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J R Soc Med. 2004 January; 97(1): 48.
PMCID: PMC1079279

Biochemical investigations in geophagia

Woywodt and Kiss’ historical review of geophagia1 prompts us to report a study illustrating how, through absorption or liberation of essential minerals, earth when consumed habitually can be either harmful or, possibly, nutritious. The amount of material ingested probably contributes as well.

An African woman admitted to hospital with limb paralysis had a serum potassium of 1.3 mmol/L (reference range, 3.5–5.0). Other serum electrolytes were normal. The hypokalaemia and a concurrent iron deficiency anaemia (haemoglobin 7.9 g/dL, serum iron 3.5 μmol/L) remained unaccounted for. Further enquiry disclosed a 10-year history of geophagia.

The earth she ingested was obtained from her garden. It was black with white speckles, malleable, rough in texture and composed of granular particles (‘black earth’). Earth excavated locally for eating was obtained from a nearby market. This was dusky pink, firm, fine in texture and composed of flaky particles (‘red earth’). Samples of black earth and red earth weighing 0.5, 1.0 and 2.0 g were mixed with 5 mL iron-enriched Ringer-lactate. Studies were done at both pH 2.0 (addition of 6 mol/L HCl) and pH 6.2. Analyses were performed on a standard system.

Black earth absorbed sodium, potassium and iron. It also liberated calcium and magnesium. These effects were present at either pH. Red earth absorbed potassium, slightly, at either pH. Iron was liberated at pH 2.0 and absorbed at pH 6.2. Other elements showed small differences only. Changes for black earth and for red earth were proportionate to mass concentration. (Full details can be had from RFG.)

Our patient’s intake of potassium conformed to the recommended dietary allowance (RDA) and that of iron was deficient (44% of RDA). The demonstrated absorbent properties for potassium and for iron of black earth indeed could thereby have caused her hypokalaemic paralysis and contributed to her iron deficiency anaemia.

Absorbent properties for sodium of black earth, though notable, were not homoeostatically significant. Intake was estimated at only 7.5% of dietary guidelines, yet the serum concentration was normal. The same applies to magnesium. This was liberated from black earth in quite large amounts, dietary intake exceeded the RDA (120%) and yet the serum concentration again was normal. Intake of calcium was below the RDA (43.5%), while the serum concentration was normal. Possibly, the calcium liberated from black earth actually functioned as a dietary supplement.24

The local African population believes that ingestion of earth benefits women when they become pregnant. Red earth indeed had properties that might prevent iron deficiency anaemia, though the bioavailability of this (non-haem, contaminant) iron may be limited.5

Before attributing any adverse or beneficial consequences to geophagia, the ion-exchange capacity6 of the substance in question should be evaluated. That is within the scope of most hospital laboratories.


We thank Prof. H Joubert for the use of laboratory equipment, and Ms R M Baloyi for providing the specimen of red earth.


1. Woywodt A, Kiss A. Geophagia: the history of earth-eating. J R Soc Med 2002;95: 143–6 [PMC free article] [PubMed]
2. Hunter JM, de Kleine R. Geophagy in Central America. Geogr Rev 1984;74: 157–69 [PubMed]
3. Reid RM. Cultural and medical perspectives on geophagia. Med Anthropol 1992;33: 337–51 [PubMed]
4. Johns T, Duquette M. Detoxification and mineral supplementation as functions of geophagy. Am J Clin Nutr 1991;53: 448–56 [PubMed]
5. Harvey PWJ, Dexter PB, Darnton-Hill I. The impact of consuming iron from non-food sources on iron status in developing countries. Pub Health Nutr 2000;3: 375–83 [PubMed]
6. Minnich V, Okcuoglu A, Tarcon Y, et al. Pica in Turkey. II. Effect of clay on iron absorption. Am J Clin Nutr 1968;21: 78–86 [PubMed]

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