Trace Mineral Choices
6 May 2020
By Tom Best
Director of Quality & Food Safety
Essential trace elements are not inert but are chemically active in an environment, such as an animal diet and during the digestion process. Some chemical forms of nutritionally important trace elements are more active than others. Understanding how these elements react to major chemical changes during the digestion process is essential to selecting the most appropriate form of the element to use.
Solubility and Availability
Inorganic trace minerals have varying degrees of solubility and availability. In order for metals to be absorbed, they have to become soluble at some point during the digestive process. The timing of that solubility can be a protective mechanism from antagonistic reactions and beneficial for absorption or interactions with the gut microbiome.
At one time, sulfates were seen as the most available because they are highly soluble. When a metal complex becomes soluble, the metal can ionize and may react to anything for which it has an affinity. The stronger the affinity, the more likely the metal will bind. Metals will only remain free ions for nanoseconds.
Oxides and Carbonates
Oxides are much less soluble and have been considered a poorer biological source, but that may be a misconception. We now know that metal complexes have a variety of information contained in their structures. Oxides have higher metal concentration and higher levels of impurities. Carbonates are yet another inorganic source but are used markedly less commonly in the industry except for cobalt and iron.
Hydroxy Trace Minerals
A more recent upcoming form of inorganics is the hydroxy trace mineral. Their high insolubility offers a protective mechanism from negative reactions throughout much of the digestive process until a lower pH threshold slowly releases the metal, much different than oxides.
All trace minerals, including organics, will become part of the digesta as they are consumed by the animals. The high reactivity of transition metals causes them to react instantaneously in solution. Many of the reactions in the gut can render the metal insoluble and indigestible, also creating other problems, including the potential to increase soil and water pollution. When metal salts like zinc sulfate dissolve in water, the products are sulfate ions and hydrated zinc. Only water molecules are bonded to the metal in a free metal ion like hydrated zinc. Free metal ions are extremely active and can react in many possible ways, many of them undesirable. When metals become free, they will react to compounds with affinity to that metal, in order of greatest affinity, until all free metals are reacted and an equilibrium is reached. Any changes to the surrounding environment, especially pH, causes a new equilibrium to be attained. This is by Mother Nature’s design, the strongest, most stable bonds survive.
Free metal ions may bond with one or more partners to form complexes. Organic bonding partners can protect metal ions from undesirable reactions. The stability that the different organic partners exhibit determines the level of protection from negative reactions. The simple complexes have one bond between the metal ion and the organic partner through a single point of attachment. This single bond makes the simple complex prone to disassociate in solution very easily, offering minimal protection. Zinc methionine is an example of a simple complex.
Most metals can form a total of four to six bonds. A chelated metal bonds with the organic partner at two or more attachment points. This chelate, which is a type of complex, exhibits a special measure of bonding strength. Chelates protect metal ions better than simple complexes because more than one bond must be broken before the organic bonding partner will release the metal ion. The multiple attachment points to the metal of chelates create greater stability and protection.
The trace mineral industry competes by researching the level of protection that each of their chosen mineral compounds offer, whether inorganic or organic, and how it affects animal performance. Each form has a different affinity to each of the metal ions and reacts differently to the environment to which it is exposed. The pH of that environment will greatly impact how these different forms persist and the final outcome. Some organic minerals with less stable partners are only partially protected from pH modification, but Optimin® Chelated Trace Minerals, for example, remain stable even when exposed to challenging physiological pH levels.
Insolubility, complexing and chelating are all methods in which the manufacturers use to provide stability to avoid negative reactions. The level of nutrients preserved for absorption, determines the ability of the animal to carry out their metabolic functions to their genetic potential. The key to nutritional success is to have enough nutrients remain available for the animal when it needs them and in the right proportion. Challenges to this metabolic process include the level of antagonists in the diet, as well as stresses or immune challenges to that animal that require higher metabolic needs. The influences are dynamic. The longer a mineral form can keep the mineral available for absorption, the greater the probability of nutritional success.
Clydesdale, F.M. 1988. Minerals: Their chemistry and fate in food. In: Trace Minerals in Foods. Marcel Dekker, Inc. New York and Basel. Holwerda, R.A., Robert C. Albin & Fred C. Madsen. 1995 Chelation effectiveness of zinc proteinates. In: Feedstuffs, Vol. 67, No. 25. Lonnerdal, B. & Sandstrom, B. 1995. Factors influencing the uptake of metal ions from the digestive tract. In: Handbook of Metal Ligand Interaction in Biological Fluids. Marcel Dekker Publishing. New York Vol. 1.