In the discussion of the evolution of human diet, a great deal of attention has centered on the diets of past and present day foragers. It might be expected that humans would show biological adaptations to a diet based on wild plants and animals since a foraging way of life represents more than 99% of human evolutionary history. When considering the health implications of forager diets, most of the attention has focused on the health of adults. Adults in forager groups are frequently described as healthy and well nourished (Hill et al., 1984; Lee, 1979). The diets of foragers also have been shown to be have nutritional properties that would help to prevent many of the chronic diseases, such as heart disease and cancer, that are such major causes of mortality among adults in industrialized countries today (Eaton and Konner, 1985; Eaton et al., 1997).

     The health implications of forager diets for children have received much less attention. Children's health is an important concern in any discussion about the evolution of human diet because child health has major ramifications for adult well-being. Most basically, survival through childhood is necessary for reproductive success. Mortality rates are higher in infancy and early childhood than at any other time except old age, and thus we might expect that there would be a great opportunity for diet to act as a basis for natural selection during childhood. Even when poor childhood health does not result in childhood mortality, it can have negative consequences for later life (Martorell, 1989).

     The purpose of this paper is to review two indicators of child health in present day foragers: child mortality and child growth. Ideally, it would be possible to use these solely as indications of the healthfulness of the diet, but, as will be discussed further below, mortality and growth are obviously affected by factors other than just diet. In particular, child growth, which is considered to be one of the most sensitive indicators of child health and nutritional status, is open to a variety of interpretations.

     Although there are not extensive data on childhood mortality in present day foragers, the data that do exist indicate that infant and early childhood mortality rates are quite high (see Figure 1). Among the Ache and the !Kung, from 12 to 20% of infants die in the first year of life, and an additional 12 to 18% of children die between the ages of 1 and 5 years. These early childhood mortality rates are similar to those seen in groups combining foraging and horticulture, such as the Trio, and in the pastoralist Herero (see Figure 1). All of us would be dismayed if our children had the probability of death early in life that is a regular experience for members these groups.

Figure 1. Infant and early childhood mortality in Ache and !Kung foragers, mixed foraging and horticultural Trio, and Herero pastoralists. All of these populations experience high infant and childhood mortality. Data for Ache from Hill and Hurtado (1996), for !Kung from Howell (1979), for Trio from Gage et al. (1984), and for Herero from Harpending and Pennington (1991).

     Although mortality rates are among the most direct indicators of health, they are not specific in terms of identifying particular health problems. It is difficult to determine to what extent the high infant and early childhood mortality rates observed in foragers are the direct result of dietary deficiencies. Diseases resulting from pathogens introduced into food as well as inadequate access to health care are certainly major contributors to these high mortality rates.

     Growth in early childhood is a health indicator that has been tied more directly to diet and nutritional status. The infant and early childhood growth patterns in foragers are similar to those seen in modern non-foraging groups considered to be suffering from mild to moderate undernutrition. That is, the length (as height is measured when children are too young to stand upright) of young infants is close to international reference data, but older children are much shorter, and in some cases are excessively thin as well (Martorell and Habicht, 1986 and references therein).

     Figure 2 illustrates this pattern for height using data from one regional and four national samples considered to have varying proportions of mildly to moderately undernourished children. Growth measurements are expressed in terms of height-for-age, the mean height at a particular age. Height-for age is considered to be an indicator of long term nutritional status because an individual's present height is the result of many years of growth (WHO Working Group, 1986). For the five groups illustrated in the figure, height-for-age is closest to international reference data in the first year of life, and then decreases relative to reference data. In the shortest population illustrated in the figure, height-for-age decreases to over 2.5 standard deviations below international reference data by the age of four years, which is equivalent to a height below the 3rd percentile of the reference population.

