Although rarely seen nowadays in the developed world, the practice of eating clay, geophagy, is not uncommon amongst tribespeople in various parts of the undeveloped world. Medicine formally describes geophagy as an aberration, but there is compelling evidence that the curious practice of eating clay is rooted in its medicinal value.
Many interesting aspects of the origins of human diet are outlined in the book, With Bitter Herbs They Shall Eat it: chemical ecology and the origin of human diet by Timothy Jones (University of Arizona Press, 1990).
Numerous reports record the fact that during famine it is common for people to supplement their diets with clay. For example, the Russian anthropologist Pitirim A Sorokin noted that during famines in Russia and Western Europe "the people have been brought to eating all sorts of roots, grass, hay, bark, earth with a small addition of flour and clay".
However, the practice of geophagy is not limited to episodes of famine or to more widespread occurrence in the distant past. Many peoples around the world today continue to eat clay in a variety of contexts, for example people in the Amazon basin, West Africa, Guatemala, New Guinea and the Philippines eat various forms of clay as spices, relishes and condiments.
On a more widespread basis it has often been recorded that pregnant women eat clay. And there is a well known tradition of clayeating in the southern USA amongst poor African-American women.
ONE obvious possible explanation for geophagy is that it is an automatic reaction that attempts to compensate for some nutritional deficiency, such as a lack of iron or some other mineral, in the ordinary diet. However, there is little evidence to support this hypothesis.
On the other hand there is considerable evidence that the practice developed and persists as a rational behaviour with a sound biological basis. Spanish exploration records from the 16th century report the widespread consumption of clays in South American villages. Various groups in South America today eat bitter-tasting potatoes as a mainstay in their diet, but the interesting thing is, they eat clay along with the potatoes.
Peoples in the Andes mountains of Bolivia and Peru mix clay with water to form a slurry into which they dip the potatoes before eating them. They say this takes away the bitter taste of the potatoes. It also wards off the nausea that develops when largish amounts of these potatoes are consumed without any accompanying clay dip.
These bitter potatoes contain organic toxins called glycoalkaloids that induce stomach pain, vomiting and even death when large amounts of potatoes are consumed. The clay detoxifies the potatoes and protects the consumer against these very unpleasant side effects. The chemical basis for the detoxifying properties of clay is well known. Clays bind avidly to organic compounds, partly because they are composed of very fine particles and have a large surface area onto which the organic compounds can stick. The glycolalkaloids are carried through the gut with the clay and excreted in the faeces. They are not absorbed into the bloodstream.
The evidence strongly suggests that eating clay is a general adaptive response that was chosen by natural selection during evolution because of its protective value. Plants contain many toxins that narrowed the range of vegetable matter that people could eat throughout history. Eating clay provides a degree of protection against plant toxins and therefore allowed people a much greater degree of flexibility in their diet. This was particularly valuable in times of famine.
This general conclusion is supported by the fact that many animals regularly eat clay. Gorillas and chimpanzees eat clay and many ecologists believe that the purpose of this geophagy is to detoxify their diet. Many animals protect themselves after eating poison by quickly inducing vomiting - readers may have observed this practice in dogs or cats who induce the vomiting by eating grass or some other uncustomary substance.
RATS are physiologically unable to vomit and, so, they have developed special ways to protect themselves against poison. It is very difficult to poison rats. Whenever they come across a novel taste that they associate with poison, they develop a conditioned aversion to that taste and completely avoid this food. However, if this poisonous food is the only food available, the rats will eat it and will accompany the food with clay in order to neutralise the poison.
There is evidence in some cases that the ingestion of clay provides a valuable nutritional role in itself, for example by providing the mineral calcium. However, such evidence is patchy and the primary significance of geophagy is attributed to its toxin-neutralising properties.
The adoption of geophagy as a general response to toxins allowed people to make wide use of natural resources and led to the domestication of certain plants. For example wild bitter potatoes were probably of minor dietary significance in South America until the practice of geophagy made their general consumption tolerable, leading to their domestication and adoption as the staple diet of a complex pre-historic civilisation in the Andes. Eating clay may well have provided a spark for the development of early agriculture.
So, there you have it. I'll bet you're wondering now whether to wash the clay off the spuds before cooking them from now on.
William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.