Interactions between zinc ions (Zn2+) and other divalent cations such as iron (Fe2+), calcium (Ca2+) and copper (Cu2+) may interfere with normal mineral metabolism. The exact interactions between zinc and copper in the gastrointestinal tract are not known, but there is evidence that zinc can impede the proper absorption of copper through formation of metal-protein complexes that form in enterocytes. This is because zinc stimulates the synthesis of thionein polypeptides in the enterocytes, which bind to both copper and zinc (and other metals) forming metallothioneins. Copper appears to have a higher binding capacity for these thioneins, which may then trap the copper in the enterocytes of the gut and prevent it from reaching the plasma. Zinc supplementation for 6 weeks has been shown to decrease superoxide dismutase activity, suggesting that the effects of zinc on copper absorption may be short-term. In fact zinc supplementation for as little as two weeks may impair copper absorption.
Researchers1 have investigated the effects of zinc supplements on the absorption and retention of copper. Twenty young adult women were fed controlled amounts of zinc (8, 16 or 24 mg/d) and copper (2 mg/d) in a metabolic unit over a period of 18 days. During the supplementation period, zinc balance became positive for all subjects, although the authors did comment that the lower levels would have probably been insufficient to maintain positive balance if zinc losses increased. In each of the zinc supplement groups, copper excretion exceeded intake, suggesting that 2 mg/d is not enough to maintain equilibrium in adult women. Plasma levels of zinc increased in all subjects, and plasma levels of copper decreased in all subjects, however, the changes were not significant. The varying intakes of zinc did not have an effect on copper absorption, supporting previous findings that suggest the ratio of zinc to copper must exceed 500:1 in order for this to occur.
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