

Children have anatomical and physiological characteristics that differ from adults, such as distinct

values of body composition, water, and bone density. Morphologically, children have a higher ratio

between surface area and body mass, which leads to a more rapid increase in body temperature when it

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is thermally stressed by heat (Rowland, 2008 and Inoue et al, 2004). The evaporative heat loss is

crucial for the maintenance of thermal homeostasis regardless of age.

Table 1. Reported differences in immature relative to mature thermoregulation

Heat stress Effect of thermoregulation

Higher surface area to mass ratio Greater heat loss in mild and cold environments,

and possibly greater heat gain in very high heat

Lower sweating rate Less heat lost via evaporative cooling in hot

climates

Well-developed vasodilation Effective shunting of blood from periphery to

core, higher skin temps in heat stress

Lower blood volume and maximum cardiac

output

Possible cardiac instability while active in heat

stress

Smaller sweat glands Lower sweating output

Lower sweat gland sensitivity Higher sweating threshold

Greater oxygen cost of locomotion Greater metabolic heat production during exercise

Adapted from Little & Hochner, 1974 and Thomas, 1994.

!.3.2 Heat acclimation or Heat acclimatization

The improved tolerance to exercise in heat is known as heat acclimatization. It is specific to the stress

imposed on the human body. For example, passive exposure to heat induces some responses, notably

an improved ability to dissipate heat. In contrast, physical training in a cool-dry environment results in

metabolic, biochemical, hematologic, and cardiovascular adaptations. Heat acclimatization via

strenuous exercise induces responses attributed to both passive heat exposure and training in cool

environments. (Armstrong, 1998)

1.4 TEMPERATURE AND GROWTH RATE

Climate appears to influence growth and development, helping to determine body size and proportions.

According to Bergmann’s and Allen’s rules, body size and proportions of warm-blooded, polytypic

animals are related to temperature. Allen’s rule states that longer extremities and appendages relative

to body size are found in warmer climates, while the reverse is true in colder climates. Bergmann’s rule

states that a larger body size would be expected in colder versus warmer climates. (Cameron, 2002).

A link between adult human body size and environmental temperature, evolved through adaptation to

heat stress, was first recognized a century ago and is now well accepted in human biology. Increasing

heat stress favours smaller body size and an increased ratio of surface area to mass. Many developing

country populations inhabit relatively hot environments compared to industrialized populations, but

growth faltering in developing countries is invariably attributed to the combination of poor nutrition

and infection. The growth faltering can relieve heat stress in both infancy and childhood. The

hypothesis that heat stress plays a role in human growth faltering in hot environments therefore merits

empirical investigation (Wells, 2000). According to Newman (1953), the smaller statures observed

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near the Equator supports Bergmann’s rule, and the shorter legs among the Inuit supports Allen’s rule.

Finally, Schreider demonstrated that amount of body surface area tends to increase from cold to hot

climates (Bogin, 1988).

A negative correlation between adult body mass and environmental temperature (ET) has been

demonstrated by Roberts (1953), Newman (1953), Froment & Hiernaux (1984), Schreider (1964), and

Crognier (1981).

As Kleiber (1961) demonstrated in his monograph “The Fire of Life'', heat production is proportional

to the body mass, M, but heat loss is proportional to the surface area, SA. The ability of an animal to

conserve or lose thermal energy is therefore strongly influenced by the SA(surface area): M (Body

mass) ratio.

1.4.1 Adaptation to thermal stress has been invoked to explain the tall lean proportions of the Nilotic

populations of east Africa (Hiernaux, 1974), and the small stature of tropical rainforest inhabitants such

as the Mbuti (Hiernaux, 1974). Both these adaptations increase the SA: M ratio, facilitating heat loss.

In hot environments, cultural factors are less successful at mitigating the thermal stress, and greater

variation in body size and shape is seen (Crognier, 1981). Roberts (1978), for example, demonstrated a

stronger relationship between size and ET (environmental temperature) in African populations than in

populations inhabiting more temperate climates.

Eveleth (1966) studied American middle-class children reared in Brazil and found them to be more

linear than expected. Stinson & Frisancho (1978) compared the children Andean migrants to the

Amazon jungle with children living in the Andes. Those reared in the jungle were more linear, the

length of the extremities clearly greater.

Figure 4. Relationship between mean annual temperature and body weight and relative sitting height.

(Source: Roberts, 1978)

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The relationships observed between body proportion and environmental temperature can be explained

in terms of the body’s thermoregulatory process. In hot environments, heat dissipation is crucial to

avoid hyper thermic stress, or overheating. A body that has greater surface area relative to total body

size or volume more efficiently dissipates heat produced by the body’s metabolism and activity. One

method through which heat is lost is convection, which is the transfer of heat from the body to the

environment through the movement of air over the body; thus, a greater surface area over which wind

can pass is beneficial to humans in hotter climates. (Cameron, 2002)

The effect of both growth and maturation as of heat acclimation is the increase of the sweat gland

cholinergic stimulus. Moreover, it occurs concomitantly to the acceleration of stimuli toward the

peripheral sympathetic nervous system (Lee et al, 2010).

