Mineral Absorption

Of all the elements mammals need, few enter the absorptive cell by passive diffusion. Several are carried into the body by proteins to which the minerals are loosely attached. Iron, zinc, and copper are imported via these transporters. Iodide and fluoride enter by way of an anion-cation exchange mechanism. Few of the minerals are 100% absorbable. For the others, the absorption process is dependent on a number of factors: the binding capacity of the transport protein (if needed); solubility; the composition of the diet; the mixture of elements present in the gut contents; and the presence of materials such as phytate and ethylenediaminetetraacetic acid (EDTA), which bind specific elements, thus changing the mineral mixture presented to the enterocyte. All of these factors contribute to the bioavailability of the essential micronutrients. In addition, physiological factors (eg, age, hormonal status, and health status) also influence absorption and subsequent use. Those minerals that are variable in terms of their absorption are those that are either divalent or multivalent ions (ie, ions with more than one charged state). Iron and chromium are multivalent ions whereas selenium, manganese, zinc, and molybdenum are divalent.

Because of their ionic nature, minerals can form electro-valent bonds to a variety of substances. Although ingested as salts, minerals ionize to their component parts, and it is the resultant ions that are absorbed, used, stored, or excreted. If absorbed, many of the microminerals are retained because there is little excretion. As ions, minerals react with charged amino acid residues of intact proteins and peptides. Table 5 provides a list of minerals and the amino acids with which they react. Depending on their valence state, these mineral-amino acid bonds can be very strong, moderate, or very weak associations.

Many minerals are carried in the blood by specific transport proteins. The reasons for this are not fully known. However, we do know that there are preferred ligand bonding groups as shown in Table 6. Other features of these

Table 5. Mineral-Amino Acid Interactions

Minerals

Amino acid

Copper

Histidine

Selenium

Methionine, cysteine

Zinc

Cysteine, histidine

Iron

Histidine

Table 6. Preferred Ligand Binding Groups for the Microminerals

Metal

Ligand groups

Mn2 +

Carboxylase; phosphate and nitrogen donors

Fe2 +

-SH, NH2 > carboxylates

Fe3 +

Carboxylate, tyrosine, -NH2, porphyrin (a nitrogen

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