Am J Clin Nutr. 2003 May;77(5):1213-9.
Degradation of phytic acid in cereal porridges improves iron absorption by human subjects.
Hurrell RF, Reddy MB, Juillerat MA, Cook JD.
Nestec Ltd, Nestlé Research Center, Lausanne, Switzerland.
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BACKGROUND: Phytic acid in cereal-based and legume-based complementary foods inhibits iron absorption. Low iron absorption from cereal porridges contributes to the high prevalence of iron deficiency in infants from developing countries. OBJECTIVE: The objective was to measure the influence of phytic acid degradation on iron absorption from cereal porridges. DESIGN: An exogenous phytase was used to fully degrade phytic acid during the manufacture of 9 roller-dried complementary foods based on rice, wheat, maize, oat, sorghum, and a wheat-soy blend. Iron absorption from the phytate-free and native phytate porridges prepared with water or milk (wheat only) was measured in adult humans with an extrinsic-label radioiron technique. Ascorbic acid was added to some porridges. RESULTS: When the foods were reconstituted with water, dephytinization increased iron absorption from rice porridge from 1.73% to 5.34% (P < 0.001), from oat from 0.33% to 2.79% (P < 0.0001), from maize from 1.80% to 8.92% (P < 0.0001), from wheat from 0.99% to 11.54% (P < 0.0001), from the wheat-soy blend without ascorbic acid from 1.15% to 3.75% (P < 0.005), and from the wheat-soy blend with ascorbic acid from 2.40% to 8.46% (P < 0.005). Reconstituting wheat porridge with milk instead of water markedly decreased or completely removed the enhancing effect of dephytinization on iron absorption in the presence and absence of ascorbic acid. Dephytinization did not increase iron absorption from high-tannin sorghum porridge reconstituted with water but increased iron absorption from low-tannin sorghum porridge by approximately 2-fold (P < 0.01). CONCLUSIONS: Phytate degradation improves iron absorption from cereal porridges prepared with water but not with milk, except from high-tannin sorghum.

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Iron nutrition is particularly important during the weaning period, when the infant is growing rapidly and has a high demand for iron. In developing countries, the intake of absorbable iron by infants is often low, and iron deficiency anemia is common. A major consequence is retarded psychomotor and mental development, with possible long-term negative effects on school performance (1). Cereal porridges are common complementary foods during the weaning period and often provide much of the dietary iron intake because the iron contribution from human milk is low.

Cereal porridges are based on common grains, such as rice, maize, wheat, oat, or sorghum. They are often combined with milk or with leguminous seeds, such as soy, to provide infants with both adequate protein and energy. In developing countries, the cereals or cereal legume mixtures are first cooked and then fed as a watery gruel. In industrialized countries, the cereals are precooked industrially and dried by roller-drying or extrusion and then reconstituted with milk, commercial infant formula, or water before consumption. Both cereal grains and legume seeds are rich in phytic acid (myo-inositol-6-phosphate), a compound that strongly inhibits the absorption of iron and other essential minerals (2, 3). Some sorghum varieties are also rich in phenolic compounds (4), which—like phytate—strongly inhibit iron absorption (5).

Because of the high phytate content of cereal porridges, iron absorption of native iron and fortification iron may be very low (6). Absorption can be increased by the addition of ascorbic acid (4, 6–8), by the addition of EDTA (9), and by the degradation or removal of phytic acid (10). Phytic acid is highest in whole-grain flours and can be decreased considerably by removing its hull (11). Iron absorption is still low, however, even from porridges made from low-extraction flours (6), because small amounts of phytate inhibit iron absorption (10). Phytic acid in cereal foods can be degraded completely by phytases, enzymes that successively remove the phosphate groups from phytic acid until it no longer binds iron. Phytic acid has been completely degraded in weaning cereals by adding commercial exogenous phytases (12) or by activating the native phytases by a combination of soaking, germinating, and fermenting (13).

In the current study, phytic acid was fully degraded in roller-dried complementary foods prepared from rice, wheat, maize, oat, sorghum, and a wheat-soy blend by adding an exogenous phytase. The iron-fortified cereal porridges were fed after reconstitution with water or milk (wheat porridge only). Ascorbic acid was added to some porridges. Iron absorption was measured in adult human subjects by using the dual-extrinsic-label radioiron technique.

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Mean iron absorption in nonanemic adult humans (with a wide range of iron stores) from precooked, roller-dried, cereal-based complementary foods reconstituted with water was very low, ranging from 0.33% to 1.80%, and was similar to what has been previously reported (6). When the phytic acid in the complementary foods was almost completely degraded by adding a commercial phytase during manufacture, iron absorption increased 2–12-fold (Figure 1). Dephytinization, with the use of the same commercial phytase, was previously reported to increase iron absorption from soy (5) and pea (25) infant formula fed to adults and from soy formula fed to infants (12).

