The adequacy of dietary zinc depends on its amount and bioavailability in the diet. Rural diets in developing countries are predominately plant-based: consumption of cellular animal protein foods such as meat, poultry and fish, rich sources of readily available zinc, is often small because of economic, cultural and religious constraints.

To date, information on zinc intakes in developing countries is limited because of the paucity of data on the zinc content of local staple foods. Substitution of zinc values for staple foods grown in western countries is not advisable because the zinc content of plant-based foods tends to reflect local soil zinc levels. In general, diets based on starchy roots and tubers have a lower zinc content than those based on unrefined cereals and legumes because the latter contain more zinc. Such differences in food sources of dietary zinc have an important impact on the amount available for absorption. Cereals contain high levels of phytic acid (myo-inositol hexaphosphate), a potent inhibitor of zinc absorption which forms insoluble zinc-phytic acid complexes in the intestine. The negative effect of phytic acid on zinc absorption can be predicted by the phytate-to-zinc [Phy]/[Zn] molar ratio of the diet; ratios above 12 have been associated with suboptimal zinc deficiency in humans. Unrefined cereals and some legumes have [Phy]/[Zn] molar ratios above 25 compared to ratios of generally less than 10 for starchy roots and tubers. Consequently diets based on unrefined cereals have high phytic acid contents and high [Phy]/[Zn] molar ratios relative to diets based on starchy roots and tubers. Exceptions are diets based on yeast leavened or fermented cereal products; during leavening and fermentation, phytic acid is hydrolyzed.

High levels of calcium exacerbate the inhibitory effect of phytate on zinc absorption in humans by forming a Ca:Zn:phytate complex in the intestine that is even less soluble than phytate complexes formed by either ion alone. Current estimates suggest that diets with [phytate] [Ca]/Zn molar ratios>0.2 jeopardize zinc status in humans. In general, the negative effect of phytate on zinc bioavailability. Notable exceptions include diets of lacto-ovo vegetarians, those based on tortillas prepared with lime-soaked maize and possibly diets of persons who chew betal nut with lime.

Several other dietary components impact on the availability of zinc for absorption. Of these, the amount and type of dietary fibre may be an additional inhibiting factor, although its relative importance of dietary fibre in compromising zinc absorption is controversial, in part because both dietary fibre and phytate often co-exist in plant-based diets, making it difficult to establish independent effects. In contrast, the inclusion of even small amounts of animal and fish protein may enhance zinc absorption. The mechanism is unclear. Certain amino acids and cysteine-containing peptides, released during digestion of cellular animal products, may form soluble ligands with zinc and/or form complexes with zinc, thereby preventing the formation of the insoluble zinc-phytate complex.

The adequacy of intakes of dietary zinc can be evaluated by comparison with an appropriate set of dietary reference values, provided an estimate of zinc bioavailability can be made. For studies in developing countries, the newly revised requirement estimates for zinc set by WHO (1996) can be used. They include estimates for both basal and normative zinc requirements, together with a model for classifying diets as having high (50-55), moderate (i.e. 30-35%) or low (i.e. 15%) zinc bioavailability, depending on their content of animal and/or fish protein, calcium (>or<1g) and their [Phy]/[Zn] molar ratios (<5, 5-15, >15), respectively.

Several methods are available for assessing risk of inadequate intakes of zinc by comparison with the WHO requirements estimates. Of these, the probability approach is preferred because it predicts the number of persons within a group with zinc intakes below their own requirements and hence provides an estimate of the prevalence of inadequate intakes, provided reliable data on the distribution of usual zinc intakes are available. When this approach is used, risk of dietary zinc inadequacy is very high for children from developing countries such as Malawi, Ghana, Kenya, and Papua New Guinea, but less so for children from Egypt, Mexico, and Ecuador. Such inadequacies in zinc intakes are likely to occur in these plant-based diets even when predicted energy requirements are met. Clearly, dietary strategies which aim to increase the total zinc intake and/or the absorption of dietary zinc in the diets of children in developing countries are required.

Appropriate dietary strategies include consumption of zinc-dense foods and those known to enhance zinc absorption (i.e. cellular animal proteins), when feasible. In addition, several methods exist to reduce the phytic acid content of plant-based staples via enzymatic and non-enzymatic hydrolysis. Methods that promote phytase-induced enzymatic hydrolysis of hexa- and penta inositol phosphate include soaking, germination and fermentation. Phytase enzymes hydrolyze phytic acid to yield inorganic orthophosphate and myo-inositol via intermediate myoinositol phosphates (penta to monophosphates (IP-5 to IP-1). Only the higher inositol phosphates (IP-5 and IP-6) inhibit zinc (and iron) absorption (Lönnerdal et al., 1989; Svanberg et al., 1993). Endogenous phytase activity is high in rye and wheat but very low in maize and sorghum. Soaking can activate these endogenous phytases, but optimal condition must be defined as activity depends greatly on the pH of the medium and temperature; pH 5.0-4.5 appears to be the optimum for cereal phytases.

Germinating cereals also enhances phytase activity through induction and/or de novo synthesis. Reductions in IP-6 content after 2-3 days germination of cereals range from 52% for rice to 21-28% for Malawian white corn. Phytate hydrolysis can also be induced by microbial phytases which originate from the microflora on the surface of the seeds, or by use of a microbial starter. Microbial phytases have a broader pH optimal (i.e. 2.5-5.5) than cereal phytases (5.0-4.5) so that phytate hydrolysis can proceed over a longer fermentation period. Commercial phytase enzymes prepared from Aspergillus oxyzae or A. niger are stable over a wider pH (3.5-7.8) and temperature range can also be used, although their high cost probably precludes their use in many developing countries at the present time.

To date, in vivo comparisons of the bioavailability of zinc in fermented versus unfermented staple plant-based foods are not available. Increases in in vitro measurements of soluble iron have been reported after fermentation with a starter culture, with and without the addition of commercially prepared phytase enzyme (Svanberg et al., 1993).

Soaking can also be used to reduce non-enzymatically the phytic acid content of staple foods prepared from certain cereals (e.g. maize) and legumes. Phytic acid is stored in a relatively water-soluble form such as sodium or potassium phytate in these staples and hence can theoretically be removed by diffusion. Levels of water soluble phytate range from 10% in defatted sesame meal to 70-97% in California small white beans, red kidney beans, corn germ, and soya flakes.

In summary, practical dietary strategies do exist to improve the content and bioavailability of zinc which are economically feasible and sustainable. They can also be use to alleviate other micronutrient deficiencies simultaneously without risk of antagonistic interactions. They involve some simple modifications to food preparation and processing techniques used for local staple foods. Nevertheless, to be successful, their implementation requires detailed knowledge of the local dietary patterns and food preparation and processing practices, food beliefs, preferences and taboos as well as he ability to change attitudes and practices.