Most of us never considered eating the mud pies we made as kids, but for many people all over the world, dining on dirt is nothing out of the ordinary. Now an extensive meta-analysis forthcoming in the June issue of The Quarterly Review of Biology helps explain why. According to the research, the most probable explanation for human geophagy—the eating of earth—is that it protects the stomach against toxins¹, parasites², and pathogens（病原体） .

The first written account of human geophagy comes from Hippocrates more than 2,000 years ago, says Sera Young, a researcher at Cornell University and the study's lead author. Since then, the eating of earth has been reported on every inhabited continent and in almost every country.

Despite its ubiquity, scientists up to now have been unable to definitively³ explain why people crave⁴ earth. Several hypotheses had been considered plausible⁵. Some researchers think geophagy is simply a consequence of food shortage. In other words, people eat dirt to ease the pangs⁶ of hunger, even though it doesn't provide any nutritional⁷ value. Others have suggested that nutrition is exactly why dirt is consumed; perhaps people crave dirt because it provides nutrients⁸ they lack, such as iron, zinc⁹, or calcium¹⁰. Still others posit¹¹ that earth has a protective effect, working as a shield against ingested parasites, pathogens, and plant toxins.

To sort through the possible explanations, Young and her colleagues analyzed¹² reports from missionaries¹³, plantation¹⁴ doctors, explorers, and anthropologists（人类学家） to put together a database of more than 480 cultural accounts of geophagy. The database includes as many details as possible about the circumstances under which earth was consumed, and by whom. The researchers could then use patterns in the data to evaluate each potential explanation.

They found the hunger hypothesis unlikely. Studies in the database indicate that geophagy is common even when food is plentiful¹⁵. Moreover, when people eat dirt they tend to eat only small quantities that are unlikely to fill an empty stomach.

The nutrition hypothesis was also a poor fit to the data. The database shows that the kind of earth people eat most often is a type of clay that contains low amounts of nutrients like iron, zinc, and calcium. Plus, if calcium deficiency drove people to eat dirt, one would expect them to do it most often at life stages when they need calcium the most—adolescence or old age. But that isn't the case, according to the database. Reports do indicate that geophagy is often associated with anemia¹⁶, but several studies have shown that cravings for earth continue even after people are given iron supplements. What's more, some research suggests that clay can bind¹⁷ to nutrients in the stomach, making them hard to digest. If that's true, it's not a lack of nutrients that causes geophagy; rather it could be the other way around.

Overall, the protection hypothesis fits the data best, the Cornell researchers found. The database shows that geophagy is documented most commonly in women in the early stages of pregnancy¹⁸ and in pre-adolescent children. Both categories of people are especially sensitive to parasites and pathogens, according to Young and her colleagues. In addition, geophagy is most common in tropical climates where foodborne（食物传播的） microbes are abundant. Finally, the database shows that people often eat earth during episodes of gastrointestinal stress. It's unlikely the intestinal¹⁹ problems are caused by the dirt itself because the type of clay people usually eat comes from deep in the ground, where pathogens and parasites are unlikely to contaminate it. Plus, people usually boil the clay before eating it.

More study would be helpful to confirm the protection hypothesis, the researchers say, but the available data at this point clearly support it over the other explanations.

"We hope this paper stimulates²⁰ [more] research," Young and her colleagues write. "More importantly, we hope readers agree that it is time to stop regarding geophagy as a bizarre（奇异的） , non-adaptive gustatory（味觉的） mistake."

"With these data, it is clear that geophagy is a widespread behavior in humans … that occurs during both vulnerable life stages and when facing ecological²¹ conditions that require protection."