Trace Mineral Absorption

Trace minerals are clearly required for animal performance, reproduction and health, but most can have toxic effects when supplemented at higher than normal concentrations in the animal’s diet. With many trace minerals, the level of intestinal absorption is a key regulatory step in maintaining optimised mineral homeostasis. Inorganic, organic and hydroxy sources of trace minerals are all absorbed using similar mechanisms and receptors in the small intestine. These mechanisms function to facilitate the active transport and absorption of Zn, Mn, Cu, and Fe regardless of source across the intestinal wall and into the blood stream, providing a biologically available source of metal required to support optimised animal productivity and well-being. A number of factors affect absorption of trace minerals (Spears et al, 2012). Of primary concern is the negative effect that rumen antagonists such as sulfur, iron, molybdenum, phytates, fiber by-products, etc. exert on trace mineral stability while passing through the rumen.

The inherent ability of inorganic trace minerals to rapidly breakdown and solubilise in the rumen allows them to bond with the wide array of rumen antagonists, significantly reducing the amount of trace mineral that is available for absorption in the lower GI tract. Following the arrival of the trace mineral in the small intestine, recent research has clearly defined a number of specific metal transporters that function to control the transport, cellular uptake and efflux of copper, zinc, manganese and iron into the blood stream of the animal. At the point of absorption trace minerals may also interact with each other (Fe, Mn and Cu) via their need to compete for a common transporter or their ability to alter the expression of transport proteins further impairing normal absorptive processes. Current data indicates that all trace minerals are absorbed via the processes defined above, eliminating the belief that certain organic trace minerals can be absorbed in their original molecular forms. One example is work done by Beutler, et al at UVA using radiolabeled Zn methionine. The results showed that the Zn and methionine from a complex were taken up separately by epithelial cells.

Zinc Absorption  

The percentage of dietary zinc that is available for absorption (typically 5 -10%) is largely affected by its reactivity with the large variety of antagonists present within the rumen and can be further reduced when dietary zinc is increased in the ration. It is well documented that Zn absorption is homeostatically regulated resulting in the expression of Zn transporters from the enterocyte that can be affected by dietary Zn concentration in the intestinal lumen. Two families of Zn transport proteins are now known. The ZIP transporters promote the import of Zn into intestinal cells from the intestinal lumen while the ZnT proteins are involved in Zn efflux from cells or influx into intracellular vesicles. There are at least 14 members of the mammalian ZIP family and 10 members of the ZnT family. ZIP4 is considered to be the major intestinal Zn import protein required for the transportation of zinc from the intestinal lumen into the enterocyte.

Copper Absorption

Copper absorption in ruminants is low (typically 1–10%). This is largely due to complex interactions that occur with antagonist in the rumen resulting in the formulation of many insoluble compounds with little or no biological value to the animal. Like Zn, Cu absorption is homeostatically regulated resulting in the expression of Cu transporters from the enterocyte. Copper Transporter 1 (CTRl) is the predominant transport protein for cellular import of Cu by the enterocytes. This transporter is a high- affinity Cu import protein that is specific for Cu+1. Prior to absorption a reductase enzyme is involved in reducing Cu+2 to Cu+1. Copper absorption by CTRl is a non-energy dependent process. Divalent metal transporter 1 (DMT1) has been shown to transport a number of divalent metals in brush border membrane vesicles and perhaps transports Cu+2. DMT1 is known to be a major transporter of Fe+2.

Manganese Absorption

Manganese is poorly absorbed (1% or less) from ruminant diets. Little is known about the absorptive processes involving Mn, although it is believed that DMT1 can transport Mn. Competition for DMT1 by both Fe and Mn may explain the strong antagonism between Fe and Mn.