With physical growth, especially of body surface area, a decrease in the density of sweat glands

activated by heat is verified. This phenomenon does not favour increased perspiration, but, on the other

hand, glands increase in size (hypertrophy), increasing the amount of sweating rate (Inoue et al, 2004).

Growth and maturation provide increased ability to sweat glands, however, unevenly. This means that

some areas may have a higher cholinergic sensitivity compared to other ones (Inoue et al, 2004).

Crognier (1981) studied the relationship between climate and anthropometric measurements in 85 East

African, European, and Middle Eastern populations. He found that mean annual low temperature was

strongly correlated with cranial measurements, but postcranial measurements were strongly correlated

to heat and dryness.

Malina and Bouchard (1991) suggested that the typical body shapes associated with extremes in

temperature, growth in hot environments is prolonged, since there is an association between delayed

maturation and a linear body type. Studies of the mean age at menarche, however, contradict this

conclusion, because a negative correlation to annual mean temperature has been observed indicating

that maturation occurs earlier in hotter climates.

1.5 ALLEN’S RULE

Allen's rule is a biological rule posited by Joel Asaph Allen in 1877.The rule says that the body shapes

and proportions of endotherms vary by climatic temperature by either minimizing exposed surface area

to minimize heat loss in cold climates or maximizing exposed surface area to maximize heat loss in hot

climates. The rule predicts that endotherms from hot climates usually have ears, tails, limbs, snouts,

etc. that are long and thin while equivalent animals from cold climates usually have shorter and thicker

versions of those body parts (Asaph, 1877 and Holstun, 1986).

Katzmarzyk and Leonard (1998) said that there is a negative association between body mass index and

mean annual temperature for indigenous human populations, meaning that people who originate from

colder regions have a heavier build for their height and people who originate from hotter regions have

a lighter build for their height.

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Figure5. These two rectangular prisms have the same volume, but they have different surface areas.

1.6 BERGMANN’S RULE

Bergmann’s rule is an ecological principle which states that body mass increases with latitude. Human

populations, who live near the poles, including the Inuit, Aleut, and Sami people, are on average

heavier than populations from mid-latitudes, consistent with Bergmann's rule (Holiday, 2009). They

also tend to have shorter limbs and broader trunks, consistent with Allen's rule (Holliday, 2009)

According to Marshall T. Newman in a 1953 article for the Journal of the American Anthropologist,

Native American populations are generally consistent with Bergmann's rule although the cold climate

and small body size combination of the Eastern Eskimo, Canoe Indians, Yuki, Andes natives and

Harrison Lake Lillouet runs contrary to the expectations of Bergmann's rule. Newman (1953) contends

that Bergmann's rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan

Africa.

1.7 WHY IS HEAT ADAPTATION NECESSARY

The ability to respond to heat is seen in all extant human populations, regardless of the environment in

which they now live or how many generations they have been removed from the heat (Edholm &

Weiner, 1981). It is very clear that physiological adaptations remain of paramount importance in

survival. Heat adaptation are very much necessary not just for human beings but for every single

organism for their better survival in hot environments.

Humans acclimate to heat in a rapid and effective manner. Acclimatization involves increased sweating

which reduces body temperature and circulatory strain. The stimulus is elevated body temperature for

short periods over several, consecutive days. In addition to a higher rate of perspiration,

acclimatization also involves a greater sensitivity to environmental heat, as evidenced by the lower

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temperature at which sweating begins, as well as a redistribution of recruitment patterns and a greater

sodium and chloride economy (Hanna & Brown, 1983). Well adapted human beings more easily

tolerate heat and enhance the level of heat tolerance (Gisolfi et al, 1969). Acclimatization to heat has a

positive effect on work performance and improves maximum work capacity (Shvartz et al, 1977). A

linear body build is of advantage in hot climates if it promotes heat loss.

SUMMARY

Heat Acclimatization is necessary to prevent or reduce the severity of heat illness.

Humans are remarkably well adapted to tolerate heat whether derived from environmental or

from metabolic sources.

Higher surface area to mass ratio in homeotherms is beneficial in terms of heat loss.

Stature, weight and limb length are phenotypically plastic and have a significant environmental

input.

With physical growth, especially of body surface area, a decrease in the density of sweat glands

activated by heat is verified.

Allen’s rule states that longer extremities and appendages relative to body size are found in

warmer climates, while the reverse is true in colder climates.

Bergmann’s rule states that a larger body size would be expected in colder versus warmer

climates.

Heat adaptation are very much necessary not just for human beings but for every single

organism for their better survival in hot environments.

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