01 December 2006

Let them it dirt!

An interesting news article by Trevor Stokes in this week's Nature (vol. 444, pp.543-544) is about geophagia, the practice of eating soil, usually clay, by humans. The article reports that geophagia is present in many parts of the world, but from the examples mentioned, it seems to be most prevalent among African children and pregnant women. According to the article, there is even an African grocery store in New Jersey that sells clay for eating. I wonder what FDA would think of that.

Several hypotheses have tried to explain why people eat clay. One idea is that clay supplies essential trace metals such as iron and zinc. However, some experiments simulating the conditions in a human stomach have demonstrated that clay actually binds trace metals so tightly that it may end up removing them from a clay-eater's gastrointestinal tract, creating deficiencies.

According to another hypothesis, proposed by Paul Sherman, eating clay may help remove toxic chemicals found in some plants, such as solanine (misspelled as solanin in Nature) in potatoes. I have not read any of Sherman's papers, but the Nature article seems to be implying that this offers an evolutionary advantage. I can't see how what seems to be a cultural practice could be an evolutionary process unless whether or not a person will eat clay is somehow genetically determined.

The article lists nicotine in tobacco as another toxic chemical that could be removed by clay. I thought that was an irrelevant example, because people don't normally eat tobacco and tobacco use is such a recent invention in human history that I can't see how it could have anything to do with the origin of geophagia that is said to have been practiced in ancient Sumeria. It seems that Trevor Stokes couldn't think of any more relevant plant toxins. Long lists of edible plants with toxic constituents are available here and here and here.

1 comment:

Anonymous said...

Maybe it just tastes good.
have you ever had clay with Fudge topping?