The significance of zinc in the diet has only recently been appreciated. Zinc forms an important component of many enzymes known as metaloenzymes, of which many participate in the digestion and assimilation of nutrients. Zinc is also essential to the proper function of the endocrine system because it forms a structural component of the hormone receptor system.24 Moreover, zinc plays a role in the synthesis of RNA and DNA, and is essential for the proper functioning of the immune system.15
Most dietary guidelines will recommend the consumption of animal products as a source of zinc, but it has been found that, as in the case of calcium, the high protein content of diets rich in animal products impacts negatively on zinc availability.23,25 Incomplete hydrolysis of casein, the protein in milk, also inhibits the uptake of zinc.23 As in the case of calcium, phytate, found in grains and legumes, may inhibit the absorption of zinc, but this may be partially offset by the protein composition of these foods. Finally, it seems as if the toasting of foods renders the protein-phytate mineral complexes less digestible and leads to lower mineral availability.26
A vegetarian whole-food diet will supply more zinc than an omnivorous diet. Legumes and seeds have a relatively high zinc content ranging from 2.7-3.2mg/100g, and grains are also rich in zinc. The bran and the germ of wheat are the main storage area of zinc in grains and contain an average of 9.8 and 14.3mg/100g respectively.27 It is better to obtain zinc from whole foods, however, than from added bran and wheat-germ, as the high fibre content of such fortified meals will make the zinc less available. Vegetables and fruits are relatively low in zinc content, emphasizing the need for a varied diet which, beside fruits and vegetables, includes grains, seeds and legumes. The zinc content of selected plant foods is presented in table 6.1, and values for some animal products are given for comparison.
Table 6.1. The zinc content of selected foods. (Adapted from reference 27)