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Mechanisms of Mineral Absorption - James D. Richards, Ph.D.

Mineral Absorption

There has been a great deal of discussion concerning the mechanism of chelated trace mineral absorption, including the suggestion that chelates are transported intact across the absorptive epithelium. This model of absorption also allows for the prediction that the chelates can remain complexed once inside the cell, and be metabolized separately from inorganic minerals. However, recent scientific data indicate that “intact absorption” is not a significant route of chelated mineral uptake, and argue against chelates remaining intact in the cell.

The first major issue with the “intact absorption” model is that there appears to be no biological mechanism for this to occur. If a chelated mineral was to be absorbed across the intestinal epithelium intact, it would have to do so either by diffusion or by transporter-mediated uptake. It is well accepted that amino acids, even in the absence of a bound metal, are unable to be absorbed efficiently by diffusion: most amino acids are too big, and all are ionized or charged and are therefore unlikely to diffuse across the cell membrane.

Likewise, it is also very improbable that a metal-amino acid or metal-proteinate complex will be taken up by diffusion. Zinc methionine, for example, is larger than methionine and still carries a charge. This leaves transporter-mediated absorption as the only other option for “intact absorption.” Unfortunately for this model, transporters are highly specific for the molecule being transported. Amino acids are transported into cells by amino acid transporters, while zinc is taken up into cells by zinc transporters3. Likewise, copper is taken up by yet another group of transporters, specific for copper. Currently, there is no credible evidence that intact OTMs are taken up by amino acid or metal transporters.

The second issue with the “intact absorption” model is that the existing data argue against it. In order to be absorbed as a whole, the OTM must travel intact through the acidic pH of the upper GI tract to the site of absorption in the small intestine. For most OTMs, this is unlikely to happen. In a study of 15 commercially complexed or chelated copper, zinc and manganese products (not manufactured by Novus International, Inc.), the metals dissociated completely from their ligands even under mildly acidic conditions (pH 5), or were “bound so weakly as to dissociate under gentle gel filtration conditions.”5 Similar results were also reported by other researchers. If dissociation between the metal and ligand occurs in the upper GI tract, then it becomes impossible for the complex to be absorbed intact in the small intestine. (Note that research performed at Novus International has demonstrated that MINTREX® chelated trace minerals are more stable at low pH than competing OTMs.)


Furthermore, in vitro absorption experiments with zinc methionine indicated that the zinc and methionine were absorbed separately. Again, this argues against the “intact absorption” hypothesis. Finally, it should be noted that even if an OTM is absorbed intact, it will not remain intact in the cell. For example, the intracellular ligands for zinc, such as metallothionein and glutathione disulfide, have a much higher affinity for zinc than do commercial ligands such as methionine. As such, these intracellular ligands will dissociate the metal from the OTM ligand. A much more reasonable model for metal absorption across the intestinal epithelium is shown in Figure 1. In this model, an OTM with strong bonds between the metal and the ligands (MINTREX® Zn chelated trace mineral in this example) travels intact through the upper GI tract to the small intestine. When the complex enters the unstirred layer (pH 2), it can begin to dissociate. The zinc will be captured by a zinc transporter and transported into the cell. Even in the absence of dissociation, the zinc transporter will be able to separate the zinc away from the MINTREX® trace mineral chelate, because the zinc transporter has a higher binding affinity for zinc than does the HMTBa ligand(10). The zinc can be used by the cell, stored as zinc-metallothionein, or exported out into the bloodstream. The OTM ligand will be absorbed separately. In the case of MINTREX® Zn chelated trace mineral, the ligand is HMTBa, a lipophilic organic acid that can be absorbed by diffusion or by a carrier system and converted to l-methionine by the animal.


Scientific evidence indicates that OTMs are not generally absorbed intact. No cellular receptors have been shown to transport metal-amino acid or metal-proteinate complexes. Furthermore, data from biochemical and cell biological experiments indicate that separate absorption of metal and ligand is the norm. In the rare case where “intact absorption” might occur, the complex will not stay intact because the intracellular metal ligands have a higher affinity for the metal than do the commercial ligands. Therefore, the idea that OTMs like zinc-methionine enter the body intact, and remain intact, is not supported by the data.


Metal Proteinates

Metal proteinates result from complexing a soluble mineral salt with a peptide chain of varying length resulting from the partial hydrolization of a protein. There is no individual amino acid guarantee, as well as no definitive structure. Due to the varying nature of amino acid and/or hydrolyzed protein (ligands) used to make a metal proteinate, chelation has not been proven.

Metal Polysaccharide Complexes

Metal polysaccharide complexes result from complexing a soluble salt with a polysaccharide (carbohydrate). The product is generally an organic mineral matrix with chemical bonding between the metal and the polysaccharide. These are large molecules based on chains of simple sugars. No structure or value of the ligand has been determined.


As you can see, by definition there are large differences in the organic mineral category. These differences include stability between single bonding (complexing) and double bonding (chelation). As you are purchasing your trace minerals, look for highly bioavailable trace minerals that allow you to deliver more to your animals.


References available on request