The detailed structural studies of several biological examples of polynuclear iron(III) discussed in the preceding chapter can be correlated with the biological roles of polynuclear iron(III) (see chapter 22). Phosvitin is a major dietary constituent of avian egg yolk, the inhibition of iron absorption of which is associated with resistance to digestive proteolytic enzymesl06 and with binding of about fifty moles of iron/mole protein in a polynuclear fashion97. Nutritional studies have shown that dietary phytic acid (inositolhexaphosphoric acid) and phosphates greatly depress iron uptake1, presumably by forming highly insoluble iron(III) phosphate compounds. Binding of iron by phosvitin—primarily through the phosphate groups of the serine phosphate side chains—does not lead to precipitation but still effectively makes the iron unavailable for absorption by maintaining it in a macromolecular complex. However, scant information is available on the competition for dietary iron among the variety of potential ligands and chelates available in the diet.