ECOLOGY OF DISEASE & NUTRITION 

Introduction

Disease and nutrition are always important aspects of ecological adaptation; indeed, these two elements may be the major determinants of population dynamics in many human populations

Ecological perspective is not concerned with clinical and physiological details of disease and nutrition per se

Rather, it focuses on the ways in which behavior and culture adjust to these adaptive problems, as well as the ways in which other aspects of human ecology affect disease and nutrition

It's appropriate to consider disease and nutrition together, because they so often interact:

1) nutritional stress ® increased freq. and severity of disease

2) diseases often ® increased nutritional demands (e.g., protein to repair cell damage, or calories to feed parasite or fuel fever)

3) populations often face trade-offs between increasing nutritional intake vs. avoiding pathogens

Classic examples of the last point:  the use of "night soil" (human feces) for fertilizer increases agricultural productivity while elevating risk of disease transmission; crop irrigation can increase rates of water-borne diseases like schistosomiasis; interaction of sickle cell and agriculture in W. Africa (see below)

Thus, examining nutrition and disease together can reveal some very interesting ecological interactions

Case studies summarized here also illustrate interaction of genetic/physiological processes with behavioral/cultural ones

Thus one can consider the ecology of disease and nutrition in relation to broader conceptual issues re human adaptive strategies

Sickle-Cell Case

First example concerns classic case of sickle-cell anemia, a genetically transmitted blood disease common in some sub-Saharan African and So. Asian populations

Sickle-cell gene involves change in molecular structure of hemoglobin, which alters shape of red blood cells, especially when oxygen supply drops (e.g., during strenuous exertion)

Homozygotes (H_sH_s) suffer from sickle-cell anemia and other complications, and thus have very low fitness (usually die before reproducing)

Heterozygotes (H_nH_s) have "sickle cell trait" -- a much less severe impairment, primarily evident under physiological stress of some kind

Yet H_s allele frequency is up to 30-40% in many W. African populations (mostly in heterozygotes)

Such high gene frequencies could only be maintained by strong, consistent, and long-term selection in favor of heterozygotes, creating what population geneticists call a "balanced polymorphism"

In turns out that the "balance" is due to fact that heterozygotes have lower rates of 1) malarial infection, and 2) malarial mortality if infected, compared to "normals" (H_nH_n)

In fact, studies have revealed that, because heterozygotes have higher reproductive success than normals in malaria-infested areas, the distribution of H_s gene closely matches region of endemic malaria = broad band across Sub-Saharan Africa (as well as areas in E. Mediterranean, parts of So. Asia, etc.)

In W. Africa, malaria is transmitted by Anopheles mosquitoes, which breed best in cleared areas (i.e., in sunlit pools, which are common in rainy season)

Swidden agriculture increases cleared areas, and thus the breeding habitat of Anopheles, thereby indirectly increasing the incidence of malaria

Supporting this ecological hypothesis is fact that areas without swidden agriculture but otherwise environmentally similar have significantly lower incidence of malaria

Swidden agriculture + root-crop complex allowed Bantu peoples (an ethnolinguistic group) to expand over much of central Africa, replacing foragers in most forest areas

Result included a tremendous population increase (densest of continent)

Denser population means more rapid transmission of malaria, allowing it to become an endemic disease (i.e., one chronically present in the population)

Thus, there are two ways in which swidden agric. increases malaria transmission:  by creating more breeding habitat for Anopheles vector, and by increasing human population density (thus increasing the transmission rate between human hosts)

This set of observations led to the hypothesis that swidden agriculture ® strong selection for sickle-cell gene as adaptation to endemic malaria

This hypothesis was first elaborated by anthropologist Frank Livingstone, and then tested by epidemiologist Stephen Wiesenfeld

Using a sample of 60 W. African societies, Wiesenfeld found that the frequency of sickle-cell trait is strongly correlated with degree of dependence on agriculture (especially swidden/root agriculture):

These data provide strong support to hypothesis linking malaria, swidden agriculture, and evolution of the sickle-cell gene

We can view this as the outcome of a coevolutionary interaction between human populations and the malarial parasite:  the spread of agric. was adaptive (increased human carrying capacity in tropical forest), but it also increased malaria (an unintended and probably unperceived consequence); humans adapted (via natural selection on genetic variation) to this altered adaptive situation with a marked increase in the frequency of otherwise deleterious gene (H_s)

This co-evolutionary process apparently explains the evolution of the H_s gene; but the trade-off between resistance to malaria and the deleterious effects of sickle-cell gene involves severe costs

Specifically, the fact that heterozygotes have maximum fitness in malarial areas means that a significant fraction of the population is excluded from these benefits

