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First published online December 10, 2003
Journal of Experimental Biology 207, 319-324 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00758

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Adaptive function of soil consumption: an in vitro study modeling the human stomach and small intestine

Nathaniel J. Dominy^1,*, Estelle Davoust² and Mans Minekus³

¹ Department of Ecology and Evolution, University of Chicago, 1101 East 57th Street, Chicago, IL 60637, USA
² Université d'Auvergne, 28 place Henri Dunant, 63300 Clermont-Ferrand, France
³ TNO Nutrition and Food Research Institute, PO Box 360, NL-3700 AJ Zeist, The Netherlands

^* Author for correspondence (e-mail: njdominy{at}uchicago.edu)

Accepted 16 October 2003

Despite occurring in a wide variety of taxa, deliberate soil consumption (geophagy) is a poorly understood behavior. In humans, geophagy is sometimes considered aberrant or a sign of metabolic dysfunction. However, geophagy is normally assigned an adaptive function in nonhuman primates and various other organisms. One hypothesis submits that clay-rich soil adsorbs intestinal insults, namely plant metabolites or diarrhoea-causing enterotoxins. Here we test the capacity of kaolin, a commonly ingested clay, to adsorb quinine (an alkaloid) and two types of tannin (digestion-inhibitors). Trials were conducted in vitro using the TNO Intestinal Model, a device that closely simulates digestion by the human stomach and small intestine. Kaolin reduced the bioavailability of each compound by 30%. However, because we could not replicate clay-epithelial adhesion and reduced motility, these results may underestimate adsorption in vivo. We also show that kaolin fails to render calcium oxalate soluble. We conclude that gastrointestinal adsorption is the most plausible function of human geophagy. Adaptive advantages include greater exploitation of marginal plant foods and reduced energetic costs of diarrhoea, factors that could account for the high frequency of geophagy in children and pregnant women across the tropics.

Key words: geophagy, pica, diet, tannin, alkaloid, human

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