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 geophagythe eating of earthis that it protects the stomach against toxins, parasites, and pathogens.
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 definitively explain why people crave earth. Several hypotheses had been considered plausible. Some researchers think geophagy is simply a consequence of food shortage. In other words, people eat dirt to ease the pangs of hunger, even though it doesn't provide any nutritional value. Others have suggested that nutrition is exactly why dirt is consumed; perhaps people crave dirt because it provides nutrients they lack, such as iron, zinc, or calcium. Still others posit 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 analyzed reports from missionaries, plantation doctors, explorers, and anthropologists 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 plentiful. Moreo
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