Geophagy is (or was) common, from the equator to northern Europe. Although pica comes in many forms, most examples of geophagy ("earth eating") involve consumption of clay soils. This article suggests an adaptive explanation:
http://www.laphamsquarterly.org/essays/balanced-diets.php?page=all
The gist is that clay stops the absorption of antinuntrients. In times of scarcity the behavior permits the eating of otherwise unpalatable foods. Could a clay supplement "shore up" a damaged gut in much the same way?