Thus, for example, whenever two heterozygotes mate, Mendelian inheritance means that on average 1/4 of their offspring will be homozygous for normal hemoglobin (thus vulnerable to malarial infection, which is often fatal) and another 1/4 will be homozygous for H_s (thus suffering even worse fate of sickle-cell anemia)

This is a very high price to pay for the malarial resistance afforded by H_s

However, some intruiging evidence suggests that cultural adaptation may have led to reduction of the costs of the genetic adaptation, through yet another trade-off between nutrition and disease

Yams, Malaria, and Harvest Festivals

"Yam Belt" of sub-Saharan Africa = area of endemic malaria

Over 100 different ethnolinguistic groups found here, differing greatly in linguistic affiliation, culture, and sociopolitical organization (village-level to small kingdoms)

One cultural institution cuts through that diversity:  rituals and beliefs surrounding cultivation and harvest of yams

In particular, almost all groups here celebrate an annual harvest ceremony (New Yam Festival), which is preceded for several weeks by a complete ban on yam consumption

Beliefs vary, but all share notion that if someone violates the preharvest taboo on yam eating, great harm will befall them

Prohibition period determined by priests or chiefs each year; falls in rainy season, time of relative hunger; festival date sometime in June-Oct

Purpose of pre-festival yam consumption taboo is not conservation, since it applies even to already harvested yams

Rainy season also happens to be time of maximal malaria (Anopheles breeding season)

One can actually predict timing of New Yam Festival by occurrence of "little dry" season, which follows main rainy season and immediately precedes festival; correlation in timing of the two events is very high (r = 0.82, p < .01), despite fact that it varies from year to year

Why this curious correlation between religious law and malarial seasonality?  One hypothesis involves an intricate interaction between disease and nutrition

Yams naturally contain thiocyanate = a chemical which greatly reduces deleterious effects of sickle-cell gene

Evidence shows that H_s gene is more frequent in yam-cultivating areas, even controlling for mosquito density

Catch is that thiocyanate also reduces resistance to malaria (makes red blood cells more normal, hence more easily parasitized)

Thus, pre-harvest (rainy season) taboo on yam consumption should make sickle-cell heterozygotes more resistant to malaria, while eating yams rest of the year might reduce penalty of this genetic adaptation, or even allow some H_sH_s homozygotes to survive and reproduce

If this hypothesis is correct, then, yam cultivation, coupled with ritual control over timing of yam consumption, makes genetic sickle-cell adaptations much less costly, increasing individual fitness of those who adopt these cultural practices (though no evidence that this is consciously recognized)

In sum, sickle-cell complex looks like an intricate example of "coevolution" of genetic and cultural adaptation:

1) Cultural/ecological adaptation of swidden agriculture leads to increased resource base and hence increased RS

2) But this gain was purchased at cost of endemic malaria (unintended consequence)

3) Endemic malaria in turn led to natural selection for genetic adaptation to malaria = sickle-cell

4) But this adaptation has its own negative trade-offs:  heterozygote may be subject to disease under stress, and up to 1/4 of offspring (H_sH_s homozygotes) die in childhood from sickle-cell disease while another 1/4 (H_nH_n homozygotes) lack genetic protection from malaria

5) Yam consumption may ameliorate harmful "side-effects" of sickle-cell trait (even allowing some homozygote sicklers to survive and reproduce)

6) But since these benefits of yams may also increase chances of malarial transmission, taboo on their consumption during height of malarial season may fine-tune the adaptation even further

This example illustrates the intricacy of connections between culture and biology, and between nutrition and disease

Maize Processing in New World

Yams are part of a swidden-based system focused on root crops

However, the major agricultural civilizations are based on cereal grains, domesticated in 4 different regions:  1) wheat = Middle East; 2) rice = Asia; 3) maize = MesoAmerica; 4) millet = W. Africa

Maize (a.k.a. "corn") = least nutritionally balanced of cereal staples, yet came to be staple of many New World societies, including empires of Mayans, Aztecs, and others

Maize is deficient in 2 essential amino acids (lysine and tryptophan), as well as in niacin (a B vitamin); shortage of amino acids could cause protein deficiency, while niacin deficiency causes pellagra

One way of reducing this problem = "complementary cuisine" -- i.e., combining maize with beans in most meals (e.g., tortillas con frijoles)

Another way involves various traditional methods of maize preparation via alkali processing (using limestone, wood ash...)

Native explanations for processing vary:  softens kernels (making them easier to grind), improves taste (post-grinding processing), etc.

Nutritionists discovered that alkali processing also doubles available lysine and tryptophan, increases niacin -- thus, makes maize much more nutritious

Anthropologist Solomon Katz examined sample 39 of Amerindian societies, found that alkali processing correlated with importance of maize in traditional diet, such that all societies where maize = staple engage in such processing, most where maize is of moderate importance do so, but almost no societies where maize is of minor importance bother with processing   [see GRAPH]

These data support hypothesis that processing is more likely to occur just where it would have greatest nutritional benefit, and hence adaptive value (increased growth, resistance to disease, etc.)

