Original ArticleContent of zinc, iron, calcium and their absorption inhibitors in foods commonly consumed in Ethiopia☆,☆☆
Introduction
Zinc and iron are two of the micronutrients that are most often deficient in developing countries, with children and women of reproductive age especially at risk of such deficiencies (Gibson, 1994; Sandstead, 1991). In children, zinc deficiency has been shown to lead to poor growth (Umeta et al., 2000; Brown et al., 2002), impaired immunity, increased morbidity from common infectious diseases (Zinc Investigators’ Collaborative Group, 1999) and increased mortality (Sazawal et al., 1998). Zinc deficiency arises to a large extent from impaired bioavailability of dietary zinc, largely attributable to the high phytic acid content of diets (Gibson, 1994). Iron deficiency is the most important cause of nutritional anaemia. This arises from the low bioavailability of non-haem iron (Hallberg and Hulthén, 2000) caused not only by phytate but also tannins in the diet. In general, intakes of calcium in developing countries, although low, are adequate to meet requirements (Latham, 1997). However, dietary calcium has been implicated in reducing the bioavailability of iron, both of non-haem iron and haem iron (Hallberg and Hulthén, 2000).
The diets of rural populations in most developing countries are based on cereals and legumes rich in phytate while the consumption of animal-based foods rich in zinc and iron with high bioavailability is low (Gibson, 1994). Phytic acid exerts its inhibitory effect on the absorption of zinc and iron by forming insoluble complexes in the gut under physiological condition (Wise, 1995). The formation of such chelates depends on the ratio of the content of zinc, iron or calcium relative to that of phytate in the food. Other minerals of nutritional importance that are chelated by phytate are copper and manganese (Wise, 1983; Hallberg et al., 1987).
It is possible to predict the relative bioavailability of zinc from the molar ratio of phytate to zinc in the diet (Fordyce et al., 1987; Morris and Ellis, 1989) and ratios >15 have been negatively associated with growth in children (Ferguson et al., 1989; Allen et al., 1991) and suboptimal zinc status in adults (Gibson et al., 1997). High levels of calcium in the diet have also been shown to exacerbate the inhibitory effect of phytate on zinc absorption in humans by forming insoluble complexes with calcium and zinc in the intestine (Morris and Ellis, 1989; Wise, 1983). Such complexes are even less soluble than complexes of phytate with zinc or with calcium alone. In addition, tannins also interfere with iron absorption thus reducing iron bioavailability (Hallberg and Hulthén, 2000).
In rural Ethiopia, the diets of the population are predominantly plant-based and low in animal products. Daily consumption of such diets for longer periods may increase the relative risk of deficiencies of zinc and iron. We have demonstrated that zinc deficiency is a problem in infants in an area of rural Ethiopia (Umeta et al., 2000) but there is very little data on the extent or severity of deficiencies of iron or calcium. There is also a lack of information on the zinc, iron, calcium, phytate and tannin content of local foods. Thus, we have collected such data and assessed the relative bioavailability of zinc and iron of some commonly consumed foods in rural Ethiopia.
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Sample collection
Food samples were collected in rural areas throughout the country. For prepared foods, at least five households at each site were visited to observe the way they were prepared and samples (ca. 1 kg) of foods were collected from households that were regarded as preparing foods in a traditional way. No specific guidance was given to those preparing the food. Fruits were purchased from open markets. The samples were packed in plastic bags of which the opening was sealed using a lighted candle,
Results
A total of 138 food samples of 36 different foods were collected. The local and scientific names, description and place of collection of the foods analysed are presented in Table 1. The zinc, iron, calcium, phosphorus, phytate and tannin content of the foods analysed are presented in Table 2, Table 3, Table 4, Table 5.
The content of zinc was highest in wheat and lowest in sorghum and maize with the zinc content of tef being between the two. Fermentation of enjera reduced the zinc concentration.
Discussion
Diets in Ethiopia are based to a large extent on cereals, starchy roots and tubers, and on legumes. The contribution of these foods varies throughout the country. Tef is the main staple in the northern, western and central parts of the country; in the lowland areas, maize and sorghum are important; while in the south and southwest, starchy tubers and roots including enset are the principal foods (Bezuneh and Feleke, 1966; Pijls et al., 1995). These foods not only provide energy and protein but
Conclusions
In conclusion, this paper provides data on the content of zinc, iron, calcium, phytate and tannin and on the relative bioavailability with particular reference to zinc of foods commonly consumed in rural Ethiopia. Although there are foods relatively rich in zinc and iron, high levels of inhibitors of absorption, especially phytic acid but also tannins, can impair bioavailability of these trace elements. Thus, the consumption of diets based on cereals and legumes, without animal products that
Acknowledgements
We would like to thank Mengistu Gebre-Tsadik, Benyam Tedla and Yohanes Tesfaye for their technical assistance in the laboratory.
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This article is dedicated to the co-author Prof. Clive E. West, who passed away on August 27, 2004.
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The study was supported by a grant from the Nestlé Foundation for the Study of the Problems of Nutrition in the World.