Zinc is an important trace mineral required as a cofactor in a number of enzymes including the extensively researched antioxidant enzyme copper-zinc superoxide dismutase. Iron too is a trace mineral and is most well known for its role as a component of haemoglobin and myoglobin. Zinc and iron are known to interact with one another during the process of absorption through competitive inhibition for transporters, and high intakes of iron are thought to be able to inhibit the absorption of zinc in humans. This interaction is of interest nutritionally as both zinc and iron are often taken at high concentration in supplements, and problems can arise with mineral balance unless this interaction is considered. The situation is complicated further by the evidence that the form of zinc and iron, be it organic or inorganically bound versions of the minerals, as well as the concentration, can have a significant influence on the absorption of both minerals. To date many studies have investigated these interactions.
For example, in one study, researchers used a variety of forms of both zinc and iron at different concentrations in order to understand the factors that alter the absorption kinetics of zinc1. When the authors used non-haem iron as ferrous sulphate and zinc sulphate in a ratio of 1:1 (25 mg zinc and 25 mg of iron in a cola drink) there was little evidence of changes to the absorption of zinc. However, when this ratio was increased to 2:1 and 3:1 (iron to zinc) there was significant inhibition of the absorption of the zinc. Interestingly, when the iron was present as haem iron, in the form of haem chloride even at an iron to zinc ratio of 3:1 there was no inhibition of zinc absorption. When 100 mg of ferrous iron was consumed along with 54 mg of zinc from the consumption of Atlantic oysters there was no inhibition of zinc at this 2:1 ratio. Based on these results it appears that interactions exist between non-haem iron and inorganic zinc, but that organic forms of both minerals do not interact at the levels of the gut.
Dr Robert Barrington’s Nutritional Recommendation: This study illustrates nicely why it is always better to supply trace minerals from foods rather than supplements. It also highlights the benefits of haem iron over non-haem sources of the mineral. In this regard consuming red meat as a source of haem iron would likely have less of an effect on the absorption of zinc compared with legumes, the latter being a good source of non-haem iron. Supplements often contain inorganic forms of minerals and therefore interactions between zinc and iron are likely. Taking supplements necessitates consideration of these effects. Most multivitamins would be expected to contain ~15 mg of both zinc and iron in their inorganic forms (with iron as non-haem iron) and therefore absorption of zinc might not be affected. However, supplementing with high concentrations of either mineral might alter the absorption of the other and such supplementation should therefore involve careful thought and close monitoring.
RdB
