THEY ARE COMPETING FOR ABSORPTION
Divalent metals, including iron, copper, and zinc, play important roles in the nutrition and growth of swine and poultry
These metals are required in small amounts to support various metabolic processes in the body, including enzyme function, oxygen transport, and immune function. There are many sources of minerals available to nutritionists with different particle size, solubilization patterns, density, surface area or purity. However, when it comes to absorption of these metals in the gut, there is competition between them due to their similar absorption pathways. In this article, we will discuss the common absorption pathways of iron, copper, and zinc in the enterocytes, and explain how to limit competition for absorption.
Iron, copper, and zinc are absorbed in the gut by similar mechanisms, primarily through two pathways: the divalent metal transporter 1 (DMT1) pathway and the Zip family transporter pathway.
The DMT1 pathway is responsible for the absorption of both iron and divalent copper and is located in the brush border membrane of the enterocytes. The DMT1 pathway involves the transport of the metal ion across the membrane and into the cytoplasm of the enterocyte.
The Zip family transporter pathway is responsible for the absorption of both divalent zinc and copper, and is located in the apical membrane of the enterocytes.
Once inside the cytoplasm, the metal ion can either be stored or transported across the basolateral membrane of the enterocyte and into the bloodstream.
Due to the similar absorption pathways of iron, copper, and zinc, there is competition for absorption in the gut. When these metals are present in high concentrations, they can compete for the same transporters, which can lead to reduced absorption and decreased availability of these metals to the body. This is particularly a problem for copper who is the less concentrated minerals in proportion to zinc and iron as illustrated in the pie chart above. It is becoming really critical in piglets phase when using pharmacological level of Zinc Oxide as animals will receive 100 times less copper than zinc (the red portion is hardly visible on the piglet chart above). This competition can even be more problematic in diets that are high in phytate, a compound found in many plant-based feeds that can bind to these metals and reduce their bioavailability.
Monovalent copper, which is present in the form of cuprous ions (Cu+), has an advantage over divalent copper for absorption due to its ability to use a specific pathway. The apical membrane of the enterocytes contains a specific transporter, known as the Copper Transporter 1 (CTR1), which is responsible for the uptake of cuprous ions. This specific transporter allows monovalent copper to be absorbed more efficiently than divalent copper, which must compete with other divalent metals for absorption through the DMT1 pathway.
In contrast, divalent copper is more susceptible to competition for absorption with other divalent metals, particularly iron and zinc. This competition can lead to reduced absorption of divalent copper, which can result in decreased availability of this important micronutrient for swine and poultry.
In order to be transported through the CTR1 transporter, copper must be in the form of cuprous ions (Cu+). However, most of the copper in the diet is present in the form of cupric ions (Cu2+). To overcome this hurdle, copper is reduced by a reductase enzyme, namely the Steap 2, into cuprous ions, which can then be transported into the enterocytes by the CTR1 transporter. This reduction is critical for the absorption of copper, as the CTR1 transporter is highly specific to cuprous ions (Cu+).
However, even for this reduction reaction, divalent copper is still in competition with other divalent metals such as ferric ions (Fe3+). Ferric ions are reduced to ferrous ions (Fe2+) by the same reductase enzymes that reduces cupric ions (Cu2+) to cuprous ions (Cu+). This competition can reduce the efficiency of copper absorption in the gut and can lead to reduced availability of copper.
There are some strategies to minimize this competition between minerals. We can increase the chances for copper to be absorbed by reducing the contribution of zinc, especially in piglet diets where some nutritionists are still using high level of zinc at 2 to 3,000 ppm. The second option for nutritionists is to provide to animals a source of monovalent copper (Cu+) to shortcut the traffic waiting in front of DMT1, Zip transporter and reductases and offer a readily absorbable source of zinc through CTR1 transporter.