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Earth Materials and Health: Research Priorities for Earth Science and Public Health
sumption of soil or clay as food—known variously as geophagia, geophagy, or pica—is a classic example of the intersection of earth science and public health. Human consumption of earth materials has been documented from historical times, and both involuntary and voluntary consumption of soil or clay occurs today (Abrahams, 2003, 2005). Because of immigration, the tradition of geophagia has been introduced and is increasing in Western societies, and imported soils can often be found in local ethnic food stores in this country for sale to immigrants. Geophagia is a potential route for transmission of pathogens (e.g., helminthes, see below) directly to the human host through ingestion of soil (Magnaval et al., 2001; Santamaria and Toranzos, 2003).
Geophagia is considered by many human and animal nutritionists to be either:
an acquired habitual response in which clays and soil minerals are specifically ingested to reduce the toxicity of various dietary components common to the local environment (e.g., in tropical rain forests, where many plants and fruits contain toxins to reduce their palatability) or
an innate response to nutritional deficiencies resulting from a poor diet, typically rich in fiber but deficient in magnesium, iron, and zinc. Such diets are common in tropical countries, particularly where the typical diet is dominated by starchy fiber-rich foods such as sweet potatoes and cassava.
From an historical perspective, geophagia has also been commonly associated with various mental disorders and afflictions that have a wide variety of rather unpleasant cures. Even today, the theory of geophagia as a subconscious response to dietary toxins or stress must be balanced against the habitual eating of soil that has been reported to develop into extreme, often obsessive, cravings. These cravings generally occur immediately after rain. Typical quantities of soil eaten by geophagics in Kenya have been reported to be 20 g per day—almost 400 times more than typical quantities of soil thought to be inadvertently ingested through hand-to-mouth contact (i.e., about 50 mg per day) or with leafy vegetables. Although eating such large quantities of soil increases exposure to essential trace nutrients, it also significantly increases exposure to biological pathogens and to potentially toxic trace elements, especially in areas associated with mineral extraction or in polluted urban environments.