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The Best Science Writing Online 2012
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In the fall of 2009 a group of biology students at Tufts University sat down together and ate some dirt. They ground up small clay tablets and swallowed the powder to find out, firsthand, what clay tastes like. This unusual taste test was part of a Darwinian medicine class taught by one of us (Starks). The students were studying the evolution of geophagia—the practice of eating dirt, especially claylike soils, which is something animals and people have been doing for millennia.
The standard reference guide for psychiatrists—the fourth edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM-IV)—classifies geophagia as a subtype of pica, an eating disorder in which people consume things that are not food, such as cigarette ash and paint chips. But as the students would learn, studies of animals and human cultures suggest that geophagia is not necessarily abnormal—in fact, it may well be adaptive. Researchers are taking another look at dirt eating and discovering that the behavior often provides people and animals with vital minerals and inactivates toxins from food and the environment.
EVOLUTIONARY APPROACH
One way to decide whether geophagia is abnormal or adaptive is to determine how common the behavior is in animals and across human societies. If many different species and cultures demonstrate the same behavior, then it is probably beneficial in some way.
Today it is clear that geophagia is even more widespread in the animal kingdom than previously thought. Investigators have observed geophagia in more than 200 species of animals, including parrots, deer, elephants, bats, rabbits, baboons, gorillas and chimpanzees. Geophagia is also well documented in humans, with records dating to at least the time of Greek physician Hippocrates (460 B.C.). The Mesopotamians and ancient Egyptians used clay medicinally: they plastered wounds with mud and ate dirt to treat various ailments, especially of the gut. Some indigenous peoples in the Americas used dirt as a spice and prepared naturally bitter foods such as acorns and potatoes with a little clay to counteract the acerbic taste. Geophagia was a frequent practice in Europe until the 19th century, and some societies, such as the Tiv tribe of Nigeria, still rely on cravings for dirt as a sign of pregnancy.
A common explanation for why animals and people eat dirt is that soil contains minerals, such as calcium, sodium and iron, which support energy production and other vital biological processes. The fact that an animal’s need for these minerals changes with the seasons, with age and with overall health may explain why geophagia is especially common when an animal’s diet does not provide enough minerals or when the challenges of the environment demand extra energy. Mountain gorillas and African buffalo that live at high altitudes may, for example, ingest earth as a source of iron that promotes red blood cell development. Elephants, gorillas and bats eat sodium-rich clays when they do not get enough sodium in their diet. One elephant population is known to continually visit underground caves where the animals dig up and eat salt-enriched rock.
Among human populations in Africa, those who have ready access to calcium do not practice geophagia as often as those deprived of calcium. The need for calcium may also partly explain why geophagia is most commonly associated with pregnancy: a mother needs extra calcium as the fetal skeleton develops.
Mineral acquisition does not fully explain geophagia, though. In an extensive review paper published in the 2011 Quarterly Review of Biology, Sera L. Young of Cornell University and her colleagues conclude that eating earth rarely adds significant amounts of minerals to one’s diet and, in many cases, interferes with the absorption of digested food from the gut into the bloodstream, sometimes resulting in nutrient deficiency.
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15 Comments
Add CommentVery cool. We've accepted clay to cleanse the skin, why not the rest of the body? I've noticed that my green clay face masque actually tastes pretty good- maybe I'll try a dollop the next time I have a hangover :)
Reply | Report Abuse | Link to this-Caitlin
Dirt eating and mud wallowing comes natural to young children and dogs. There is two kinds of grass and the dirt around that grass that a dog will eat if its stomach is upset or it has gas build up. To rid its body from parasites, a dog will wallow in black mud - the kind that is found in swampy places. A child, most often, will seek out sandstone and smooth ash from burnt clay or coal if it has a food toxin build up. If the child cannot find the smooth ash, then they will eat a little bit of powdery sand - the kind that is most often found around creek banks, or burnt plant ask. If you accidently scorch the edges of your garden greens at dinner time, and if you notice your child eating that part first, then give them an anti-inflammatory drug.
