Determine Factors affecting Zn Absorption - Micro Minerals?
In the last unit you have studied that absorption of various minerals (bioavailability) is influenced by number of factors. Similarly in case of zinc, different constituents of the diet, commonly known as dietary ligands may bind to zinc and either inhibit or enhance its absorption. It has been observed that citric acid, picolinic acid, glutathione, amino acids especially histidine and cysteine and possibly lysine and glycine serve as ligands and appear to enhance zinc absorption, especially in the presence of inhibitors. Zinc histidine and cysteine complexes are absorbed 30-40% more efficiently than zinc sulphate. These two amino acids appear to be also the preferred ligands for zinc. Isotope studies with human subjects have identified two factors that, together with the total zinc content of the diet, are major determinants of absorption and utilization of dietary zinc. The first is the content of inositol hexaphosphate (phytate) in the diet and the second is the level and source of dietary protein. Phytates are present in whole-grain cereals and legunles and in smaller amounts in other vegetables. They have a strong potential for binding divalent cations and their depressive effect on zinc absorption has been demonstrated in humans. The molar ratio between phytates and zinc in meals or diets is a useful indicator of the effect of phytates in depressing zinc absorption. At molar ratios above the range of 6 -10, zinc absorption starts to decline; at ratios above 15, absorption is typically less than 15%.
It has been observed that phytates, in the presence of high i~ltralurninal calcium, has a greater inhibitory effect than phytates alone. Provisionally it has been suggested that if phytate to zinc molar ratio is greater than 15, the content of available zinc in the diet is likely to be low (less than 15%). Available evidence shows that only hexa and penta-phosphorylated foims of phytic acid inhibit zinc absorption. The phytate content can also be reduced by activating the phytase present in most phytate- containing foods or through the addition of microbial or fungal phytases. Phytases hydrolyze the phytate to lower inositol phosphates, resulting in improved zinc absorption. The activity of phylases in tropical cereals such as maize and sorghum is lower than that in wheat and rye. Germination of cereals and legumes increases phytase activity and addition of some germinated flour to ungerminated maize or sorghum followed by soaking at ambient temperature for 12-24 hours can reduce the phytate content substantially.
Additional reduction can be achieved by the fermentation of porridge for weaning foods, or dough for bread making. Thus, fermentation which promotes extensive degradation of dietary phytates can significantly improve the bioavailability of zinc. The effect of phytate is, however, modified by the source and amount of dietary proteins consumed. Animal proteins improve zinc absorption from a phyiate-containing diet. Zinc absorption from some legume-based diets (e.g. white beans and lupin protein) is comparable with that from animal protein-based diets despite a higher phytate content in the former, as in case of iron, absorption of zinc generally is higher from foods of animal origin as compared to that from plant foods. Also, absorption appears to be enhanced by low zinc status, especially carrier - mediated mechanism. This indicates that the amount of zinc absorbed is homeostatically regulated.