Zinc binds tenaciously to proteins at near neutral pH. Consequently, the amount of protein in the diet is a factor contributing to the efficiency of zinc absorption. As protein digestion proceeds, zinc becomes more accessible for zinc transport mechanisms of intestinal cells. The relative abundance of zinc as small molecular weight complexes of low binding affinity enhances the process. Small changes in protein digestion may produce significant changes in zinc absorption (Sandstrom and Lonnerdal, 1989). These changes in absorption may explain the correlation between zinc deficiency symptoms and certain malabsorption disorders (Cousins, 1996). The markedly greater bioavailability of zinc from human milk than from cow’s milk is an example of how protein digestibility, which is much lower in casein-rich cow’s milk than in human milk, influences zinc absorption (Roth and Kirchgessner, 1985). In general, zinc absorption from a diet high in animal protein will be greater than from a diet rich in proteins of plant origin such as soy (King and Keen, 1999).
Plants contain phytic acid (myo-inositol hexaphosphate) for use as a storage form of phosphorus. Consequently, plant-based foods, particularly grains and legumes, have a significant phytic acid content. Enzymatic action of yeast during the leavening of bread and other fermentations reduce phytate levels, whereas extrusion processes (used in preparation of some breakfast cereals), may not (Williams and Erdman, 1999). In Caco-2 cells, the metal binding property of phytic acid decreases proportionally as fewer than six phosphate groups are bound to each inositol molecule (Han et al., 1994). Phytate binding of zinc has been demonstrated as a contributing factor for the zinc deficiency related to consumption of unleavened bread seen in certain population groups in the Middle East (Prasad, 1991). The overall effect of phytate is to reduce zinc absorption from the gastrointestinal tract through complexation and precipitation (Oberleas et al., 1966). These chemical effects appear to be enhanced by simultaneous binding of calcium. Phytate binding in the intestinal lumen includes zinc of both food origin and endogenous origin. Since zinc homeostasis is controlled in part by endogenous secretions, consumption of phytate-rich foods may