Bioavailability of zinc from diet and the recommended dietary allowance

BIOAVAILABILITY OF ZINC FROM DIET AND THE RECOMMENDED DIETARY ALLOWANCE 

The results of dietary zinc analysis need to be considered in terms of the availability of the zinc in the food for intestinal absorption. The zinc content of whole meals and the total daily zinc intake are not sufficient information on their own, without knowledge of factors which inhibit or promote intestinal absorption (O Dell, 1984). Free ionic zinc probably does not exist in the intestinal tract, zinc being bound to molecular species such as protein, amino acids, phytic acid, citrate and others. The bioavailability of the metal is determined by the nature of these zinc binding ligands. When the zinc complex is insolubie as in Zn-phytate, the uptake from diet is poor, whereas zinc-protein or zinc-amino acid complexes are more easily dissociated and are a good source of available zinc. Other dietary components affect zinc absorption such as the amount of iron, calcium and phosphate. 

When the ratio of the concentration of these substances to zinc is excessive, zinc is poorly absorbed. 

Absorption experiments in which the fraction of a known amount of a zinc tracer, usually Zn, crossing the intestinal wall is measured. The tracer can be added as an inorganic salt and mixed into the diet very thoroughly this is extrinsic labelling. Or the label can be incorporated into the growing piant or animal crop and thus into the processed foodstuff this is Intrinsic labelling. 

A volunteer consumes the labelled food as part of the diet under study and the whole body radioactivity is determined in an appropriate whole body counter, and this is repeated after 14 days, by which time all non-absorbed material will have been eliminated in the faeces. The fraction of the original activity retained gives a measure of zinc absorption from that diet. 

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