Thus, maize-processing case again shows intricate adaptive relationship between physiology (nutrition), ecology (subsistence base), and culture (processing technology, food production system, culturally-prescribed cuisine)

How might this relationship between physiology, ecology, and culture have evolved?

Unlikely to have been through conscious choice:

1) Nutritional effects of processing are not detectable without biochemical analysis

2) Effects on health and survival likely to be relatively subtle:  given all the other sources of child mortality, even a 10% increase in child survival would require elaborate epidemiological research design to verify, and not be noticeable with small samples available over any one individual's own lifetime

Hence, more likely explanation is that maize processing arose for other reasons (e.g., kernel-softening) in individual households (probably independently several different times, given different methods used in different parts of the Americas), then spread through process of natural selection (on culture, not genes) in areas where maize was dietary staple

That is, those individuals who engaged in alkali processing had slightly more surviving children, who in turn passed on the technique to their offspring (given 10% selective advantage, it would take 74 generations, or about 1500 years, to spread to fixation in a population of 1000)

This interpretation strengthened by fact that food preparation traditions tend to be passed from mother to daughter, since such "vertical" (parent-to-offspring) cultural transmission mimics genetic transmission, and practice will spread because it "creates more little heads to be copied into" (see lecnotes on cultural evolution)

(Why didn't processing spread everywhere?  Perhaps where maize is not a staple, extra work involved in alkali processing discouraged its adoption; and certainly natural selection for processing would have been much weaker)

Lactose Absorption

Last example also concerns interaction between nutrition and health, but instead of an Amerindian staple, we look at a staple of Northern Europeans -- milk

Many Europeans and their No. American descendants hold belief that milk is an ideal food, even for adults

This belief reinforced by Dairy Association ads claiming that "Everybody Needs Milk" and "You Never Outgrow Your Need for Milk"

Perhaps because of the deep historical roots of this belief and fact that most scientists = Europeans or Euroamericans, it wasn't until 1965 that scientific documentation of variation in ability to digest milk began to accumulate

In the next few years, we learned that the great majority of humans not only outgrow their need for milk, but indeed their very ability to digest it!

That is, most adults lack the enzyme (lactase) needed to break down complex carbohydrate of "milk sugar" (lactose) into simpler sugars (glucose and galactose) that can be absorbed by human gut

Almost all human infants produce lactase, hence can digest lactose, but in most human populations (and all other mammalian species except domestic cats) this ability atrophies after weaning

Without lactase, much of the nutritional content of milk is unavailable, and lactose intolerance is evident (fermentative diarrhea ® cramps, gas, etc.)

People w/o lactase are termed "lactose malabsorbers" (easily identified by standard clinical test); others termed "lactose absorbers" (LA for short)

Most human populations can be classified as either consisting primarily of malabsorbers (0-20% LA among adults) or of absorbers (adult LA = 90-100%), but there are some populations with intermediate rates of LA

Worldwide distribution of LA phenotype shows following patterns:

1) High LA among N and W Europeans and their descendants

2) High LA among a few African, Mideast, and Mediterranean populations

3) Intermediate LA among some populations in Old and New World, mostly those with recent mixed ancestry

4) Low LA among all indigenous populations of Americas, Oceania, E and SE Asia, and much of Mediterranean and Near East

Thus, most "Third World" populations = malabsorbers, which should explain why milk powder in relief shipments often ends up being used for whitewashing walls, or discarded after reports it is poison are confirmed by diarrhea

Since there may be a student presentation on the lactose absorption issue in class, I'll say no more on it here...

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Sources:

Durham, William H. (1983) Testing the malaria hypothesis in West Africa.  In Distribution and Evolution of Hemoglobin and Globin Loci.  New York: Elsevier Science Publishing Co., Inc.

Durham, William H. (1991) Coevolution:  Genes, Culture, and Human Diversity.  Stanford: Stanford University Press.

Katz, S.H., M.L. Hediger and L.A. Valleroy (1974) Traditional maize processing in the New World.  Science 184: 765-73.

Katz, S.H., M.L. Heduger, and L.A. Valleroy (1975) The anthropological and nutritional significance of traditional maize processing techniques in the New World.  In Biosocial Interrelations and Population Adaptation, ed. E.S. Watts, F.E. Johnston, and G.W.

McCracken, Robert D. (1971) Lactase deficiency: an example of dietary evolution.  Current Anthropology 12: 479-517.

Wiesenfeld, Stephen L. (1967) Sickle-cell trait in human biological and cultural evolution.  Science 157: 1134-40.