Geophagia (also known as 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.:33 Human geophagia may be related to pica, an eating disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM) characterized by abnormal cravings for non-nutritive items.
Anthropological and historical evidence
Some researchers believe that humans first ate soil in Africa:
The oldest evidence of geophagia 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).—Peter Abrahams, Geophagy and the Involuntary Ingestion of Soil:446
Geophagia is nearly universal around the world in tribal and traditional rural societies (although apparently it has not been documented in Japan and Korea). In the ancient world, several writers noted the use of geopagia. Pliny is said to have noted the use of soil on the Greek Island of Lemnos and the use of the soils from this island have been noted until the 14th century.  The textbook of Hippocrates (460-377 BCE) mentions geophagia and the famous medical textbook called De Medicina edited by A Cornelius Celsus (14-37 CE) seems to link anaemia to geophagia.
Early explorers in the Americas noted the use of geophagy amongst Native Americans including the Gabriel Soares de Sousa who reported in 1587 of a tribe in Brazil using it in suicide and von Humboldt who said that a tribe called the Otomacs ate large amounts of soil. In Africa, Livingston wrote about slaves eating soil in Zanzibar and it is also thought that large number of slaves brought with them soil eating practices when they were shipped as part of the transatlantic slave trade.  Geophagia 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.
In more recent times, according to Dixie's Forgotten People: the South's Poor Whites, geophagia was common among poor whites in the South-eastern United States in the 19th and early 20th centuries, and 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." Geophagia among southerners may have been caused by the high prevalence of hookworm disease, in which the desire to consume soil is a symptom.
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, people afflicted by poverty are known to eat biscuits made from soil, salt, and vegetable shortening. This is reported to cause stomach pains, and long-term consumption of these biscuits could be a source of malnutrition and is not recommended by doctors. 
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 USA, cooked, baked, and processed dirt and clay are sold in health food stores and rural flea markets in the South. However, geophagia has become less prevalent as rural Americans assimilate into urban culture.
Geophagia 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 geophagia has been documented in "many species of mammals, birds, reptiles, butterflies and isopods, especially among herbivores."
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 geophagia.
There are several hypotheses about the importance of geophagia in bats and primates.:436 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.
There is debate over whether geophagia 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. 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 geophagia in bats. Additionally, bat presence at salt licks increases during periods of high energy demand. Voigt et al. 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.
Impact on health
Clay minerals have been reported to have beneficial microbiological effects. Humans are not able to synthesize vitamin B12 (cobalamin), so geophagia may be a behavioral adaption to obtain it from bacteria in the soil.:195
There are obvious health risks in the consumption of soil that is contaminated by animal or human feces; in particular, helminth eggs, such as Ascaris, that can stay viable in the soil for years, can lead to helminth infections. Tetanus poses a further risk. Lead poisoning is also associated with soil ingestion.
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