Figure 2. Height-for-age in selected populations in which a portion of children are mildly to moderately undernourished. The zero line represents the median for international reference data based on U.S. children. Heights are presented as standard deviations from the U.S. median (Z scores). Height is most similar to international reference data in the first year of life, but at older ages, most children are substantially shorter. The data for Mali are from Dettwyler (1991), and are for several rural villages. The data for other countries are unpublished data from national surveys kindly supplies by Macro International, Demographic and Health Surveys.

     Figure 3 illustrates weight-for-height in these same populations. Although there is some debate on the subject, weight-for-height is frequently considered to be a better indicator of current nutritional status than is height-for-age since weight can be quickly gained or lost (Victora, 1992; Waterlow et al. 1977; WHO Working Group, 1986). As shown in the figure, children in some populations maintain the same weight-height relationship as in the reference populations, while in other populations children become quite thin. In no case, however, is weight-for-height more standard deviations below international reference data than is height-for age.

Figure 3. Weight-for-height in selected populations in which a portion of children are mildly to moderately undernourished. The zero line represents the median for international reference data based on U.S. children. Weight-for-height is presented as standard deviations from the U.S. median (Z scores). Children in some populations maintain weight-for-height similar to the reference data, but in other populations, children older than one year are thin compared to reference data. The data for Mali are from Dettwyler (1991), and are for several rural villages. The data for other countries are unpublished data from national surveys kindly supplies by Macro International, Demographic and Health Surveys.

     These same patterns of growth illustrated in figures 2 and 3 are evident in present day foragers. Truswell and Hansen (1976), reporting on their studies of the !Kung in the late 1960s, found that about 40% of !Kung infants under one year of age were below the 3rd percentile of reference data for weight and 50% were below reference data for length. After the age of one year, about 80% were below the 3rd percentile for height and from 60% to almost 90% were below the 3rd percentile of reference data for weight (see Figure 4). Similarly, studies of the Ache indicate that Ache children are similar in size to U.S. children at birth, but by 18 months of age they are below the U.S. fifth percentile for height, and at or slightly above the U.S. fifth percentile for weight (Hill and Hurtado, 1996). Compared to the populations presented in Figure 2, the growth of Ache and !Kung children is most similar to that of the smallest children shown in the figure, those in Nigeria and Pakistan.

Figure 4. Percentage of !Kung children who were smaller than the 3rd percentile of international reference data for height and weight in the late 1960s. A large percentage of !Kung children were very small compared to international reference data. Data from Truswell and Hansen (1976)

     Several questions are raised by these comparisons with international reference data. The first involves the reference data themselves, which are based on the growth of U.S. children. Could or should children in diverse populations be expected to show the same pattern of growth as U.S. children? The question of could relates to what can be termed the genetic growth potential of a population. Would children with different genetic makeups grow to be the same size as U.S. children if they were raised in the same environmental circumstances? Studies comparing the growth of affluent children of differing ethnic groups indicate no major differences in their growth patterns, at least prior to puberty (see Figure 5). These studies, which suggest there are not substantial genetic differences in growth in early childhood, have been the justification for using one set of international reference data for growth comparisons (Martorell and Habicht, 1986). However, there are many populations in the world for which affluent representatives do not exist. For these populations, we do not yet know whether they could grow as U.S. children do.

Figure 5. Heights of well off and poorly off boys at age 7.5 in selected populations compared to international reference data based on U.S. children. For these populations, there are not major differences in early childhood growth among affluent children. Data for Nigeria and India from Eveleth and Tanner (1976), for Jamaica from Ashcroft et al. (1966), for Costa Rica from Villarejos et al. (1971), for China from Lin et al. (1992), and for Guatemala from Johnston et al. (1975) and Bogin and MacVean (1976).

     For populations such as the !Kung and the Ache, we do not know if they would grow exactly as U.S. children if they were raised in the same environment, but we do have indications that their growth pattern when leading their more or less traditional foraging way of life can change when environmental circumstances change. As shown in Figure 6, the percentage of young !Kung children whose heights were below the U.S. 5th percentile decreased markedly from the late 1960s to the mid 1970s as they moved from a foraging diet to a mixed diet of domestic and wild foods (Hausman and Wilmsen, 1985). Ache children adopted by U.S. families are much taller than children raised in their traditional environment (Hill and Hurtado, 1996). These findings indicate that the growth patterns of foraging populations do not indicate their genetic growth potential.