Reply | Report Abuse | Link to thisTo correct an error in your report: Acorns and potatoes are not bitter herbs or plants. If a potato is sun burnt and has a shade of green to it, it will have a slight bitter taste and it is poisonous. The slight bitter taste tells a child and an animal not to eat it because it will harm them. Acorns have a slight bitter taste, which is not actually a bitter taste, but a tree sweet taste, which is a different sweet than sugar; the taste is caused by the tannins and quercitrin. Children and animals will eat acorns if they have an infection of some kind because it is an astringent with anti-inflammatory and antiseptic properties. If a child has an infection somewhere in its body, an acorn will not taste bitter to it, but have a slight sweet taste. If a child takes a bite of the acorn and it tastes sweet, the child will chew that part and swallow it. When the child takes another bite, the acorn will taste bitter and the child will spat it out. That is how nature protects us. It is good to see that science is catching up with the common person's knowledge and understanding of the world around us.
Many Americans are unnaturally obsessed with over cleanliness and "purity." This can be seen in the over use of anti-bacterial soaps to the over purification of foods.
Reply | Report Abuse | Link to thisHygiene and being clean is one thing, but being obsessively concerned with it is counter productive yet very common.
In Denmark we have an old saying "You need seven pounds of dirt a year", this article seems to validate this old adage, and explain why pregnant women sometimes eat dirt.
Reply | Report Abuse | Link to thisAt the same time, studies point to excessive cleanliness, not living with animals etc. as a possible cause of allergies etc.
In India this mud eating is a common practise amongst pregnat women.Even the local shops selling traditional medicines(called as "Pasarewale"s shop")do sell this dried mud for them. Usually this is heated in the source of heat(Shegadi) before eating(I think to get rid of germs)
Reply | Report Abuse | Link to thisMud eating is a natural response to get sufficient iron and other minerals.Like the cats who eat leafs of certain plants when they are ill.As a pregnant woman requires extra supply of these elements body demands in this form of response.Children develope a habit of eating chalks or earthen pencils when their body demands extra calcium or iron.
Reply | Report Abuse | Link to thisThe studies by Gilardi on geophagia among parrots did not explore the possibility that, although the particular clays they consume may not have been rich sources of "minerals" in general, whatever that means, or not as rich as other nearby clays, there might be a particular trace element (or two) they need that is present in higher concentration in the preferred clay. Although clays may very well reduce the blood level of quinidine when fed to parrots administered quinidine, there is some question in my mind as to how good a model this is. I want to know what the actual typical levels of assorted alkaloids are in the wild parrot diet, as well as how toxic these various alkaloid mixtures are, and I also want to know whether clay consumption varies seasonally with seasonal alkaloid level, as well as how efficacious the clay in question is at reducing alkaloid level of the real world alkaloidal mixtures. Clay eating is not universal among parrots throughout the world, and one might well wonder exactly what dietary differences exist between those which do and those which don't consume clay. What exactly is it in those particular parrots' diets that they need to be protected against that is insignificant elsewhere in the world? I'm aware that alkaloidal components are wildly varied throughout the plant world, but this explanation would make more sense if one could identify a particular toxin or group of toxins. I'd like to see a study of the alkaloidal composition of samples taken from the crops of wild parrots in the region. Are there, say, certain plant species in that region that are peculiarly toxic but that the parrots get away with consuming because of the clay eating? Show me. The possibility exists; chirimoya is a fruit that grows in the region that has toxic seeds, for instance. If you showed me that the parrots there eat chirimoya seeds and get away with it, then I would be more prone to believe Gilardi's hypothesis. But I don't buy these vague generalizations about toxins in the seeds and food they eat, as if that were a problem unique to that one area.
Reply | Report Abuse | Link to thisI grew up in the red clay hills of North Mississippi, and it was common, 50 or 60 years ago, to see people, predominantly women, take a chunk of firm red clay from the side of a road cut and proceed to eat it. These were people who appeared to be in an economic situation such that they did not receive regular medical care. A doctor once told me that he thought that they were doing this as a treatment for intestinal parasites, either to eliminate the parasites or alleviate discomfort. I assume this was what might be called a form of folk medicine.
