Publisher Summary This chapter discusses that the knowledge of phytic acid had its beginning in the discovery by Hartig, who isolated small particles or grains (which were not starch grains) from the seeds of various plants. Phytic acid has been generally regarded as the primary storage form of both phosphate and inositol in almost all seeds. The amount of phytic acid varies from 0.50% to 1.89% in cereals (except polished rice), from 0.40% to 2.06% in legumes, from 2.00% to 5.20% in oil seeds except soybeans and peanuts (grouped under legumes), and from 0.40% to 7.50% in protein products. Many foods and seeds contain myo-inositol hexaphosphate as an important source of phosphorus, and accurate methods for its determination are needed. Phytase (meso-inositol hexaphosphate phosphohydrolase, EC 3.1.3.8) is widely distributed in plants, animals, and fungi. In mature cereal grains, legumes, and oil seeds, the major portion of the total phosphorus is present in the form of phytic acid (phytate). Experiments with animals have suggested that phytic acid in plant foods complexes with dietary essential minerals such as calcium, zinc, iron, and magnesium and makes them biologically unavailable for absorption. The correlation of phytate with the cooking quality of peas was first suggested by Mattson. Phytic acid, myo-inositol 1,2,3,4,5,6-hexakis dihydrogen phosphate serves as the main phosphorus store in mature seeds and grains. In cereals and legumes, phytic acid content ranges from 0.14% to 2.05%, which accounts for 18 to 88% of the total phosphorus.

Phytates in Legumes and Cereals

Phytic acid is present in many plant systems, constituting about 1 to 5% by weight of many cereals and legumes. Concern about its presence in food arises from evidence that it decreases the bioavailability of many essential minerals by interacting with multivalent cations and/or proteins to form complexes that may be insoluble or otherwise unavailable under physiologic conditions. The precise structure of phytic acid and its salts is still a matter of controversy and lack of a good method of analysis is also a problem. It forms fairly stable chelates with almost all multivalent cations which are insoluble above pH 6 to 7, although pH, type, and concentration of cation have a tremendous influence on their solubility characteristics. In addition, at low pH and low cation concentration, phytate‐protein complexes are formed due to direct electrostatic interaction, while at pH >6 to 7, a ternary phytic acid‐mineral‐protein complex is formed which dissociates at high Na concentrations. These complexes appear to...

Phytic acid interactions in food systems

Marginal zinc deficiency and suboptimal zinc status have been recognized in many groups of the population in both less developed and industrialized countries. Although the cause in some cases may be inadequate dietary intake of zinc, inhibitors of zinc absorption are most likely the most common causative factor. Phytate, which is present in staple foods like cereals, corn and rice, has a strong negative effect on zinc absorption from composite meals. Inositol hexaphosphates and pentaphosphates are the phytate forms that exert these negative effects, whereas the lower phosphates have no or little effect on zinc absorption. The removal or reduction of phytate by enzyme (phytase) treatment, precipitation methods, germination, fermentation or plant breeding/genetic engineering markedly improves zinc absorption. Iron can have a negative effect on zinc absorption, if given together in a supplement, whereas no effect is observed when the same amounts are present in a meal as fortificants. Cadmium, which is increasing in the environment, also inhibits zinc absorption. The amount of protein in a meal has a positive effect on zinc absorption, but individual proteins may act differently; e.g., casein has a modest inhibitory effect of zinc absorption compared with other protein sources. Amino acids, such as histidine and methionine, and other low-molecular-weight ions, such as EDTA and organic acids (e.g., citrate), are known to have a positive effect on zinc absorption and have been used for zinc supplements. Knowledge about dietary factors that inhibit zinc absorption and about ways to overcome or remove these factors is essential when designing strategies to improve the zinc nutrition of vulnerable groups.

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Dietary Factors Influencing Zinc Absorption

A rapid method is described for the colorimetric determination of 1.5–15 μg phytate phosphorus in concentrations as low as 3 μg ml−1 in extracts of cereal grains and cereal products. The phytic acid is precipitated with an acidic iron-III-solution of known iron content. The decrease of iron in the supernatant is a measure for the phyticacid content.

Sensitive method for the rapid determination of phytate in cereals and cereal products

There are a number of components present in soybeans that exert a negative impact on the nutritional quality of the protein. Among those factors that are destroyed by heat treatment are the protease inhibitors and lectins. Protease inhibitors exert their antinutritional effect by causing pancreatic hypertrophy/hyperplasia, which ultimately results in an inhibition of growth. The lectin, by virtue of its ability to bind to glycoprotein receptors on the epithelial cells lining the intestinal mucosa, inhibits growth by interfering with the absorption of nutrients. Of lesser significance are the antinutritional effects produced by relatively heat stable factors, such as goitrogens, tannins, phytoestrogens, flatus-producing oligosaccharides, phytate, and saponins. Other diverse but ill-defined factors appear to increase the requirements for vitamins A, B12, D, and E. The processing of soybeans under severe alkaline conditions leads to the formation of lysinoalanine, which has been shown to damage the kidneys of rats. This is not generally true, however, for edible soy protein that has been produced under milder alkaline conditions. Also meriting consideration is the allergenic response that may sometimes occur in humans, as well as calves and piglets, on dietary exposure to soybeans.

