IT IS PROBABLY ONLY A QUESTION OF DISSOCIATION
Thirty years ago, were introduced to animal feed nutritionists as an alternative to standard zinc, iron, copper and manganese sources. Recently, more sophisticated source of inorganic minerals demonstrated as well superior bioavailability and performance. Nowadays, animal nutritionists need to have a clear understanding on the characteristics of these different sources of minerals to select to optimal solutions for their diets.
There are several proposed theories to explain the enhanced availability of chelated minerals. These theories have never been confirmed scientifically but they are helping to explain the differences of bioavailability that we find on the field when comparing different sources of minerals, whether they are sulfate, oxide or chelates.
The first theory considers that that complexing the mineral with a chelated component may increase the water and lipid solubility of the mineral which may enhance passive absorption of the mineral.
Some other companies advanced the hypothesis that organically bound trace minerals may be absorbed via amino acid or peptide transport pathways and that the minerals would then be released after absorption.
But it important to keep in mind that every flux is controlled by homeostatic mechanism to protect animal organism against moderate excess and deficiency. In the case of the absorption of minerals, the up- and down-regulation comes from the transcription of minerals transporters to control both the influx of zinc into the cell and the efflux out of the cell. Any source of product with a claimed different absorption pathway would raise concerns of by-passed homeostasis and of potential toxicity. This point will tend to favor the theory that minerals are all absorbed in the same way, through intestinal zinc transporters. Any difference in final bioavailability may should originate before absorption in relation to what happens to the ingredient in the gut environment.
Similar remarks apply as well to the oxide group. If you take as example the zinc oxide, we easily demonstrate that the dissociation kinetic in acidic pH highly depends on the specific surface area of the product together with other elements as particle size and density. The more contact surface between the zinc oxide crystals and the liquid in the gut and the faster will be the dissociation
That dissociation pattern partly explains the bioavailability of the different source of Zinc source. Zinc Sulfate is by chemical nature very easy to dissociate and be absorbed. That is why Zinc Sulfate is used in most of bioavailability studies as the reference compound. At the opposite, standard dense zinc oxide has a very flat solubilization curve. It is indeed a less available source of Zinc. It can dissociate only in acidic conditions and with a limited surface of 3 m2 per gram of product, it solubilizes much slower than Zinc sulfate.
But when we increase the contact surface of the zinc oxide from 3 to 50 m2/g by creating porous surface, we are accelerating the dissociation kinetics. In only few hours, that porous zinc oxide reaches an equivalent quantity of Zn2+ ions compared to the Zinc sulfate source.
That intermediate solubilization patterns (as illustrated in the blue in the diagram below) has a strong advantage. It acts as a slow-release device to supply Zn2+ ions on a longer period instead of a peak like Zinc Sulfate. It is indeed not very positive to have a quick and high release of Zn2+ ions in the stomach and small intestine as they will complex with other molecules namely phytates. Not only it will reduce the amount of Zn2+ ions available for absorption but when attached to phytates, Zn2+ ions will prevent the phytase enzyme to act effectively. Many studies prove that an excess of Zn2+ will reduce the level of available phosphorus from neutralizing the action of phytase.
The very slow dissociation of standard zinc oxide is the reason why the dosage to obtain the anti-bacterial effect needs to be between 2,000 to 3,000 ppm whereas we reach similar efficacy from only 200 ppm for the porous zinc oxide because of a faster release. For the coated zinc oxide, the fat coating prevents the solubilization in the stomach and when the product arrives in the small intestine where the fat is removed, then the pH is not acidic anymore to enable any dissociation of Zn2+. Coming back to the bioavailability of chelated minerals, the same concept of slow release could well apply to organic minerals to explain their superior bioavailability as it is used for the porous zinc oxide. Given the chemical complexity of chelated minerals, the process of separating Zn2+ from its ligands may actually slow down its release and prevent the Zn2+ ions from binding with phytates. That theory can be summarized in the graph above where we see 3 patterns of kinetics. When we are comparing the bioavailability of the different sources of Zinc, we noticed indeed that chelated minerals and porous Zinc Oxide present similar bioavailability performance similar than the reference Zinc Sulfate and even higher for some sources.
The graph above illustrates as well that the chelated minerals is a very heterogeneous family with very variable performance even between batches of similar product.
In conclusion, the situation is more complex than what it was initially presented by chelated minerals supplier. The idea that chelated minerals were using alternative absorption pathways needs to be seriously challenged. The superior bioavailability demonstrated on the field may actually not be explained by a different absorption channel but rather by a dissociation kinetics different from common minerals source. The best model to demonstrate that theory is illustrated by the porous zinc oxide. Thanks to an improve dissociation kinetics, that porous technology demonstrates similar or even better bioavailability and performance as chelated sources while still using classic absorption pathways.
As I mentioned earlier, these theories still need to be confirmed scientifically. But they already give a direction scientists can work on. In the past 30 years, companies marketing chelated minerals were the only ones to promote their theories and finance research. In the last 10 years, new players arrived on the market proposing porous solution based on inorganic technologies. The arrival of these new players will contribute to enrich the discussion. We should therefore expect in the coming years to learn more about mineral absorption pathways.