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Geophagy is the practice of eating earth or soil-like substrates such as clay or chalk. It occurs in non-human animals where it may be a normal or abnormal behaviour, and also in humans, most often in rural or preindustrial societies among children and pregnant women. Human geophagy may be related to pica, a classified eating disorder in the DSM-IV characterized by abnormal cravings for nonfood items.
Geophagy is widespread in the Animal Kingdom. Galen, the Greek philosopher and physician, was the first to record the use of clay by sick or injured animals in the second century AD. This type of geophagy has been documented in "many species of mammals, birds, reptiles, butterflies and isopods, especially among herbivores."
Many species of South American parrots have been observed at clay licks, and Sulphur-crested Cockatoos have been observed ingesting clays in Papua New Guinea (Discover, 1998) as well as in Glenbrook in the Blue Mountains of Australia (Parrots Magazine, 2000). Analysis of most soils consumed by wild birds show that they prefer soils with high clay content, often with the smectite and bentonite clay families being well represented.
The preference for certain types of clay or soil can lead to unusual feeding behaviour. For example, Peruvian Amazon rainforest parrots congregate not just at one particular bend of the Manu River but at one specific layer of soil which runs hundreds of metres horizontally along that bend. The parrots avoid eating the substrate in layers one metre above or below the preferred layer. These parrots regularly eat seeds and unripe fruits containing alkaloids and other toxins that render them bitter and even lethal. Because many of these chemicals become positively charged in the acidic stomach, they bind to clay minerals which have negatively charged cation-exchange sites, and are thereby rendered safe. Their preferred soils have a much higher cation-exchange capacity than the adjacent layers of soils that were rejected because they are rich in the minerals smectite, kaolin and mica. The preferred soils surpass the pure mineral kaolinate and surpass or approach pure bentonite in their capacity to bind quinine and tannic acid. In vitro and in vivo tests of these soils indicate that they also release nutritionally important quantities of minerals such as calcium and sodium. It remains unknown which function is the more important in avian geophagy.
There are several hypotheses about the importance of geophagy in bats and primates.  Benefits of geophagy include nutrition, detoxification, antimicrobial and anti-parasitic agent. However, the reasons for geophagy are largely species-specific. Krishnamani and Mahaney evaluated various hypotheses that could lead to routine geophagy in different species of primates including Japanese macaques and chimpanzees. Reasons for geophagy are largely dependent on the species as well as the habitat. For instance, mountain gorillas demonstrate geophagy as a result of an increased need for iron. Iron supplementation from the soil helps to meet the demands of decreased oxygen partial pressure at high altitudes.
There is debate over whether geophagy in bats is primarily for nutritional supplementation or detoxification. It is known that some species of bats regularly visit mineral or salt licks to increase mineral consumption. However, Voigt et al. 2008 demonstrated that both mineral-deficient and healthy bats visit salt licks at the same rate. Therefore, mineral supplementation is unlikely to be the primary reason for geophagy in bats. Additionally, bat presence at salt licks increases during periods of high energy demand. Voigt et al. 2008 concluded that the primary purpose for bat presence at salt licks is for detoxification purposes, compensating for the increased consumption of toxic fruit and seeds. This was shown to be especially evident in lactating and pregnant bats, as their food intake increases to meet higher energy demands.
"The oldest evidence of geophagy practised by humans comes from the prehistoric site at Kalambo Falls on the border between Zambia and Tanzania (Root-Bernstein & Root-Bernstein, 2000)." Here, a calcium-rich white clay was found alongside the bones of Homo habilis (the immediate predecessor of Homo sapiens).
Anthropological and historical evidence
Geophagy is nearly universal around the world in tribal and traditional rural societies (although apparently it has not been documented in Japan and Korea). Also, the eating of clay (a form of geophagy) has been documented in historical sources beginning with Ancient Mesopotamia and Egypt. (Also see under medicinal clay.)
Geophagy was practised by Native Americans in California and Peru who would eat earth with acorns and potatoes to neutralize potentially harmful alkaloids. Clay was used in the production of acorn bread in California and Sardinia, Italy. Among the Jews in the second and third centuries, a type of earth was consumed for medical purposes, but the Talmud warns about possible physiological damage from eating it Babylonian Talmud, Shabbat 113b.
In parts of Africa, rural areas of the United States, and villages in India, clay consumption is correlated with pregnancy and some women eat clay to eliminate nausea, possibly because the clay coats the gastrointestinal tract and may absorb dangerous toxins. The clay may also provide critical calcium for fetal development (Vermeer).
In Africa, kaolin, sometimes known as kalaba (in Gabon and Cameroon), calaba, and calabachop (in Equatorial Guinea), is eaten for pleasure or to suppress hunger. Consumption is greater among women, especially during pregnancy.
In Haiti, geophagy is widespread. The clay mud is worked into what looks like pancakes or cookies, called "bon bons de terres" (earthy bon bons), that are dried in the sun and sold throughout the poorer areas. Small amounts of other ingredients, vegetable shortening, salt and sometimes sugar, are also added to the mix.
Bentonite clay is available worldwide as a digestive aid; kaolin is also widely used as a digestive aid and as the base for some medicines. Attapulgite, another type of clay, is an active ingredient in many anti-diarrheal medicines.
In the United States
According to Dixie's Forgotten People: the South's Poor Whites, by J. Wayne Flynt, geophagy was quite common among poor whites in the South-eastern United States. In the 19th and early 20th centuries, this was often ridiculed in popular literature. The literature also states that "Many men believed that eating clay increased sexual prowess, and some females claimed that eating clay helped pregnant women to have an easy delivery." Geophagy being a common practice among southerners may have been caused by the high prevalence of hookworm disease, in which the desire to consume soil is a symptom. Geophagy was common among slaves who were nicknamed "clay-eaters" because they had been known to consume clay, as well as spices, ash, chalk, grass, plaster, paint, and starch. The author cites a recent survey by Obstetrics-Gynecology Clinic at Duke Medical Center in North Carolina, according to which one quarter of patients were clay-eaters.
Cooked, baked, and processed dirt and clay are sold in health food stores and rural flea markets in the South. Clays like activated attapulgite and diosmectite have been used in active ingredients in over-the-counter antidiarrheal medications. The US version of Kaopectate, for example, contained kaolinite clay until a reformulation in 2003. Researchers have noticed that geophagy is not as prevalent as it once was as rural Americans assimilate into urban culture.
Impact on health
In a Science Digest article (Paraquat: a Potent Weed Killer is Killing People), it is recommended that a paraquat poisoning victim promptly swallow dirt, even at the risk of salmonella, because paraquat is deactivated upon contact with soil.
Chimpanzees in Kibale National Park, Uganda, have been observed to consume soil rich in kaolinite clay shortly before or after consuming plants including Trichilia rubescens, which possesses antimalarial properties in the laboratory. Simulated mastication and digestion reveals that the clay helps to release active antimalarial components from the leaves. The same type of soil is used by local healers to treat diarrhea, presumably by the same mechanism as over-the-counter antidiarrheal preparations.
There are obvious risks in the consumption of earth that is contaminated by animal or human feces; in particular, parasite eggs, such as roundworm, that can stay dormant for years, can present a problem. Tetanus poses a further risk.
There is a psychological hypothesis, which is centred on the cravings reported by clay eaters. Researchers' attention was directed mainly towards pregnant and postpartum women and their emotional states. Geophagy was attributed to feelings of misery, homesickness, depression, and alienation.
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