5 Adaption of Growth Rates to High Altitude

Dr. Meenal Dhall

Contents

1.1 Adaptation

1.2 Growth and environmental stress

1.3 Human adaptation to high altitude

1.4 Changes in Physical Growth in High Altitude Populations

1.5 Conclusion

1.1 Adaptation

Most of the empirical studies that analyzed the relationship between altitude and human growth compared anthropometric data of genetically similar populations living in two different villages or communities, one at high altitude (above 3500 or 4000 meters) and the other at low altitude (at sea level or below 1500 meters) (Marini, ). The results of the different studies have been equivocal with respect to specific effects of hypoxia on growth. Mueller et al. (1978) suggested that the lack of consistencies in the results of human studies could reflect the failure to keep constant other factors that vary with altitude, beside hypoxia.

1.2 Growth and environmental stress

Humans can respond to environmental stress with physiological and growth adjustments. These changes are rapid and reversible as in the accommodation of the eye to light or darkness. Some are slower, such as an increase in the number of red blood cells in adjustment to a high altitude. Some long term changes as those that occur during childhood growth are irreversible. Studies of highland Indian populations in Peru have shown that chest dimensions and lung volume are greater at all ages in the people studied. The high altitude populations were also shorter at most ages than low altitude populations. The shorter stature is related to delayed maturation, whereas the increase in chest size is a functional adaptation to a high altitude (The Cambridge Encyclopedia of Human Evolution).

1.3 Human adaptation to high altitude

Studies of human adaptation have long sought to determine whether the unique physiology that characterizes native altitude populations is the result of genetic adaptation to high altitude. Several unique features of the high-altitude environment make it well suited for studying genetic adaptation. Unlike the majority of environmental stressors such as temperature, malnutrition or disease threats, the hypoxia of high altitude is pervasive in that it affects all residents, all the time (Julian et al, 2009). Oxygen, even more than food or water, is essential to human life. But unknown is the extent to which the ability to adapt — that is, the ability to survive and reproduce successfully — to high altitude is due to time-dependent changes that can be acquired by all persons (i.e., acclimatization), changes acquired during growth and development (i.e., developmental adaptation) and/or to genetic factors; it is only the latter set of events that can be truly called evolutionary adaptation.

The phrase is used to signify irreversible, long-term physiological responses to high-altitude environments, associated with heritable behavioral and genetic changes. While the rest of human population would suffer serious health consequences, these native inhabitants thrive well in the highest parts of the world. These people have undergone extensive physiological and genetic changes, particularly in the regulatory systems of respiration and circulation, when compared to the general lowland population (Wikipedia).

1.4 Changes in Physical Growth in High Altitude Populations

High altitude results in lower birth rates in all races. Newborns are shorter and have smaller head diameters. However, Tibetan newborns seem to weigh relatively more, a possible consequence of a longer history of occupation in highlands. This is a result of natural selection. While birth weights decline, the weight of the placenta increases a compensatory response to reduced oxygen levels. High altitude doesn’t seem to increase–or decrease life expectancy.

Child growth rate is decreased at high altitude during infancy and adolescence. Interestingly, at high altitudes, adolescent growth can extend into their early twenties. Skeletal age is significantly delayed. This extended maturation enables many children to achieve the heights of their low altitude cousins. Peruvian Indian females found to have delayed menarche, but it was not true for Tibetans. It appears that Tibetans have had a much longer adaptation to high altitude than have Peruvians.

Hemoglobin values increase and cerebral blood flow is reduced at high altitude. For lowland people, acute mountain sickness and pulmonary edema are two syndromes of altitude. Malnutrition also seems to contribute to growth depression of some Peruvians at high altitude.

It has long been observed that lung capacity and chest size are greater for persons at high altitude. Life long residents of the high Andes tend to be short legged, usually grow slowly, and to have a larger thoracic volume. They also have more red bone marrow (Bunny, 1994).

The three mechanisms that involves in adjustment to high altitude hypoxia are (Barron et al., 1964):

• Increase in surface area available for diffusion of oxygen transfer between maternal and fetal bloods,
• Decrease in resistance of the placental barrier to transfer of oxygen
• A combination of both

Morphological studies of the placenta of Peruvian populations indicate that at high altitudes the frequency of “irregular-shape” rather than the usual round or oval placentas is three times greater than at sea level (Rendon, 1964; Sanchez, 1963). A significant difference in birth weight was observed in Colorado and Denver attributed to high altitude hypoxic effect (Litchy et al, 1957 and Howard et al, 1957).

The most distinctive morphological and physiological characteristics of the adult high altitude native are:

• Small stature
• Increased lung volumes
• Enlarged chest size
• Predominance of the right ventricle of the heart

A comparison of the three studies shows that, while hypoxia and other environmental agents do influence certain growth parameters, specific effects demonstrated in one high altitude population cannot be directly applied to another (Pawson, 1976).

Studies on human growth proved that altitude influences anthropometric outcomes but did not show uniform and uncontroversial results on the influence of altitude on height. Pawson (1977) does not find a significant difference between the high altitude Sherpa and the low altitude Tibetan (living in Nepal). Malik and Singh (1978) find that in India children living at high altitude are taller than low altitude children in late adolescence but the opposite is true for children in early adolescence. The patterns of physical growth of members of the indigenous population of Nufioa are characterized by late sexual dimorphism, slow and prolonged growth in body size, late and poorly defined adolescent stature spurt in both males and females, and accelerated development in chest size. The socio-economic factors associated with urban-rural and altitude differences appear to be reflected in greater deposition of subcutaneous fat and increased weight but do not seem to influence the development of stature. We suggest the pattern of growth of this population is related to the hypoxic effects of high altitude, and/or reflects a genetic adaptation to such stress. The anthropometric and physiological studies conducted during this and previous studies and the comparative data from Peruvian populations situated at lower altitudes document the specific adaptive response of the chest wall to the hypoxic effects of high altitude (Frisancho and Baker, 1970).

