Phytic acid

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Phytic acid, the hexaphosphoric ester of myo-inositol, is a strong acid. It forms insoluble salts with many types of bivalent and tervalent heavy metal ions. In that way, phytic acid reduces the availability of many minerals and essential trace elements.

Phytic acid has been shown to have a negative effect on iron absorption in humans. The absorption of iron depends mainly on the levels in the iron pools, the amount and the chemical form ingested, and the presence of ascorbic acid.

Ferric phytate is least soluble in diluted acid, i.e., it is insoluble in the stomach. At the pH in the duodenum, ferric phytate dissolves in the form of ferric hydroxide.

Phytic acid prevents the complexation between iron and gastroferrium, an iron-binding protein secreted in the stomach. Results from animal experiments and human studies indicate interference of phytic acid with the absorption of magnesium, zinc, copper, and manganese.

An important factor in the precipitation of phytates is the synergistic effect of two or more different cations, which can act together to increase the quantity of phytate that precipitates. For instance, zinc-calcium phytate precipitates maximally at pH 6, which is also the pH of the duodenum, where mainly calcium and trace metals are absorbed. An element makes its deficiency felt as soon as it becomes limiting due to binding to phytic acid.

The results of chemical analysis of the mineral and trace element contents of diets may give a false impression because of the interaction of phytic acid with the elements concerned at the levels of exposure or absorption. The highest levels of phytate occur in grains and some legumes. Generally, grains make up the bulk of the diet especially in developing countries where the diet is often deficient. Therefore, the presence of phytates in grains is a cause for concern. The phytate contents of several foods are listed in Table 3.1.

The phytate of plant seeds is located primarily in the bran and germ. Therefore, brown and wholemeal bread contain more phytate than white bread. In human studies, a diet of

Table 3.1 Phytate contents of selected foods

Food

mg%

Grains

Wheat

170-280

Rye

247

Maize

146-353

Rice

157-240

Barley

70-300

Oats

208-355

Sorghum

206-280

Buckwheat

322

Millet

83

Wheat bran

1170-1439

Legumes and vegetables

Green bean (Phaseolus vulgaris)

52

Bean (Phaseolus vulgaris)

269

Bean (Phaseolus lunatus)

152

Soybean

402

Lentil

295

Green pea (Pisum sativum)

12

Pea (Pisum sativum)

117

Pea (Lathyrus sativum)

82

Chick pea

140-354

Vetch

500

Potato

14

Carrot

0-4

Nuts and seeds

Walnut

120

Hazelnut

104

Almond

189

Peanut

205

Cocoa bean

169

Pistachio nut

176

Rapeseed

795

Cottonseed

368

Spices and flavoring agents

Millet

83

Caraway

297

Coriander

320

Cumin

153

Mustard

392

Nutmeg

162

Black pepper

115

Pepper

56

Paprika

71

brown bread (containing 214 mg phytic acid per 100 g) resulted in a 33 to 62% decrease in calcium absorption after 3 to 4 weeks, compared to a white bread diet. Addition of bran to white flour gave similar results. Only if the calcium intake was increased to 1.0 to 1.4 g per day, the calcium absorption improved. Calcium absorption is influenced not only by dietary phytate but also by vitamin D and lipids. If vitamin D is limiting in the diet, calcium absorption will be less efficient and the phytate effect will become more pronounced.

In many foodstuffs phytase activity can reduce the phytic acid level. Phytase is an enzyme occurring in plants. It catalyzes the dephosphorylation of phytic acid. Soybeans show weak phytase activity. Rye contains the most active phytase of all cereal grains. The activity of phytase drastically reduces the phytate content of dough during bread-making. Dephosphorylation of phytic acid is facilitated by the increase in acidity of bread dough caused by the reactivity of the yeast. Phytase is added to animal feeds so that no extra phosphate needs to be added. Also, in this way the animals will excrete less phosphate, which may contribute to reduction of environmental pollution.

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