Phytate bioavailable forms

Monoferric phytate is the major fraction of iron in wheat bran, and is a highly bioavailable form of dietary iron in contrast to insoluble di- or tetra-ferric phytate. Monoferric phytate equilibrates with the miscible nonheme iron pool of a meal in extrinsic label iron absorption tests. Whole wheat bran depressed absorption by humans of nonheme iron in a meal. Dephytinized wheat bran also inhibited nonheme iron absorption by humans and the inhibition could not be clearly attributed to either the insoluble or soluble fractions of the dephytinized bran. Adult men who consumed 36 g of wheat bran per day had positive iron balances. Iron balance was not increased when dephytinized bran was consumed. The form of ferric phytate must be known to properly explain the effect of phytic acid on iron absorption. The overall meal composition must be considered to evaluate the effect of wheat bran on iron nutrition of humans. 

In conclusion, phytic acid forms soluble complexes with Ca2+ at intestinal pH under a variety of conditions and fails to inhibit Ca2 bioavailability to mice in our experimental system. Despite the hazard in direct extrapolation of results obtained with animals kept on a well-defined dietary regimen to humans consuming a complex diet, many elements of which affect Ca2+ bioavailability, our data demonstrate the need for a reevaluation of the putative antinutritional properties of dietary phytate. Our further contention that adequate levels of dietary phytate may actually be beneficial due to its food preserving properties and its protection against colonic cancer will warrant a prospective epidemiological human study designed to assess the longterm effects of dietary phytate on mineral bioavailability and inflammatory bowel diseases. 

Wheat bran has been the fiber source most commonly used to study effects of dietary fiber on calcium absorption in controlled laboratory studies. However, wheat bran and other forms of fiber as they occur in food products present several disadvantages in terms of definition and by concurrently altering intakes of other substances or materials known or suspected of having an adverse effect on the bioavailability of calcium such as phytates and oxalates (5,13,17,22-28). Several studies have been conducted which have sought to separate or compare the effects of phytate and fiber... 

Phytase offers significant promise as a means to reduce phosphorus levels in animal waste by 30-35%, while also reducing the cost of phosphorus supplementation. The enzyme hydrolyzes phytate (myo-inositol hexakisphosphate), the primary storage form of phosphorus in plant seeds and pollen, in several steps into inositol and inorganic phosphorus, which is readily bioavailable to the farm animals. Phytases can also have non-specific phosphorus monoester activity. Addition of phytases to farm animals diets significantly enhances bioavailability of plant phosphorus for the animals while reducing phosphorus in the waste and simultaneously allowing a reduction of total phosphoms in the feed 500 units of phytase... 

Other organic acids which have been tested for their impact on iron absorption include oxalic, succinic, fumaric and lactic acid (Table V). The iron from iron phytates was bioavailable only in the soluble monoferric form, whereas the less soluble diand tetra-ferric phytates were very poor iron sources (37,... 

Iron bioavailability is affected by valence state, form, solubility, particle size, and com-plexation which in turn may be affected by the food matrix. Complexation of iron has been found to have either a positive or negative effect on availability, with such compounds as ascorbic acid and fructose increasing availability and oxalates, phytates, phosphates and food fibers perhaps decreasing availability. Availability has also been shown to be directly correlated to acid solubility. We have found that acidity tends to increase ionization as well as favoring the ferrous state which has greater solubility at... 

Based on USDA estimates of per capita consumption of wheat flour, one-third of the adult woman s Recommended Dietary Allowance (RDA) for iron could be obtained if we consumed whole wheat products Q). The iron in wheat, however, is thought to be poorly bioavailable to humans, primarily attributable to the effect of phytate. British investigators found that the iron balance of individuals was lower when they ate largely whole meal bread than when they ate bread made with white flour (2). When the test bread made with white flour contained either sodium or ferric phytate, postprandial serum iron rise was depressed ( ). They theorized that the phytate present in the brown bread formed an insoluble iron salt and rendered the iron unabsorbable. That theory was supported by the work of Moore et al. (4) at Washington University, who tested the response of anemic individuals administered therapuetic doses of ferric... 

About a decade ago we began studies to identify the chemical nature of iron in wheat as possibly a first step in devising a means to improve the assimilation of the iron in wheat-based foods. In this communication we will discuss the evidence for our belief that monoferric phytate is the endogenous form of iron in bran, some physical-chemical characteristics of monoferric phytate, and studies in animals and humans of the bioavailability of iron in wheat bran and monoferric phytate. 

Human studies. Simpson et al. (14) measured absorption by humans of free monoferric phytate from meals of both high and low bioavailability. Absorption was measured by the extrinsic tag method (15). Iron (2 mg) was added to the meals in each pair of absorption tests as either monoferric phytate or FeClj, each compound was labeled with either 55 Fe or 59Fe. The absorption ratio of monoferric phytate to FeClj was not significantly different from unity (Table III). The free biological form of iron in wheat bran, monoferric phytate, was no less well absorbed than the dietary nonheme iron in the meals. Absorption of both compounds was several fold greater from the standard (STD) than from the semisynthetic (SS) meal reflecting the relative bioavailability of nonheme iron from the two types of meals. 