There were large differences between grains in the magnitude of the improvement in iron absorption on dephytinization. Iron absorption from wheat porridge increased 12-fold, which was significantly higher than that from all of the other cereal porridges except oat. The 3–5-fold increases in absorption observed on dephytinization of the rice, wheat-soy, and maize porridges were not significantly different from each other. The 5–8-fold increases in absorption on dephytinization of maize and oat were also not significantly different from each other. Phytic acid degradation of low-tannin sorghum porridges increased iron absorption only 2-fold, and no improvement in iron absorption was observed on dephytinization of high-tannin sorghum (absorption ratio: 1.34). This latter finding was presumably due to the strong inhibitory effect of phenolic compounds (4, 5). The magnitude of the observed change in iron absorption on dephytinization could be related to the initial level of iron absorption from the native phytate–containing porridge and to differences in composition of the grains. Phenolic compounds, certain proteins, and calcium could inhibit iron absorption from cereal foods (26). Another finding of the present study was that the enhancing effect of dephytinization on iron absorption was greatly decreased or even completely removed by preparing the wheat porridges with milk instead of water (studies 3, 7, and 8). When the results from these studies were pooled (Figure 2), dephytinization of wheat-milk porridge in the absence of ascorbic acid resulted in a small nonsignificant 1.6-fold increase in iron absorption, whereas dephytinization of wheat-milk porridge with added ascorbic acid resulted in a small but significantly different (P < 0.005) 1.9-fold increase. When the 2 outliers (Figure 2) were omitted from the calculation, dephytinization of wheat-milk porridge significantly improved iron absorption both in the absence (1.9-fold; P < 0.005) and presence (1.7-fold; P < 0.005) of ascorbic acid. Nevertheless, in the presence of milk, the influence of dephytinization on iron absorption is at best modest. We previously reported that dephytinization of commercial infant cereals made from low-extraction wheat and milk and containing ascorbic acid did not improve iron absorption in human infants (12). The inhibitory effect of cow milk on iron absorption is thought to be mainly related to its high concentration of calcium (27) and casein (28).

Ascorbic acid is a well-known enhancer of iron absorption in the presence of phytic acid (6) and is usually added to commercial infant foods together with fortification iron to ensure adequate absorption (29). Ascorbic acid also enhances iron absorption from foods such as infant formulas containing no phytic acid. The magnitude of the response depends on the amount of ascorbic acid added and the food matrix (30). In our studies 8 and 9 (Table 2), 25 mg ascorbic acid further increased iron absorption from the phytate-free wheat-soy blend from 3.75% to 8.46% (P < 0.05) but not from the phytate-free wheat-milk porridge (1.47% compared with 2.25%; P > 0.05), perhaps because of the inhibitory nature of milk.

Although the current studies investigated the influence of dephytinization of complementary foods on iron absorption in adults, not infants, we previously showed that iron absorption in infants is inhibited by phytic acid in a way similar to iron absorption in adults (31). The current studies, however, were single-meal studies, which have been reported to overemphasize the influence of enhancers and inhibitors on iron absorption in comparison with multimeal studies (32); therefore, care should be taken in the interpretation of these results. Nevertheless, the findings of the current studies confirm the very low iron absorption from cereal porridges and indicate that phytate degradation would be a useful means for improving iron absorption from cereal-based complementary foods, provided that these foods are fed mixed with water, not milk. In industrialized countries, most dried infant cereals are prepared with cow milk or cow milk–based infant formula, and, with these products, the addition of ascorbic acid and not phytate degradation would be the best means of ensuring adequate iron absorption (29). Industrially manufactured weaning foods containing blends of cereals and pulses, however, would be expected to have a substantially improved iron bioavailability if the phytic acid were degraded.

The main advantage of dephytinization is seen in developing countries, where infant porridges are usually consumed with water and where infants have difficulty obtaining an adequate supply of absorbable iron. Phytic acid degradation, however, is not suited for home-prepared complementary foods and is best achieved on an industrial scale by adding commercial phytases (12, 29) or by activating native phytase (33) and then drying. Traditional food-processing methods will also activate cereal phytases, and soaking of pounded maize flour was reported to decrease the phytate content by 49% (34), whereas germinating and dehusking rice and mung beans reduced phytic acid by 92% (13) and a combination of soaking, germinating, and fermenting degraded phytic acid in sorghum completely (35).

One mole of phytic acid binds 6 mol ferric iron so that even relatively small quantities of residual phytate are still strongly inhibitory (10). Hallberg et al (36) found that adding 10 mg/100 g phytic acid to bread rolls decreased iron absorption by 20% and that adding 20 mg/100 g decreased iron absorption by 40%. More recently, Mendoza et al (37, 38) reported little or no improvement in iron absorption from maize meals prepared from maize that had been genetically modified to contain 30–50% less phytic acid.

In conclusion, the magnitude of iron absorption from cereal-based porridges depends on the contents of the different components that enhance or inhibit iron uptake. Phytic acid, polyphenolic compounds, and milk are the major inhibitors, whereas ascorbic acid enhances iron absorption. In the absence of milk and polyphenols, phytic acid degradation greatly improves iron absorption from cereal-based complementary foods and, in developing countries, dephytinization should be considered as a major strategy to improve iron nutrition during the weaning period.

Categories: 2003, Iron, Iron absorption, Grains, Legumes, Soy, Phytates, Tannins, Calcium, Vitamin C, Dairy, Nutrition and diet