Function of geophagy as supplementation of micronutrients in Tanzania

Geophagy is defined as the practice of eating soil and is observed worldwide. In Tanzania, edible soil sticks called pemba are consumed mainly by pregnant women, but the physiological function of eating pemba has not yet been elucidated. The objectives of the present study were to evaluate the physicochemical properties of edible soil sticks compared with ordinary soils in Tanzania and to evaluate the function of geophagy in terms of micronutrient supply and the adsorption capacity of materials such as toxins. The color of eight pemba samples collected from various markets was reddish or whitish and their shape was generally columnar with an average length, width and weight of 6.1 cm, 1.8 cm and 22 g, respectively. Pemba had a more clayey texture (48% clay on average) than the ordinary soils investigated for comparison, and the clay composition was generally dominated by kaolinite. The concentrations of available nutrients in pemba, extracted with 0.1 mol L-1 NaCl (pH 2), were 391 mg Ca kg(-1), 234 mg Mg kg(-1), 17 mg Mn kg(-1), 6.5 mg Fe kg(-1), 4.9 mg Cu kg(-1), 2.8 mg Co kg(-1), 1.9 mg Zn kg(-1) and 1.1 mg Ni kg(-1), and extraction with reductant drastically increased the amounts of Fe and Mn released. The amount of available nutrients supplied by pemba consumption at a rate of 50 g day(-1) amounted to 99% of Mn and 13% of Fe in the case of reddish pemba and 75% of Cu in the case of whitish pemba of the daily requirement by pregnant women, suggesting the potential of pemba to supply these micronutrients. A moderate cation exchange capacity (CEC) level (11.1 cmolc kg(-1)) and lower ratio of CEC to clay content for the pemba compared with the soils indicated that the adsorption capacity was not the main criteria for choosing soil materials and instead fine-textured soils with kaolinitic clay mineralogy were deliberately chosen for pemba. In conclusion, the main function of eating pemba in Tanzania, and probably the original function of geophagy, is the supply of micronutrients, such as Mn, Cu and Fe, rather than the adsorption of toxic materials.