Eating dirt. Human clay consumption
Excerpted from the CDC


This earth is blessed. Do not play in it

Sign on the wall of El Santuario de Chimayo, New Mexico

This place feels old beyond human recollection. The carvings and paintings were surely done by human hands, but no one remembers whose hands those were. The work is striking, especially in the alcove behind the altar. There, the colors of surrounding hills have been transferred onto nearly luminous wooden reredos full of Catholic symbolism. Above the altar hangs a most intricate ancient Christ crucified on a green cross. Overhead, the roof is held in place by massive carved wooden beams, big around as human bodies and blackened by nearly two centuries of incense and candle smoke. The air is rich with the memory of thousands of benedictions and baptisms. Threadbare trousers have polished the pews to a high varnish that this afternoon ripples with a low orange glow from dozens of votive candles burning purposefully in back of the church.

This is El Santuario de Chimayo, an old adobe-brick and stucco structure in the hills of northern New Mexico. This chapel was built in 1816, but a sanctuary has been at this site for much longer. The locals offer many legends about its origins, fanciful tales of miraculous crucifixes and Santo Niños. But the truth is buried beneath the murk of time. One thing is clear though, as beautiful as the sanctuary is and as striking as the crucifix (El Senior de Esquipalas) above the altar is, nearly none of those in the pews today have come to see the sanctuary or the crucifix. Instead, they have come from all over the world to this place in New Mexico to eat the dirt that lies beneath the adobe floor.

According to legend, that dirt is sacred, consecrated by Christ himself. Crutches cast off by the newly healed fill the anteroom, and on some days, the line of pilgrims stretches for blocks. Some call this place the Lourdes of America, but in Chimayo the miracle can be seen each day by anyone who peers into a low-ceilinged room off the main entrance. There, a hole (the posito), half a meter across, pierces the floor. Beside it, someone has left a plastic spoon to aid the faithful. Beyond the spoon, beneath the opening, lies only dirt, only the deep-red dirt of Chimayo.

Most of the faithful here today have come to eat that dirt. This religious tradition is practiced, as far as I know, only at one other place—a Catholic shrine in Esquipalas, Guatemala. But pilgrims to these shrines are not the only humans who eat dirt. Nor are religious reasons the only reasons to imagine that dirt may have special powers.

Geophagy (Eating Dirt) and Its Reasons

Other than water, what little stuff we humans have inside us is largely dirt. Admittedly, this dirt is sometimes highly processed before we receive it, but most solids that make up humans and other creatures either are now or recently were dirt (the simple stuff that stripes the outer surface of our world, the thin paste that raises us above rocks) transformed by sunlight into plants or animals. Most of us prefer the dirt we eat in the form of cows and sheep and carrots and squash and bison and sorghum. Other dirt we’d just as soon scrape from our feet and leave at the door.

But not everyone wishes to be so far removed from the stuff of mud pies and mucilage. On every continent (except, possibly, Antarctica), some of us intentionally eat dirt, and we are joined in this practice by a myriad of rats, mice, mule deer, birds, elephants, African buffalo, cattle, tapirs, pacas, and several species of primates. Most scientists consider animal geophagy "normal," probably because most soil consumption by animals has no obvious adverse effects and is sometimes beneficial; however, some of these same scientists consider most (or all) human geophagy "abnormal."

Abnormal Behavior

In the United States, many of us believe that humans should only eat food. We consider the consumption of nonfood items pathological, even though we know that what people define as "food" varies dramatically by region and ethnicity. We call the pathological act of eating nonfood items pica. Pica is a disease, but a disease different from polio or smallpox. No infectious agent is obviously associated with pica. Pica is a disease only because we believe normal "undiseased" persons would not eat anything but traditional human foods; some of those who do, some of the time, are at considerable risk because of their unusual appetites.

Pathological consumption of soil, "soil pica," is associated with several psychological abnormalities. But all ingestion of soil is not soil pica. How much soil a person has to eat to be considered ill is not known. One report described soil pica in a developmentally disabled person who regularly consumed more than 50 g of soil per day. Most of us would consider that level of geophagy at least potentially pathological, although I am not sure why.

