GEOPHAGY. Geophagy, the consumption of earth, is widespread in various animal taxa, including birds, reptiles, and mammals. Among the latter it is reported in rats, ungulates, and primates, and in many human populations. The most frequently consumed soils are generally rich in clay, and the qualities of clay appear to be the stimulus for geophagy. The prevalence of this practice suggests that it is not aberrant behavior, but rather that it may have some functional significance related to diet and ecology. Several overlapping hypotheses have been proposed to explain the existence of geophagy, and clay consumption in particular: (1) clay provides supplemental minerals that may be lacking in the routine diet; (2) clay has the capacity to adsorb toxic secondary compounds widely distributed in plant foods; (3) clay protects the gastrointestinal tract from chemical and biological insult, and thus counters gastrointestinal disease. Support exists for all of these hypotheses depending on ecological context and taxon and indicates that geophagy is likely to be associated with positive biological effects.
The unique chemical structure of clays allows them to have these biological functions. Clays are associated with a variety of soil types, and are formed by routine weathering forces. All clays have similar properties: a large surface area, which derives from the organization of silicon-oxygen tetrahedrons in hexagonal networks, and an ability to bind and exchange minerals because of the dense localization of hydroxyl ions and oxygen in the tetrahedron structures. Clays are commonly composed of aluminum, magnesium, iron, and calcium, which can engage in mineral exchanges. Clays also have colloidal properties that make them adsorbent of water and other organic compounds. Commonly consumed clay types include: kaolin, smectite, montmorillonite, halloysite, and allophane.
Geophagy is well-described among ungulates, which seem to derive significant mineral nutrition (especially sodium, calcium, and magnesium) from eating earth, and among rats, who appear to use clay to detoxify a highly omnivorous and opportunistic diet. Gilardi and others found that parrots in Peru consumed large amounts of clay-rich soils, which served to detoxify a seed-based diet that was high in secondary compounds and to protect the cells that line the gastrointestinal tract from these toxic chemicals. Within the primate order, apes and monkeys consume soils that contain valuable minerals along with the clay metahalloysite, which counteracts diarrhea.
Among humans, the consumption of clay takes a variety of forms. In some cultural contexts, clay is an integral part of cuisine. Timothy Johns has documented the use of clay sauces with potatoes among highland Andean populations. Consumed in this way, clay adsorbs the toxic glycoalkaloid (solanine) in the potato cultigens that are the staple foods of this region. Clay is also used in the production of acorn bread by both Native Americans and Sicilians (Johns and Duquette, 1991). In this example, baking with clay reduces the toxicity of tannins in acorns, and improves the overall nutrient composition of this food. Clay is also used for specific medicinal purposes, most often to counteract gastrointestinal illness such as nausea, heart-burn, or diarrhea (Vermeer and Ferrell Jr., 1985). Consumed in tablet or liquid suspension (as in Kaopectate®), clay has these effects by slowing gastrointestinal motility, binding toxins or pathogenic microorganisms, and buffering acids of the upper gastrointestinal tract. It is important to note that since clay can adsorb a variety of chemical compounds, it can also interfere with the absorption of pharmaceuticals such as antimalarial drugs (chloroquine).
Beyond the routine use of clay in cuisine, clay consumption is often correlated with pregnancy in humans (Lagercrantz, 1958). The practice is reported most frequently among Africans and African Americans, although it is found in many other populations. Women report that clay eases the nausea and vomiting that often occur during the first trimester. This is likely due to the ability of clay to buffer the gastrointestinal tract and adsorb toxins, to which the embryo is especially vulnerable during early development. Clay consumption often continues throughout pregnancy, and clay may provide supplemental calcium, the demand for which increases during pregnancy to form the fetal skeleton. Support for this analysis comes from Andrea S. Wiley and Solomon H. Katz's study (1998) of geophagy in African populations, which demonstrated that clay consumption was significantly more common in populations that did not consume milk and that relied heavily on toxin-rich plant foods. Hence clay may serve as a detoxicant as well as a source of calcium; both are particularly important for nondairying, agricultural populations. Many sub-Saharan African clays (especially those derived from termite mounds) have been found to be rich in calcium (Hunter, 1993). Importantly, clays are frequently baked before consumption, thus reducing the potential for microbial contamination. When clay is not readily available, laundry starch is sometimes consumed by pregnant women, although this is not likely to be associated with the same health benefits as clay.
Hunter, John M. "Macroterme Geophagy and Pregnancy Clays in Southern Africa." Journal of Cultural Geography 14, no. 1 (1993): 69–92.
Johns, Timothy, and Martin Duquette. "Detoxification and Mineral Supplementation as Functions of Geophagy." American Journal of Clinical Nutrition 53 (1991): 448–456.
Lagercrantz, Sture. "Geophagical Customs in Africa and among the Negroes in America." Studia Ethnographica Upsaliensia 17 (1958): 24–81.
Vermeer, Donald E., and Ray E. Ferrell Jr. "Nigerian Geophagical Clay: A Traditional Antidiarrheal Pharmaceutical." Science 227 (1985): 634–636.
Wiley, Andrea S., and Solomon H. Katz. "Geophagy in Pregnancy: A Test of a Hypothesis." Current Anthropology 39, no. 4 (1998): 532–545.
Andrea S. Wiley
Processing of acorns for consumption is laborious and time-consuming, but the large quantity of tannins in acorns makes them inedible unless some mechanism for their removal is employed. In traditional Native American cuisine, acorns were first hulled (sometimes after boiling, to make it easier to extract the nutmeats) and pounded into a coarse flour with a stone mortar and pestle. Then, a variety of leaching techniques could be employed, including putting the flour in a basket or woven bag or digging a hole in a sandy bank along a river and allowing water to flow through the flour for up to several days. Or the flour could be put in a hole in the ground that was lined with leaves or pine boughs; water was then poured over it numerous times. The leaching removed the bitter tannins, thereby making the acorn flour both more palatable and more digestible. The flour was then placed into a tightly woven basket with water and very hot rocks to make a boiled acorn mush. It could also be molded into patties and fried, or the flour could be dried and then made into a stiff dough and slowly baked in a smoldering fire to produce bread. In contexts where leaching was not practiced or was insufficient to reduce the bitterness of acorns, acorn meal was mixed with clay and water and baked in an earth oven for several hours to produce acorn bread.