Geophagy is the intentional eating of earth. It’s a type of pica, the craving for and consumption of non-food items. Pica is often considered a rare psychological disorder, yet the consumption of soil, dirt, clay, sand, chalk, and other forms of earth is widely practiced across the globe. Hippocrates wrote about it some 2,400 years ago, while archaeological evidence suggests it may stretch back over a million years to our Homo habilis relatives. And it isn’t only humans: other primates (at least fifty-seven species of them) do it. So do other vertebrates (240 species, at least), including other mammals, birds, and reptiles.

The question of why humans and other vertebrates consume earth has long puzzled anthropologists, ecologists, biochemists, ethologists, geographers, medical professionals, and others. “Yet, even after much investigation, geophagy remains an enigma,” write Sera L. Young, Paul W. Sherman, Julius B. Lucks, and Gretel H. Pelto. They note that “single-discipline approaches” and under-reporting (there can be stigma associated with it) have led to a “scarcity of hypothesis-driven approaches to its study.”

There are three existing hypotheses about geophagy. The most well-known is that people eat dirt because of dietary deficiencies: they aren’t getting enough mineral micronutrients like zinc and iron.

The second hypothesis is that people eat dirt because it may offer protection against harmful chemicals, parasites, and pathogens by making the gut wall less permeable to toxins and pathogens. The earth may also bind directly to toxins and pathogens, making them too big to be absorbed.

These first two hypotheses consider geophagy a helpful, adaptive behavior. The third hypothesis holds that it’s a non-adaptive or counterproductive. People do it “to ease hunger pains when no other food is available” and/or because “nutrient deficiencies have caused neurological or sensory problems,” including confusing the body’s appetite-controlling chemistry.

Young et al. reviewed 482 published accounts of human geophagy and 330 published accounts of other animal geophagy to “systematically evaluate the three hypotheses for the etiology of geophagy.” They note that their paper does not address the

cultural forces and beliefs that shape geophagy (the proximate determiners of behavior), but rather explores the ecological triggers and physiological underpinnings of geophagy (the ultimate causes.) Socioeconomic status may be an underlying cause of hunger, nutrient deficiency, or increased exposure to toxins and pathogens, but it is not a direct cause of geophagy itself.

In their definition of geophagy, the authors exclude “mouthing behavior of young children” (this is “environmental exploration”); purely symbolic instances, as in “oaths, mourning, tests of innocence or for religious purposes”; and such preparations of food as nixtamalization, in which corn is soaked in a limestone-derived solution.

For non-human animals, lithophagy (ingesting rocks or grit) was excluded: some vertebrates regularly do this to aid their digestive system in breaking down tough foods. While clay, mineral, and salt licks were all considered aspects of geophagy for other animals, human salt consumption—it’s the only rock/mineral we eat as a regular part of our varied diets—isn’t considered geophagy.

Considering how well-known the nutrient hypothesis is, the authors conclude that “few of the available data support the hypothesis that geophagy functions to ameliorate mineral nutrient deficiencies.” Minerals in soils just aren’t all that bioavailable, meaning they’re not easily absorbed in the body. (As with multivitamins and supplements, people may just think they’re doing good.) The consumption of clay, specifically, “might actually cause nutrient deficiencies” because it acts to bind nutrients in the body.

The authors also conclude that human geophagy is “best explained as providing protection from dietary chemicals, parasites, and pathogens.” For other animals, however, “geophagy may involve both micronutrient acquisition and protection.” In their pitch for more study and consideration of geophagy, they argue that it’s time to stop regarding geophagy as “a bizarre, non-adaptive gustatory mistake.”

The data tell a clear story: “geophagy is a widespread behavior in humans and other vertebrates that occurs during both vulnerable life stages and when facing ecological conditions that require protection.”

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