Phytic acid zinc absorption

No effects of phytic acid on absorption absorption depended on zinc level (39)... 

If one had to state an overall role of copper in the body, one might say oxygen metabolism. One major factor shared by both zinc and copper is that both metal ions occur bound to metallothionein. The function of metallothionein is not firmly established. Copper is bound to another protein, ceruloplasmin, which occurs in the cell and plasma. The function of this protein is not clear either. Zinc absorption, as iron absorption, is impaired by high levels of phytic acid. Copper absorption is not inhibited by phytic acid. The major route of excretion of both metal ions is fecal, rather than urinary. 

No effect on apparent absorption of iron, manganese or copper although not statistically significant, absorption of zinc tended to decrease with increased phytic acid intake (44)... 

Simpson et al. (24) did not observe increased non-heme iron absorption when dephytinzed bran was included in a test meal compared to the non-dephytinized bran. Removal of phytic acid from soy protein likewise did not improve non-heme absorption (38). Decreased iron balances, observed by Bodwell et al. (40) with soy isolate diets, did not appear to be associated with the level of phytic acid a tendency for zinc balances to decrease with increased phytic acid was observed. 

Especially phytic acid has been claimed to be a potent Inhibitor of zinc absorption and zinc deficiency on high intake of phytic acid have been shown in different animals (9, 10). 

To study the effect of phytic acid, calcium and protein in bread meals in various combination, on the zinc absorption in man. 

When adding animal protein in the form of milk, egg and cheese to the whole flour meal, the zinc absorption was increased. (Table II). It seems like the binding of zinc to phytic acid and fibercomponents is overcome if enough animal protein is served together with the whole flour bread. 

In animal experiments it has been shown that a high calcium content in combination with the phytic acid in whole flour bread will decrease the zinc absorption (10), When adding milk and milk products to the whole flour bread in Dr. Sandstr0ms experiments the same positive effect was seen from the protein despite of the higher calcium content (Table III). 

It is possible that the protein competes with the binding to phytic acid or fiber comnonents in whole flour bread rather than facilitating zinc absorption. 

By increasing the fermentation period when making the dough for the bread, the decreased zinc absorption in whole flour bread can be overcome. The phytic acid is in this way broken down, and the zinc better available absorption. 

Most of our knowledge about factors affect zinc absorption ( , 10, 18) comes from studies on animals. The most extensive studies are made on the affect of phytic acid, which is present in vegetables and unrefined cereal products. Decreased zinc absorption has been found both when substantial amounts of sodium-phytate are added to the diet and when the diets are based on phytic-containing food, such as soy beans. The explanation of this is that a complex between Zn and phytic acid is made, resulting in a decreased bioavailability. 

From the data obtained for zinc absorption in the studies presented here, the contents of zinc, protein and phytic acid were useful in estimating the degree of zinc absorption. The main determinant, however, seems to be the total zinc content. 

Interaction between zinc and calcium has been demonstrated In several animal studies (l- ). It has been shown that calcium antagonizes the biological effects of zinc and that calcium reduces the availability of zinc for absorption. This decrease In zinc absorption resulted In severe malnutrition and parakeratosis (1.-7.) Several studies have conclusively shown that the calclum-zlnc antagonism studied In animals Is due to excess phytic acid In the diet (, 9). However, In the absence of phytic acid, excess dietary calcium per se has also been shown to decrease the Intestinal absorption and the retention of zinc In rats (10). This Inhibitory effect on zinc absorption. Induced by calcium, was further enhanced by the addition of phosphorus to the high calcium Intake (11). 

Proteins and Meals. Nutritional properties of the oilseed protein meals and their derived products are determined by the amino acid compositions, content of biologically active proteins, and various nonprotein constituents found in the defatted meals. Phytic acid (3), present as salts in all four meals, is beheved to interfere with dietary absorption of minerals such as zinc, calcium, and iron (67) (see Food TOXICANTS, NATURALLY OCCURRING Mineral nutrients). ... 

Zinc deficiency has been found among peasant populations in rural areas of the Middle East. Unleavened whole wheat bread can account for 75% of the energy intake in these areas, that is, Iran, Egypt, and Turkey. This diet, which does not contain meat, does contain zinc, but it also contains phytic acid at a level of about 3 g/day. The phytic acid inhibits zinc absorption. The yeast used to leaven bread produces phosphatases that hydrolyze the phosphate groups from phytic acid. 

The ability of zinc ions to block copper absorption, possibly by formation of- intestinal metallothionein that strongly binds copper, has led to its use in pharmacological doses in the management of Wilson s disease/ Similarly molybdate ion can form insoluble copper-molybdate complexes in the intestine that limit copper absorption. The detrimental effects of organic phosphate (phytic acid) in limiting zinc absorption are aggravated by excess dietary calcium, probably by formation of a highly insoluble Ca-Zn-phytate complex. The subject of these and other interactions has been reviewed, ... 

It has been shown in animal experiments that phytic acid, iron and zinc may decrease the gastrointestinal absorption of lead. In animals, concomitant intake of vitamin D, protein and fat may increase lead uptake, whereas in man a deficient iron status and alcohol may increase the gastrointestinal absorption of lead. 

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. 

The direct measurement in the diet of substances known to adversely affect zinc absorption. This most commonly would require determination of the phytic acid content. Substances such as dietary fibre, calcium, iron, tin, oxalate and others, are also important in certain instances. 

In animal cells, it occurs as a component of phospholipids, substances containing phosphorus, fatty acids, and nitrogenous bases. In plant cells, it is found as phytic acid, an organic acid that binds calcium, iron, and zinc in an insoluble complex and interferes with their absorption. 

ABSORPTION, METABOLISM, EXCRETION. Zinc is poorly absorbed less than 10% of dietary zinc is taken into the body, primarily in the duodenum. It appears that metallic zinc and zinc in its carbonate, sulfate, and oxide forms are all absorbed equally well. Large amounts of calcium, phytic acid, or copper inhibit zinc absorption. Cadmium appears to be a zinc antimetabolite. 

Additional detail concerning the composition of the egg white biotin-fortified diet and the method for analysis of the zinc content of the diets is supplied in a recent publication by Leucke and Fraker (1979). Diets containing phytic acid or soybean protein contaminated with phytic acid should be avoided since the amount of zinc available for absorption is unknown and phytic acid can chelate metals other than zinc. 

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