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Effect of Excess Dietary Zinc on Tissue Storage of Iron in Rats

Physiological Chemistry Laboratories, Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, MA 02139 and USDA Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111

Earlier investigators observed that addition of large amounts of zinc to the diet of rats can retard growth, lower their hemoglobin levels and reduce storage of iron. In the present studies, addition of 0.75% zinc to a synthetic diet confirmed the reduced storage of iron in the livers and spleens of growing rats, but failed to show an effect on growth rate or hemoglobin level. The adverse effects of zinc excess on growth and hemoglobin level could, however, be reproduced by replacing the Rogers-Harper salt mixture used in the present studies with the Wesson salt mixture used in the earlier studies. Rats fed excess zinc along with the Wesson salt mixture grew less well, had anemia and also had low levels of copper in their livers. It is suggested that addition of zinc to the low copper Wesson salt mixture reduced copper absorption sufficiently to deplete liver copper to a level at which mobilization of liver iron stores by a copper-dependent mechanism became impaired, thus depriving red cell production and tissue enzymes of stored iron. The mechanism by which iron stores are depleted by addition of zinc to the better balanced Rogers-Harper salt mixture remains unexplained. It is not due to interference by zinc with iron absorption from the diet nor with cellular uptake of iron from circulating transferrin, and the capacity of tissues to store iron as ferritin is not impaired.

KEY WORDS: • iron • zinc • salt mixture

Manuscript received 8 December 1980.