Zinc can reduce the absorption of copper. Typical intakes of zinc do not interfere with copper bioavailability. However, large quantities of supplemental zinc, well over the tolerable upper intake level (UL), can reduce the bioavailability of copper in the body. Too much zinc can cause the synthesis of a copper-binding protein called metallothionein in the intestines. This metallothionein traps copper and prevents its absorption. High copper intakes have not, however, been found to affect zinc absorption.
Zinc absorption may be influenced by iron. Higher levels of supplementary iron may decrease zinc absorption. Iron in food has not been shown to interfere with zinc absorption. Pregnant and breastfeeding women who take more than 60 mg of elemental iron per day may need supplemental zinc because of this interaction.
Calcium may reduce zinc absorption in diets high in phytates. Plants store phosphates in phytates (phytic acid). Tortillas are high in phytates and are made with calcium oxide (lime). In diets consisting largely of tortillas, a slight increase in zinc consumption may be recommended to offset the slight decrease in zinc bioavailability.
Zinc is needed to produce an active form of vitamin A, retinal, which is used in visual pigments. Zinc is also needed by the retinol-binding protein that transports vitamin A. Zinc deficiency can cause signs of vitamin A deficiency, even when there is sufficient vitamin A.
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