The solubility of monoferric phytate in water likely explains the high bioavailability of the iron from monoferric compared to iron from the relatively insoluble di or tetraferric phytates. Possibly the ferric phytates used by Moore et al. (4) for humans and by Bremner and Dalgarno (20) for calves were mixtures of all possible ferric phytates, but were predominately insoluble forms and the iron was poorly bioavailable. We found the iron of an insoluble calcium ferric phytate product to be of low bioavailability. Sharpe et al. (21) added sodium phytate and ferric chloride labeled with radioiron to milk and found that the radioiron was poorly absorbed by adolescent boys. Possibly insoluble calcium ferric phytate formed in that experiment. 

Approximately 70f of the monoferric phytate baked into bread could be recovered in extracts of the bread. He have not tested the bioavailability or the chemical form of iron when an iron salt and sodium phytate are baked into bread as was done by McCance et al. (3). If an insoluble ferric phytate of any ion composition was formed, we would expect the iron to be poorly bioavailable. Hussain and Patwardhan (22) did not specify whether they added sodium phytate to the food during preparation or administered it orally when young men ate the meals in their study using a vegetarian menu. They found decreased iron balance when the percentage of phytate phosphorus to total phosphorus was increased from 8 to 40 by addition of sodium phytate. 

The major fraction of iron in wheat bran is monoferric phytate, which is soluble and equilibrates with the miscible dietary nonheme iron pool of a meal. The di- and tetra-ferric phytates are much less soluble than monoferric phytate and probably do not equilibrate with the miscible nonheme iron pool of a meal. Bioavailability studies using either ferric phytate or sodium phytate must be evaluated in light of the form of ferric phytate or whether an insoluble ferric phytate may have been produced in the food. Wheat bran depresses absorption of dietary nonheme iron by humans. On the basis of available evidence, that effect of wheat bran cannot be unequivocally attributed to either the phytate or fiber component alone and might be influenced by interactions between the fiber, phytate and iron in the whole bran. Adult men maintained adequate positive iron balance when they consumed 36 g of whole bran each... 

Phosphorus is an important component of feed, as it is crucial for bone and skeleton formation. About 70 % of phosphorus in vegetable feed ingredients is present in the form of phytate, an inositol-bound organic form of phosphorus that has a low bioavailability in monogastric animals. For this reason, the diet for monogastric animals like pigs and chickens is supplemented... 

Heme-iron, only present in animal foods, is more bioavailable compared to other sources contained in grains and vegetables. The consumption of animal products enhances iron absorption associated with grains. Another important enhancer of iron absorption is vitamin C because it chelates nonheme-iron under stomach acidic conditions, and keeps it soluble under the relatively neutral pH conditions of the duodenum. Vitamin C reduces ferric iron (Fe+ ) into ferrous iron (Fe+ ) in the stomach. The ferrous form is more efficiently absorbed by the duodenum epithelial cells. Heme-iron enters the epithelial cells complexed to porphyrin myoglobin and hemoglobin. The absorbed iron is stored in ferritin molecules located inside the intestinal cells and transported bound to transferrin. There are known inhibitors of iron absorption, the most important being phytates and fiber. 

Non-heme iron exists in plant products and its bioavailability is compromised by the concurrent ingestion of tannins, phytates, soy, and other plant constituents, that decrease its solubility in the intestinal lumen. Bioavailability of non-heme iron is increased by concurrent ingestion of ascorbic acid and meat products. Nonheme iron is reduced from the ferric to the ferrous form in the intestinal lumen and transported into enterocytes via the divalent metal transporter (DMT-1). Once inside the enterocyte, iron from heme and nonheme sources is similarly transported through the cell and across the basolateral membrane by the ferroportin transporter in conjunction with the ferroxidase hephaestin after which it can be taken up by transferrin into the circulation. The regulation of iron across the basolateral membrane of the enterocyte is considered the most important aspect of iron absorption. 

The issue of bioavailability from food sources and the interactions between food groups and copper availability remains a critical question. Lonnerdal et al. demonstrated that heat treatment of cows milk formula decreases the copper bioavailability. Transitional complexes form in the milk upon heating that have a similar configuration to copper and thereby directly inhibit copper absorption. High doses of zinc also reduce copper bioavailability, as does combined iron and zinc supplementation. The dilemma is how to prepare an infant formula containing adequate copper, iron, and zinc that will meet the RDA for copper. Other nutrients dramatically affect copper absorption from foods. Soy protein-based diets promote less copper retention in tissues than lactalbumin-based diets. However, it is unclear if this effect is solely due to the soy protein composition or to the higher zinc in these soy-based formulas. In animals, phytate causes a drop in serum copper but human stable isotope studies reveal no... 

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