Implications of antinutritional components in soybean foods

1. The inclusion of phytate (10 g/kg) in a purified diet containing zinc (15 mg/kg) fed to young male rats significantly reduced growth rate and food intake, and promoted a cyclic pattern of food intake characteristic of an uncomplicated Zn deficiency. The decreased growth rate could be accounted for by the reduced food consumption. 2. Rats maintained on a Zn-deficient diet (0.5 mg Zn/kg) were found to have a cyclic pattern of food intake and a very slight weight gain. The addition of phytate (10 g/kg) to the Zn-deficient diet promoted a net loss of mean body-weight. 3. Rats maintained on the Zn-supplemented diet containing phytate excreted significantly more Zn in their faeces than either pair-fed or ad lib.-fed control rats. Rats given the Zn-deficient diet supplemented with phytate excreted more Zn in their faeces than Zn-deficient control rats. 4. Dietary phytate significantly reduced the average daily accumulation (mug/d) and whole-body retention (relative to dietary intake) of iron, copper, manganese and Zn, whether or not the diet was supplemented with Zn. 5. The addition of phytate to the lumen fluid of ligated loops of rat duodenum maintained in situ significantly inhibited 65Zn absorption, compared with the control systems without added phytate. 6. Other studies using ligated duodenal and ileal loops indicated that Zn is secreted into the gut lumen and approximately one-third of this is normally reabsorbed. Recycling of endogenous Zn may be a significant process in the over-all body economy of this trace element. 7. The absorption of 65Zn added to the diet was significantly reduced by dietary phytate. Dietary phytate also reduced the biological half-life of body 65Zn from 61 to 211 h post-administration, possibly by inhibiting reabsorption of endogenous 65Zn and thus promoting a more rapid loss from the body.

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The effects of phytate on intestinal absorption and secretion of zinc, and whole-body retention of Zn, copper, iron and manganese in rats.

1. A study has been made of the zinc, copper, iron, manganese, protein (nitrogen X 6.25) and phytic acid contents of nineteen soya-bean-based textured-vegetable-protein (TVP) meat-extenders and meat-substitutes and of three "ready-prepared" canned meals containing TVP. 2. Phytate analysis was performed using a newly-developed method based on Holt's (1955) procedure. This method enabled the phytate content of milligram quantities of TVP to be estimated, with an SD for six replicates of 3%. 3. The Fe, Cu and Mn contents (mg/kg) of the meat extenders or meat-substitutes varied, with values of 59.4-144, 14.1-19.7 and 19.5-29.1 respectively. The protein content of these products was approximately 500 g/kg. 4. The phytate content of the meat-extenders and meat-substitutes ranged from 11.0 to 20.2 g/kg and the Zn content from 35.0 to 49.4 mg Zn/kg. The calculated molar ratio, phytate: Zn varied from 25 to 42. 5. The trace element, phytate and protein contents of the "ready-prepared" canned meals were 30-50% lower than the meat-extenders and meat-substitutes. 6. Cooking the "ready-prepared" meals as specified by the manufacturers was without effect on the trace element or phytate content. 7. When TVP was fed to rats as the only protein source, they had significantly lower growth rates and plasma Zn concentrations than rats given an egg-albumen-based diet of similar Zn content (14.5 mg Zn/kg). Supplementation of the TVP diet with Zn (100 mg Zn/kg) significantly increased growth rate and plasma Zn concentration whereas Zn supplementation of the albumen diet was without effect. 8. The possible implications of consumption of TVP products in relation to Zn status of the human population is discussed.

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An evaluation of the phytate, zinc, copper, iron and manganese contents of, and Zn availability from, soya-based textured-vegetable-protein meat-substitutes or meat-extenders

1. Studies were carried out in vitro to examine the effects of phytate on the solubility of the trace elements zinc, copper and manganese. Appropriate volumes of a solution of sodium phytate were added to a mineral solution to achieve phytate: Zn values of from 0 : 1 to 45 : 1. In a second series the same values for phytate: Zn were achieved by varying the amount of added Zn at a fixed phytate concentration. 2. In both experiments greater than 85% of the Zn was rendered insoluble at pH 6.5 even at the lowest value for phytate: Zn (5 : 1). The effect of phytate on Zn solubility was greater than effects on Cu or Mn. 3. In a dietary study, rats were offered a semi-synthetic egg-albumin-based diet with added phytate. Two series of diets were prepared, the first had a constant Zn content (18.5 mg Zn/kg) and the amount of sodium phytate varied so as to achieve values for phytate: Zn of from 0 : 1 to 40 : 1 (series 1). In the second series, the same values for phytate: Zn were achieved by adding a fixed amount of phytate (7.4 g phytic acid/kg) while the amount of Zn was varied (series 2). 4. Dietary phytate caused significant reductions in growth rates, plasma Zn concentrations and hair Zn concentrations and greying of the coat at values for phytate: Zn of 15 : 1, 10 : 1, 15 : 1 and 15 : 1, respectively. 5. While phytate was apparently slightly more effective in reducing Zn status when phytate: Zn values were achieved at the lower absolute levels of phytate and zn (series 1 diets), the differences at equivalent phytate: Zn values were small. It was concluded that phytate: Zn values can be used as an indicator of Zn availability from phytate-rich diets. 6. Rats offered three diets containing soya-bean-based textured-vegetable-protein (TVP) exhibited low rates of weight gain compared with rats offered an egg-albumen-based diet of similar Zn content (14.5 mg Zn/kg). Additional Zn supplied in drinking-water (25 mg Zn//l) was without effect on rats consuming the egg-albumin diet but significantly improved the weight gain of rats on the TVP diets. 7. It was concluded that phytate naturally present in TVP behaves similarly to phytate added to an otherwise phytate-free diet and that the reduced availability of Zn in TVP diets can be accounted for entirely by their phytate contents.

