Q. When looking at the ingredients in various feeds, I notice there seem to be different types of the same mineral. For example, I’ve seen zinc oxide, zinc proteinate, zinc methionine, and zinc amino acid complex. What is the difference between these, and is one better than the others?
A. First of all, well done for looking at feed labels in such detail and noticing this difference! You are correct that the same mineral exists in different forms. This not only applies to zinc but also other trace minerals, such as copper and manganese. Less frequently you will find different versions of macro minerals such as calcium and magnesium.
The bioavailability of minerals, or the proportion of the mineral that is absorbed from the diet into circulation, varies based on a number of factors, one of which is their chemical form. Minerals bonded to an inorganic compound (one that does not contain carbon) are known as inorganic minerals. Examples of inorganic minerals include oxides and sulfates, and they tend to be easy and inexpensive to produce. Generally, inorganic sources of macrominerals (calcium, phosphorus, magnesium, sodium, chloride, and potassium) have relatively high bioavailability; however, inorganic trace minerals are less bioavailable.
The other forms of minerals you mentioned, such as proteinates, amino acid complexes, etc., are known as organic minerals, because the mineral is bound to an organic compound. The Association of American Feed Control Officials defines them as follows:
Metal proteinate—The product resulting from the chelation of a soluble salt with amino acids and/or partially hydrolyzed protein. It must be listed as a specific metal proteinate such as manganese proteinate. (Note that chelation, pronounced key-lay-shun, is the result of a metal binding to an organic substance in a ring structure. This protects the metal and facilitates absorption.)
Metal polysaccharide complexes—The product of complexing a soluble salt with a polysaccharide solution declared as an ingredient as the specific metal complex such as zinc polysaccharide complex.
Metal amino acid chelate—The product resulting from the reaction of a metal ion from a soluble metal salt with amino acids with a mole ratio of one mole of metal to one to three moles of amino acids to form coordinate covalent bonds. It must be declared as a specific metal amino acid chelate such as copper amino acid chelate.
Metal amino acid complex—Results from complexing a soluble metal salt with an amino acid(s). The ingredient must be listed as a specific amino acid complex such as zinc amino acid complex.
Metal (amino acid specific) complex—Results from complexing the soluble metal salt with a specific amino acid and must be declared as the specific amino acid complex such as copper lysine or zinc methionine.
These definitions show that even within a definition there is the potential for quite large variability in the resulting product’s chemical composition. So, the metal amino acid chelate made by one company can differ significantly from that made by another, even if they are both copper amino acid chelates.
The purpose of these complexes and chelates is to make the minerals more bioavailable. One of the reasons this is thought to work is that the organic forms of these minerals are protected from unwanted interactions in the gastrointestinal tract that would reduce their absorption. Another cause of better bioavailability is that the organic forms might be absorbed through additional pathways not available to inorganic forms. For example, the copper in copper lysine might be able to be absorbed not only through regular copper channels but also with lysine through lysine absorption pathways. This becomes particularly useful if the regular mineral absorption pathway is for some reason blocked, perhaps by another competing mineral. Some organic forms are potentially more stable than others and might be better candidates for use in certain situations.