Dietary Metal-complexing Agents and Zinc Availability in the Rat

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Abstract
The growth rate of weanling rats was used to determine the effects of phytate, calcium and ethylenediaminetetraacetate (EDTA) upon the physiological availability of zinc. Phytate decreased availability and the effect was augmented by excess dietary calcium. Calcium had no effect in the absence of phytate so that its effect must be mediated through an interaction with phytate. EDTA increased zinc availability when the diet contained phytate but had no significant effect upon the growth rate in the absence of phytate. In vitro experiments showed that zinc phytate is highly insoluble at the pH range encountered in the small intestine. Addition of calcium to the medium produced an even more insoluble complex containing zinc, calcium and phytate. The results suggest that the formation of such complexes with phytate is the mechanism whereby zinc is made less available and the more complete precipitation of zinc in the presence of calcium explains the effect of excess calcium. In vitro studies with intestinal strips and 65Zn showed that zinc uptake was progressively decreased as the ratio of calcium to phytate was increased. This effect was counteracted in part by the addition of EDTA. It appears that EDTA increases zinc availability by competing with phytate and forming a soluble complex which allows absorption across membranes.