Inhibitory effect of a soybean-protein--related moiety on iron absorption in humans

The inhibitory effect of soybean protein isolates on nonheme-iron absorption was studied in 34 human subjects. Iron absorption was measured by using an extrinsic radioiron label in liquid-formula meals containing hydrolyzed corn starch, corn oil, and either egg white or a series of soybean-protein derivatives. The unmodified soybean-protein isolate markedly inhibited iron absorption. Percentage absorption was 19-fold higher when an extensively enzyme-hydrolyzed preparation with very little phytate was used as the protein source. Both the glycinin (11S) and conglycinin (7S) fractions of soybean protein were inhibitory to iron absorption. Dephytinization removed the inhibitory effect of the glycinin but not of the conglycinin fraction. We conclude that there are two major inhibitors of iron absorption in soybean-protein isolates, phytic acid and a protein-related moiety contained in the conglycinin (7S) fraction.