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The study of the importance of trace elements in animal nutrition has been ongoing for nearly a century. Early on it was discovered that commercially available diets were often deficient in several of the nutritionally important trace elements. Initially it was found that the addition of trace elements in the form of minerals (generally from mining or other industries) was sufficient to meet the animal’s biological needs. A lot of research was conducted on the solubility and nutritional availability of these “mineral” sources and it was found that some mineral sources were more nutritionally available than others.
This research was used to formulate diets with the most appropriate sources of added minerals. More available forms of trace elements were needed for improved reproduction and overall health. Today it is generally accepted that animal performance can be improved by judicious use of organic forms of essential elements (especially transition metals and selenium). However, there is much debate concerning which organic forms are best utilized by the animal.
Defining some of the major, and most important, differences in organic trace minerals is the basis for this article. 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 deciding on the most appropriate form of the element to use.
What happens to metal complexes during digestion?
All organic trace minerals
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, while 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, some 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 causes a new equilibrium to be attained. This is by Mother Nature’s design, so that the strongest bonds survive. Metal ions may bond with one or more partners to form complexes.
Organic bonding partners can protect metal ions from undesirable reactions, keeping the metal soluble and increasing the probability that the mineral will be absorbed. The simple complex (Figure 1),
has one bond between the metal ion and one organic partner through a single point of attachment. This single bond makes the simple complex prone to dissociate (separate the metal from its escort) in solution very easily, defeating the purpose of complexing. Zinc methionine is an example of a simple complex.
Most metals can form a total of two 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 can release the metal ion. The chelate (Figure 2),
shows a zinc ion, Zn2+, linked to three different partners: ammonia (H3N), the amino acid glycine and the amino acid cysteine.
Ammonia complexes or attaches to zinc through its single nitrogen (N) atom. In contrast, glycine chelates zinc. Zinc is chelated because it is linked to glycine through both nitrogen and oxygen (O) atoms. Cysteine also chelates zinc, but through three points of attachment (nitrogen, oxygen, and sulfur (S) atoms). Taken together, the three partners form six bonds with zinc. It is not unusual for a metal to have several different bonding partners, some chelating and others simply bonding with a single point of attachment. But the real answer is “in the solution”!!
Commercial Metal Complexes and their Stability in Solution
Each organic partner, or amino acid in this case, has a different affinity to each of the metal ions. Methionine is a relatively weak bonding partner with most transition metals and results in a complex that can ionize easily in solution. As the pH drops, so does the ability of all complexes to maintain their bonds. Chelating can be a method to keep metals bonded in solution, maintaining solubility and improving the probability they will be absorbed by the animal. Solubility is enhanced in chelates with metal-binding partners that are strongly attracted to water. Optimins utilize these bonding partners to maximize its bond strength in solution and the probability that the metal is available for absorption.
Optimins are proteinates. Metal proteinates are complexes that have chelation in their structure. All chelates are complexes, but not all complexes are chelates. Not all products advertised as proteinates retain the distinctive cyclic bond structure of chelates when dissolved in water. In fact, the figure below shows both “proteinates” show a poor stability at a pH7. As pH drops (digesta is about pH4 at the site of trace mineral absorption in the small intestine), the percent of remaining chelated metal declines. Optimins on the other hand, were engineered to remain chelated in solution, mimicking what Mother Nature does naturally. In a neutral solution, 100% of the Optimins remain chelated and 95% of Optimins still remain bonded at a pH of 4.
As illustrated, an organic mineral’s nutritional performance is strongly affected by its form. In solution, a mineral’s form can and will change dramatically as this environment changes on its journey toward absorption in the animal. The solution is to have the right amino acids exposed to the right metal ions at the right time. Optimins were designed to improve the probability of absorption of essential trace elements.
Trouw Nutrition. 2019. Dr. Fred Madsen, Thomas Best and Dr. Kate Jackson, "Is there a difference in organic trace mineral sources?! The answer is "in the solution"!!".