Figure 6. Increase in the height and weight of young !Kung children between the late 1960s when their diet was based mainly on hunting and gathering and the mid-1970s when they were eating a diet of mixed wild and domestic foods. Data from Hausman and Wilmsen (1985).

     The second question raised by comparisons with international reference data based on U.S. children is should children in diverse populations show growth patterns similar to U.S. children? Many have argued that the high growth rates and large body size of children in affluent countries should not be presented as an ideal because they are associated with diets that lead to numerous health problems later in life (see for example, Walker et al., 1994). It has also been suggested that shortness, not associated with excessive thinness, should be considered an adaptation to environments with marginal nutritional resources because smaller individuals need fewer nutrients (Seckler, 1980; Stini 1971), or that small body size is advantageous for foragers because it increases hunting success (Lee, 1979).

     As Beaton (1989) and Martorell (1989) have so ably pointed out, however, we must separate the state of being small from the process that leads to becoming small. While there are possible advantages and disadvantages to small adult body size, the poor nutritional status and high rates of disease that lead to slow growth in early childhood are certainly unhealthy. There are no living human populations that combine slow growth in early childhood with measures indicating high levels of health, such as low mortality rates. While the growth rates of U.S. children may not be optimal, neither are the slow early childhood growth rates typical of forager populations indicative of good health in early life.

     To what extent is the poor childhood health of foragers the result of dietary problems? This is a difficult question to answer because we have no data specifically on the dietary intake of young children in foraging groups. It is notoriously difficult to collect accurate measurements of individual food intake because of the disruption to normal behavior that any such attempt necessitates. Studies of forager diet have therefore relied on measuring the total food brought into camp, and estimating per capita intake by dividing the total nutrients by the number of individuals in the group. In general, these studies do not indicate major deficiencies in the average diet (Hill et al., 1984; Lee, 1979), but an adequate average diet does not necessarily mean that the nutritional intake of young children is satisfactory.

     Even in cases where ample food is available, young children may not have acceptable nutrient intakes because of factors such as unequal food distribution, beliefs that certain foods are inappropriate for young children, and expectations that young children should be responsible for feeding themselves (see Dettwyler, 1991 for examples from rural Mali). In addition to specific dietary inadequacies, poor nutritional status in early childhood is also linked to disease (Martorell and Habicht, 1996 and references therein). Pathogens may be present in the supplementary foods introduced when breast milk alone becomes insufficient to support nutritional requirements, and as infants become more mobile, they come into contact with increasing numbers of pathogens in their environment. Poor nutritional status decreases immune function and thus can make children more susceptible to disease, and disease makes children more likely to become undernourished. Disease is frequently associated with loss of appetite or, especially in the case of diarrheal disease, with direct nutrient losses. The synergistic relationship between nutrition and infection makes it extremely difficult to separate their contributions to poor early childhood nutritional status.

     Based on data available for the !Kung and the Ache, children in contemporary foraging populations experience poor health in early childhood as indicated by slow rates of growth and high mortality rates. Of course, we cannot extrapolate two well-studied cases to all present or past foragers. While the preponderance of bioarchaeological data indicates that health declined with the transition from foraging to agriculture, these same data document that health conditions for prehistoric foragers were far from ideal (Cohen and Armelagos, 1984). Dietary deficiencies are one potential cause of poor early childhood health, although we cannot eliminate factors such as disease and lack of access to medical services. High mortality rates in early childhood would produce substantial opportunity for natural selection to operate at this point in the life cycle. Given this fact, if might be fruitful to look for evidence of adaptation to early childhood conditions when we consider the evolution of human diet.

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