In June 2000, the U.S. Agency for Toxic Substances and Disease Registry appointed a committee to review soil pica. The committee settled on pathological levels as consumption of more than 500 mg of soil per day but conceded that the amount selected was arbitrary. Soil consumption is defined as pathological according to the amount eaten (no normal person could possibly eat that much dirt) and the severity of health consequences (lead poisoning, parasites). Because underlying psychological or biologic abnormalities are not easy to establish, I explore only what appears to be nonpathological dirt eating in pregnant women (especially in sub-Saharan Africa), migrants from sub-Saharan cultures to other parts of the world (notably the United States), and children worldwide.

Inadvertent Exposure

Why is it, that in spite of all the times we’ve been told not to, we still eat dirt? This is a very complex question with many possible answers. And while each proposed answer has its advocates, no single answer seems satisfactory to all—except one. Almost everyone agrees on one cause of geophagy, inadvertent consumption of air-, water-, and foodborne dirt. Contaminated food, soiled hands, and inhaled dust add soil to our diets. Children ingest considerable amounts of soil in these ways. My children did. Of course, my children also ate dirt on purpose. But child or adult, each of us inadvertently eats a little dirt every day. This dirt can pose a health threat, especially near sites of industrial contamination, but dirt we eat intentionally poses a greater challenge. Intention may indicate something biologic that drives some of us (sometimes regularly, sometimes religiously, sometimes ritually) to eat dirt.

Tradition and Culture

For centuries, indigenous peoples have routinely used clays (decomposed rock, silica and aluminum or magnesium salts, absorbed organic materials) in food preparation. The clays were used to remove toxins (e.g., in aboriginal acorn breads); as condiments or spices (in the Philippines, New Guinea, Costa Rica, Guatemala, the Amazon and Orinoco basins of South America); and as food during famine. Clays were also often used in medications (e.g., kaolin clay in Kaopectate). But the most common occasion for eating dirt in many societies (the only occasion in some societies) is pregnancy. When sperm and egg collide, the world changes. That is obvious. But why pregnant women eat dirt is not.

Wiley and Katz have proposed that eating clay serves different purposes during different periods of pregnancy, soothing stomach upset during morning sickness in the first trimester and supplementing nutrients (especially calcium) during the second and third trimesters, when the fetal skeleton is forming. This type of geophagy occurs most commonly in cultures of sub-Saharan Africa and their descendants. The timing of dirt ingestion and amounts consumed vary with tribes and individual persons, but soil comes consistently from certain sites. In some cultures, well-established trade routes and clay traders make rural clays available for geophagy even in urban settings. Clays from termite mounds are especially popular among traded clays, perhaps because they are rich in calcium. Whatever the underlying reason, geophagy in Africa does not appear to be a recent cultural development; it may predate Homo sapiens.

Women eat dirt during the first, second, or third trimester or throughout pregnancy, often throughout the day, as a supplement rather than a meal. Most commonly consumed are subsurface clays, especially kaolin, bentonite and montmorillonite, 30 g to 50 g a day (sometimes much more). However, eating dirt is not always confined to pregnant women, even among the cultures of sub-Saharan Africa, nor is it limited to tribes with little or no access to dairy-derived calcium, so these hypotheses do not adequately explain local tastes for dirt.

Soil, including kaolinitic and montmorillonitic(smectites) clays, contains considerable amounts of organic material, including many live microorganisms. The human gut is the largest area of direct contact between a person and the world. Gut-associated lymphoid tissue (GALT) is a major site of T-cell differentiation and selection in adults and of intense immunologic activity (including T lymphopoiesis) in children and adults 6–9). And while it is not entirely clear why some gut-introduced antigens promote tolerance of microorganisms and others immunize against them, it is clear that immunization via the gut is a major source of immunoglobulin (Ig) A, both locally and systemically 6–10).