Studies on the phytate: zinc molar contents in diets as a determinant of Zn availability to young rats.

1. A technique is described for the measurement of the extent of 65Zn absorption by different regions of the intestine in the intact rat. Using this technique it was shown that the duodenum contributed 60%, the ileum 30% and the jejunum 10% to the over-all absorption of 65Zn. Negligible absorption of 65Zn occurred from the caecum and colon. 2. Using ligated loops of rat duodenum in situ, 65Zn absorption was shown to be rapid, with 1% of a 5 microgram dose being transferred to the carcase within 1 min of intraluminal dosing. 3. When 65Zn was injected into ligated loops in a dose range of 1--200 microgram Zn/ml the rate of absorption was linear with respect to time over the first 15 min. The rates of 65Zn absorption v. dose of 65Zn exhibited saturation kinetics indicating absorption by a 'carrier' or enzyme-mediated process. 4. The binding of 65Zn to loop tissue showed biphasic kinetics which suggested that at low intraluminal concentrations of Zn (1--50 microgram Zn/ml) binding was to specific sites whereas, at higher concentrations (50--200 microgram Zn/ml), non-specific binding occurred. 5. A study of the fate of mucosally bound 65Zn showed that over the first 30 min a proportion of the 65Zn was rapidly transferred to the carcase and this was probably associated with the rapid phase of 65Zn absorption described previously. From 30 min up to 6 h after the initial binding, 65Zn was also transferred to the carcase albeit at a much slower rate indicating a slow phase of Zn absorption. A study of the kinetics of this slow phase indicated that the loss of bound 65Zn to the body was a saturable process indicating an enzyme- or 'carrier'-mediated process. A comparison of the kinetics of the slow and rapid phases of 65Zn absorption suggests that these processes are distinct. 6. Histological examination of mucosal tissue of loops exposed to 200 microgram Zn/ml revealed to discernable diamage. Similarly, no effect was observed on either arginine or glucose uptake by isolated duodenal loops in situ, although this concentration of Zn completely abolished fluid uptake. A study of the effect of different doses of Zn showed that 50 microgram Zn/ml inhibited mucosal fluid uptake by more than 50% and 100 microgram Zn/ml by more than 90%. It was concluded that these effects were due to a specific action on the fluid-transfer process and not due to a general poisoning of the mucosa.

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Studies on the absorption of zinc by rat intestine.

1. The activities of alkaline phosphatase (EC 3.1.3.1) and phytase (EC 3.1.3.8) were similarly distributed in the small intestine of rats. Regional differences in activity were reflected by similar differences in the capacity of ligated intestinal segments to hydrolyse phytate in vivo. Activities were greatest in the duodenum and lowest in the terminal ileum. 2. Specific activities of both enzymes were tenfold greater in the brush border fraction of duodenal mucosa compared with entire mucosal homogenates. 3. Brush-border alkaline phosphatase and phytase activities required both magnesium and zinc ions for maximal activity. 4. Zn deficiency induced by feeding a diet low in Zn (0.5 mg Zn/kg) caused similar reductions in activity of both enzymes. 5. Zn deficiency induced by feeding diets marginally adequate in Zn (12 mg/kg) and phytate (10 g/kg) caused reductions in alkaline phosphatase, phytase activities and phytate hydrolysis in vivo. 6. It is suggested that phytase activity is a manifestation of alkaline phosphatase and the significance of this in relation to phytate-induced in Zn deficiency is discussed.

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The similarity between alkaline phosphatase (EC 3.1.3.1) and phytase (EC 3.1.3.8) activities in rat intestine and their importance in phytate-induced zinc deficiency

N. T. Davies

Papers

The effects of phytate on intestinal absorption and secretion of zinc, and whole-body retention of Zn, copper, iron and manganese in rats.

An evaluation of the phytate, zinc, copper, iron and manganese contents of, and Zn availability from, soya-based textured-vegetable-protein meat-substitutes or meat-extenders

Studies on the phytate: zinc molar contents in diets as a determinant of Zn availability to young rats.

Studies on the absorption of zinc by rat intestine.

The similarity between alkaline phosphatase (EC 3.1.3.1) and phytase (EC 3.1.3.8) activities in rat intestine and their importance in phytate-induced zinc deficiency