Figure 2. Comparison of Chest Circumference Among Peruvian Children And Adults From Sea Level, Moderate Altitude, And High Altitude. Highland Quechuas from Nuñoa Exhibit Accelerated Growth in Chest Size. (Source: Frisancho & Baker, 1970)

In Andean studies, the heart of the high altitude native is heavier and bigger than that of the low altitude dweller, and studies suggest that this difference is primarily due to lack of involution of the right ventricle (Frisancho, 1993). At birth, the morphology of the heart at high and low altitude is similar, but after 3 months of age, the right ventricle at sea level is smaller than the left while at high altitude the right ventricle remains as large or becomes larger (Penaloza, 1964). This difference is most marked at the inferior aspect of the heart (Recavarren & Stella, 1962). The difference in developmental patterns is explained by the increased work by the right ventricle that pumps against increased pulmonary vascular resistance due to chronic hypoxia (Penaloza, 1964).

The large chest size and lowered diaphragm of the high altitude native is associated with the development of a larger heart, and greater blood and lung volumes. This characteristic of high altitude Andean populations has not been reported for other populations, such as in the Himalayas or Ethiopia. Thus, the small stature of Andean man is the result of a slow and prolonged period of growth, while his increased lung volumes and chest size are due to an accelerated development (Frisancho & Baker, 1970).

High altitude affects skeletal and sexual maturation also during development. Skeletal maturation prior to the age of 16 years is retarded in Andean, Himalayan, and Ethiopian high altitude populations relative to U.S. standards (Pawson, 1977 and Clegg, 1972). This skeletal maturation delay parallels the delay in growth and attainment of adult stature in Andean and Himalayan populations (Cameron, 2002).

Figure 4. Respiratory changes occurring in high altitude (Source: Paulev-Zubieta, New Human Physiology)

The irreversible changes reflect human genome plasticity, the ability to make adaptational changes during growth.

Pre natal responses: the fetus at high altitudes would most likely be subject to even greater stress than at sea level unless adaptive responses were made. The responses could be physiological or morphological, or a combination of both. It was demonstrated by many investigators that high altitude populations tend to have low birth weights and relatively greater placental weights than their sea level counterparts. At high altitudes there is an 18% reduction in birth weight, while the weight of the placenta relative to the weight of the neonate is about 25% greater than at sea level (Vilchez, 1954; Jara, 1961; Sanchez, 1963; Chabes et al., 1966).

Figure 5. Graduated Effects Of High Altitude Hypoxia (Values Are The Cumulative Percentage Of Babies Across Weight Intervals Shown Who Were Born At Low Or High Altitude Of Andean, Mestizo, Or European Ancestry).

Postnatal response: The pattern of postnatal growth is also altered at high altitude. Experimental animal studies show that weight increases at a slower rate under hypoxic conditions, and microscopic analyses indicate that retarded growth under hypoxic conditions is due to a smaller number of cells, rather than to the reduced cytoplasm observed in growth retardation due to malnutrition (Frisancho, 1993). Infants in La Paz were significantly shorter at 1 month old and at 6 months old and the rate of gain in recumbent length was slower at high altitude than low altitude (Hass et al, 1982). In Andean studies, a consistent reduction in growth of length and weight from birth to 2 years old exists (Moore et al, 1998). Be all et al (1977) reported that low altitude boys from Peru 6–11 years old were 4 kg heavier than their high altitude counterparts and that the weight difference was greater for adolescents 12–18 years old when the low altitude males weighed 11.6 kg more. A similar relationship was observed for females.

This pattern of delayed growth has been observed in other high altitude populations, including the Himalayas and Russian highlands (Frisancho, 1993 and Pawson, 1977). In the high altitude regions (3000 m) in Ethiopia, however, children exhibit a faster rate of growth than in the low altitude regions (1500 m), and the highland children were significantly heavier than lowland children (Clegg et al, 1972). This observation is unexpected; however, it can be explained by the better nutrition and reduced prevalence of disease at high altitude relative to low altitude in Ethiopia, reminding us that each ecological zone is complex and may contain factors both beneficial and detrimental to growth (Cameron, 2002).

Garn and Rohmann (1966) demonstrated that among Ohio White children, increased fatness during childhood and adolescence is positively and systematically associated with taller statures and advanced somatic and sexual development. Nutritional studies on animals also indicate that increased fat deposition speeds maturation and dimensional growth (Hammond, 1954).

Conclusion

The long term effects of high altitude seems to have several different effects on growth and development of human beings living in different areas, including reduced body weight and length at birth, a delay in childhood and adolescent growth, and a delay in skeletal, Menarche and sexual maturation. Since many of these effects have been observed in controlled experiments with nonhuman animals, they are likely due to hypoxia. However, in human exposure to high altitude, the effect of suboptimal nutrition may add to the reduction and delay in growth, but nutrition would not affect the other aspects of growth patterns (e.g., chest circumference) observed in high altitude populations. The growth rates show a late and poorly defined spurt for both males and females among Nuiioa Quechua Indian. Future research may sort out effects due to different stressors. Growth studies provide some evidence for a genetic adaptation to high altitude; however, this evidence consists of population comparisons showing that the growth deficiency or delay among long-term resident populations is less than the more recently migrated populations. However, specific genes responsible for better adaptation have not been identified.