Reply | Report Abuse | Link to thisWhile the terms "dirt" and "soil" are used synonymously in this article, soil scientists make an important distinction. Dirt is not soil. "Dirt" is what you find under your fingernails or sweep out from under your doormat. It's misplaced and is no longer part of the soil ecosystem (i.e., it's often biologically inactive or less active). "Soil" is a biologically active, living unit composed of mineral and organic materials and living forms (earthworms, microbes, etc) that performs many important ecosystem functions. We depend on soils for agricultural production, water quality and filtration, nutrient cycling, and ecosystem health.
Reply | Report Abuse | Link to thisIt may seem like semantics, but knowing the difference is important for understanding what's actually being discussed in this article.
Jennifer Ackerman's article "How Bacteria in Our Bodies Protect Our Health" in this (June 2012) issue argues that bacteria are a necessary part of human biology, helping us to perform many vital functions. Could geophagia perhaps be a mechanism by which the organism seeks to source the bacteria needed to form the microbiome and support these functions?
Reply | Report Abuse | Link to thisIn Germany "Heilerde" (i.e. healing earth or healing clay) is widely sold as alternative medicine - both for external and internal use. Its internal use is aimed to cure diseases such as diarrhea and gastroesophageal reflux. E.g. on the website of the company Luvos, which manufactures "Heilerde", one can see a person drinking this sort of clay dispersed is a glass of water.
Reply | Report Abuse | Link to thisFurthermore, the positive effects of "Heilerde" have been documented since medieval times under the names of "bolus armenicus", "terra armena", or "terra armenia". In the 19th century its use was
then publicized by the founders of the naturopathic medicine movement, e.g. Sebastian Kneipp, the "Lehmpfarrer" (clay priest) Emanuel Felke, and Adolf Just. According to Adolf Just this clay is nature's best medicine.
Now let me ask: Why were these circumstances not mentioned in the article?
Pregnant Australian Aboriginal women used to eat soil from termite mounds as it was very rich in Calcium. By the way, I remember a TV ad in England in the 60s trying to baffle the public with science in selling their indigestion tablets, showing the chemical symbols of the ingredients; Aluminium silicate (clay), Calcium carbonate (chalk), which could also be dug up from the ground where I lived (actually the chalk in the ground was Calcium bicarbonate).
Reply | Report Abuse | Link to thisIn reply to ParrotSlave - I agree with your points about toxins and lots of good questions. Have a look here at a new paper on this exact question about toxins in parrot diets (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0038293)
Reply | Report Abuse | Link to thisGood question, but unlikely to be the case - most animals, including humans, that eat soil do so well below the surface, often from a steep bank, river bank, road cut, or deep hole or cave. These sites tend to be 'soils' with high clay content, little organic matter, and little biological activity of the sort which would contribute microbial or probiotic factors to the consumers' gut.
Reply | Report Abuse | Link to thisI hope the authors check these comments, particularly Prof. Starks. A fellow faculty member at Tufts, Dr. Joel Weinstock MD, carries on lab research that deliberately introduces parasitic worms to modify or eliminate the effects of auto-immune syndromes such as Crohn's disease. While Prof. Starks and his co-author warn against ingesting "parasitic worms", the chemical/mineral approach on which they focus in their article is not sufficient to encompass all the dynamics of geophagia. It is conceivable, given Dr. Weinstock's research and that performed by the NHS in the UK amongst others, that human and animal dirt-eaters may be seeking instinctively to ingest biological life forms that could provide essential chemicals or perform other processes that help keep their immune systems in balance. The absence of certain so-called parasites may in fact be compromising the immune responses of increasing numbers of people, according to what is commonly termed the "hygiene hypothesis". I hope that the authors are prepared to reach out across disciplinary boundaries to contribute to a combination of chemical, biological, medical and social research strategies. Such a multi-pronged approach will be needed to develop a holistic public health response that could benefit those suffering from auto-immune syndromes ranging from asthma to multiple sclerosis, as well as those with diseases such as type-1 diabetes that may respond favourably to the introduction of specific parasites, microbes or chemicals. This is a major opportunity for scientists to consult together and combine resources in order to offer potentially significant benefits to a large number of people while benefitting the public health of the general population.
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