If animals and people are not getting much in the way of dietary minerals from dirt, what is the benefit of geophagia? A second explanation—that eating dirt is often a form of detoxification—is gaining credence.
The idea that, in most cases, eating dirt is probably a way to get rid of toxins could explain why people and animals so often prefer claylike soils to other kinds of earth. Negatively charged clay molecules easily bind to positively charged toxins in the stomach and gut—preventing those toxins from entering the bloodstream by ferrying them through the intestines and out of the body in feces. Detoxification might also explain why some indigenous peoples prepare meals of potatoes and acorns with clay—these foods are bitter because they contain small amounts of toxins.
In the 1990s James Gilardi, executive director of the World Parrot Trust, found support for the detoxification hypothesis in one of the few experimental studies on geophagia. While observing a flock of Peruvian parrots foraging on a particular band of exposed soil along the Manu River, Gilardi noticed that the birds neglected nearby stretches of soil with far more minerals. He surmised that the parrots were not ingesting soil for minerals but rather to counteract toxic alkaloids in the seeds and unripe fruit that make up a large part of their diet. Toxins prevalent in plants (and meats) often irritate the gut. To test this idea, Gilardi fed some parrots the toxic alkaloid quinidine with and without their preferred dirt and measured how much alkaloid made it into the birds’ blood after the meal. Birds that did not consume the soil had higher levels of quinidine in their blood, whereas a side dish of dirt reduced quinidine levels in the blood by 60 percent. Researchers have shown the same benefit in chimpanzees and baboons that supplement their diets with clay.
Further evidence of dirt detox comes from studies of bats. A 2011 study in PLoS ONE asked whether Amazonian bats visit clay licks—cliff sides of exposed clay—for nutrition or detoxification. Christian Voigt of the Leibniz Institute for Zoo and Wildlife Research in Berlin and his colleagues captured bats of two different species: one that eats mostly fruit and one that eats mostly insects. If the bats were eating clay for minerals, Voigt predicted, he would find fewer fruit-eating bats at the clay licks because fruits have more dietary minerals than insects. But most of the bats he captured at the clay lick were fruit-eating bats—and many of them were pregnant or lactating. Voigt concluded that the pregnant fruit bats visited the clay licks to detox because they were eating twice as much to feed their babies, which meant twice the dose of plant toxins from unripe fruits, seeds and leaves.
Like bats, pregnant women may also eat dirt for its detoxifying properties, in addition to using dirt as a supplemental source of minerals. The first trimester of pregnancy plagues many women with nausea and vomiting, and cross-cultural studies document geophagia early in pregnancies in response to morning sickness. Women in sub-Saharan nations and in the southern U.S. have reported that they consume clay to alleviate this discomfort. Some researchers have proposed that morning sickness purges the mother of toxins that might harm the fetus. Perhaps geophagia and morning sickness work together to protect the developing fetus. Because clay can bind bacteria and viruses, it may also protect both mother and fetus from food-borne pathogens such as Escherichia coli and Vibrio cholerae.
Although the scientific community has only recently accumulated enough evidence to argue that geophagia is an adaptive behavior, people—and not just pregnant women—have used clay minerals as remedies for nausea, vomiting and diarrhea for thousands of years. In the age of modern medicine, pharmaceutical companies harnessed the binding properties of kaolin, a clay mineral, to produce Kaopectate, a drug that treats diarrhea and other digestive issues. Eventually the synthetic chemical bismuth subsalicylate—also the key ingredient in Pepto-Bismol—replaced kaolin, but the clay is still used today in other ways. Kaolin and smectite bind not only harmful toxins but also pathogens. Ranchers use clay when preparing livestock feed to inhibit toxin transmission, and some researchers have proposed harnessing clay’s pathogen-binding talents to purify water.