Regular consumption of soil might boost the mother’s secretory immune system. Monkeys that regularly eat dirt have lower parasite loads. In some cultures, clays are baked before they are eaten, which could boost immunity from previous exposures. For decades we have used aluminum salts, like those found in clays, as adjuvants in human and animal vaccines. Adjuvants are compounds that nonspecifically amplify immune response, probably because of their effects on innate defenses such as macrophages, dendritic cells, and the inflammatory response. Aluminum compounds make effective adjuvants because they are relatively nontoxic, the charged surfaces of aluminum salts absorb large numbers of organic molecules, and macrophages and dendritic cells readily phagocytose the particulates produced by the combination of the adjuvants and the organic compounds. The clays that pregnant women and others consume, which are rich in aluminum compounds, likely make at least passable immunologic adjuvants. For all these reasons, clays might act as vaccines. And the IgA antibodies produced against the associated organic antigens may appear in breast milk and have a major role in mucosal protection of newborns.

In pregnant women, this type of gut immunization might produce high levels of IgA against endemic pathogens and other antigens. All this IgA would appear shortly before birth in the breast milk and would provide protection for infants against precisely the pathogens encountered immediately after birth. Furthermore, IgA antibodies prevent attachment of bacteria and some viruses at mucosal surfaces, the major contact between the infant and the infectious world. In humans, mucosal surfaces offer the only routes of natural immunization short of wounding, and dirt would seem to offer a potent vaccine containing many endemic pathogens—no needles, no sugar-cube, no gene gun.

Eating dirt, then, rather than being abnormal, may be an evolutionary adaptation acquired over millennia of productive and not-so-productive interactions with bacteria—an adaptation that enhances fetal immunity and increases calcium, eliminates gastric upset, detoxifies some plant and animal toxins, and perhaps boosts mothers’ immunity at times when the hormones of pregnancy, factors produced by the fetus, changes in the complement system, replacement of MHC class I antigens in the trophoblast, and who knows what else suppress the mother’s natural immunologic desire to destroy her fetus—a miracle, nearly.

Innate Tendency

My children ate dirt with surprising gusto, garden soil, road soil, leaf-mush soil, sod soil, bug-body soil—even gutter soil. As usual with my children, before I could talk them out of this behavior, they gave it up on their own—their behavior depending more on personal likes and dislikes than on my paternal concerns. I was pleased when they quit. Later I was reassured to discover from other parents that their children were just as taken with dirt as mine, some even more so. I felt less like the parent of a couple of dirt-eating, psychosis-ridden, nutritionally deprived children, even if my children were never quite "normal."

Eating dirt appears nearly universal among children under 2 years of age. When I asked my 2-year-old daughter why she ate dirt, she just stared at me, her eyes wide open, a thick moustache of loam limning her lips. She must have decided that either what I had asked was unfathomably abstract or her answer would be far beyond my comprehension.

Soil pica has been defined as eating 500 mg to >50 g of soil per day. But the general applicability of these numbers is widely disputed (pregnant women in Africa eat far more soil than this). By inference, however, normal soil consumption must fall into the range of 0 mg to 500 mg per day per small mouth. Soils consumed by children may differ from those consumed by adults. Generally, children consume topsoils and not the deep (60 cm- to 90-cm deep) clays adults regularly consume. And children are considerably less selective in the sites they choose for dirt to eat. But why children eat dirt remains largely obscure to all but children.

Children may eat soil for the same reasons pregnant women and some animals do. Because of their rapid growth, they have special nutritional needs and surface soils may serve as supplemental nutrients; detoxification of plant or animal toxins might be accelerated by geophagy— particularly in some parts of the world; or soil components, especially clays, may relieve gastric distress. But topsoils are probably not as effective as deep clays at gastric soothing.

Among children, too, it seems eating dirt might have immunologic consequences. Maternal immunoglobulins are secreted in breast milk shortly before birth and for 1 year or more afterwards. Children often begin eating dirt a year or two after birth. As maternal immunity wanes, eating dirt might "vaccinate" children who are losing their maternal IgA, which could stimulate production of nascent immunoglobulins, especially IgA. Eating dirt might also help populate intestinal flora.

But all of this remains speculative. No clear evidence supports a biologic benefit to geophagy among children. Its frequency and distribution, though, suggest a greater biologic involvement than the simple oral obsessions of children.

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