Chapter 9
Enhanced Iron Absorption from Cereal and Legume Grains by Phytic Acid Degradation R. F. Hurrell Laboratory for Human Nutrition, Institute of Food Science, Swiss Federal Institute of Technology Zurich, CH-8803 Rüschlikon, Switzerland
Phytic acid in cereal and legume foods is a potent inhibitor of mineral absorption but can be degraded by activating endogenous phytases during processes such as soaking, blanching, germination and fermentation. Wheat and rye are rich sources of phytases and can be used to completely degrade phytic acid in cereal legume mixtures. Alternatively, commercially available phytases can be used. Iron absorption by human subjects fed a liquid formula meal containing soy isolate was increased 4-5 fold when phytic acid was reduced to zero. Even relatively small quantities of phytate however were strongly inhibitory. Iron absorption was similarly increased 4-10 fold when wheat, maize, rice and oats were dephytinized using a commercial phytase and fed to human subjects as porridge with water. Iron absorption studies in infants using stable isotopes confirmed the beneficial effects of phytate removal from soy formula but not from infant cereals fed with milk.
Phytic acid is a potent inhibitor of mineral absorption. In the gastrointestinal tract, it forms complexes with minerals and with the peptides released on digestion of food proteins. Minerals, such as iron, zinc and calcium,
© 2002 American Chemical Society
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118 are tightly bound in these complexes and are not completely released for absorption. The absorption of iron, zinc and calcium is thus lower from phytate-containing foods such as cereals and legumes (/). Low iron absorption from cereal and legume-based diets is an important factor in the etiology of iron deficiency in developing countries (2) and is particularly important in relation to infant foods, including those manufactured in industrial countries, since infants usually consume a limited number of food items. Young infants, with a rapidly expanding blood volume, have a high iron requirement and without a good supply of bioavailable iron from weaning foods rapidly become iron deficient. Iron deficiency anemia may significantly reduce a child's psychomotor and mental development (J), reduce immune status and lead to an increased susceptibility to infection (4,5). Cereal grains and legume products are often used to produce infant foods including weaning products and soy based infant formula. The level of phytic acid in both cereal grains and legume seeds is usually around 1%; in wheat, rice, maize, barley or oats, or in legume seeds such as soybean, lima bean, navy bean and peas (6). Milling and bran removal reduce the phytic acid considerably in cereal flours, by 50% in high extraction wheat, 80-90% in low extraction wheat or polished rice and even more in degermed maize. On the other hand with legume seeds, protein concentration or isolation usually maintains or increases the phytic acid level since the phytate is in the protein bodies of the endosperm. A soybean isolate or concentrate may have twice the level of phytic acid as soybean itself.
Phytic Acid and Iron Absorption Much of the evidence that phytic acid inhibits iron absorption in human subjects has come from the demonstration that degradation of phytic acid during food preparation enhances iron absorption. The degradation of phytic acid in wheat bran by endogenous phytases almost completely removed the inhibitory effect of wheat bran on iron absorption (7). Similarly phytic acid degradation by native phytases during bread fermentation and malting of oats increased iron absorption in adults (8,9) from these cereals, and using microbial phytases to degrade phytic acid in soy protein isolate increased iron absorption in adults fed soy formula meals (10) and infants fed phytate-free soy infant formula (11). Adding phytic acid (7-890 mg) to phytic acid-free wheat rolls inhibited iron absorption in adults in a dose-dependent way (12) as did adding increasing amounts of maize bran containing increasing amounts of phytic acid (35-205 mg) (13). When soy protein isolate was included as the protein component of a
119 liquid formula meal in adults, the phytic acid level of the isolate had to be reduced from ca. 1% to 0.03% (10 mg phytic acid/meal) so as to achieve a meaningful three fold increase of iron absorption (10). The molar ratio of phytic acid to iron had to be reduced from 2.2:1 to 0.1:1 in order to achieve an almost 5 fold increase in iron absorption.
Enhanced Iron Absorption from Cereal and legume-Based Infant Foods by Phytic Acid Degradation In our laboratory, we have made extensive studies on iron absorption from soy formula and infant cereals after phytic acid removal or degradation. Phytic acid was removed from soy isolates by acid/salt washing and ultrafiltration, and completely degraded by adding a phytase from Aspergillus niger (10). Phytic acid in infant cereal was similarly completely degraded by adding an exogenous phytase (14). Absorption from the infant foods, with different levels of phytic acid, was measured either in adults using radioisotopes or in infants using stable isotopes. The adult radioiron studies used the extrinsic tag technique with Fe or Fe and, in each study, 9-10 subjects were fed 4 different meals. The infant studies were made using the extrinsic tag technique with Fe and Fe stable isotopes. There were 10-12 infants per study and 2 different meals per infant. Fourteen days following the meal, iron absorption was calculated based on the incorporation of the isotopes into blood hemoglobin. The radioisotopes were quantified using scintillation counting and the stable isotopes by thermal ionization mass spectrometry. 59
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Soy Formulas Low-phytate soybean isolates were manufactured from soy flour by first water-extraction, then isoelectric coagulation of soy proteins by dilute acid. This was followed either by phytase treatment or dialysis after an acid/salt treatment. The product was finally sterilized and dried. In the first series of radioiron absorption studies (Figure l)(/0), the effect of phytate removal on iron absorption from soy protein isolates fed in a liquid meal with maltodextrin, corn oil and water was investigated. Iron absorption was measured relative to a formula containing an egg white control protein fed in the same subject. The egg white control meal had a mean iron absorption of between 5-9% and was designated a relative absorption of 100. Reducing the phytic acid in the soybean isolate from 1000 mg/100g to 400 mg/100 g had little or no influence on relative iron absorption, and only when it was degraded using the phytase to
120 Relative 100 iron absorption (EW=100) h 80
Iron absorption substantially improved only when > 95% phytate degraded
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0CASEW
Phytic acid (mg/100g)
Figure 1. Effect of Phytic Acid Removal/Degradation on Iron Absorption by AdultsfromSoy Protein Isolates fed in a Liquid Meal Formula (10) EW= egg white; CAS = casein.
very low levels (0-30 mg/100 g) did relative iron absorption increase from around 10-20% to around 50-60% of the egg white control, the same value as given by bovine casein. Soy protein, like casein, is itself slightly inhibitory to iron absorption (15). The message to the food manufacturer from this study is that iron absorption from soy protein isolates can only be substantially improved when >95% of the phytate is degraded. Removing or degrading 50% or 60% is not beneficial. In a second study, the effect of phytic acid degradation on iron absorption in adults from soy protein fractions fed in the same liquid formula meal was investigated (75). The major fractions of soy protein (each about 40% of the total) conglycinin (7S) and glycinin (1 IS) were extracted and dephytinized with phytase. Relative iron absorption from the formula containing the 7S protein was 32% and increased to 43% on dephytinization. Relative iron absorption from the formula with the 1 IS fraction was 21% and increased to 122% with dephytinization. The conglycinin fraction (7S) is therefore responsible for the inhibitory effect of soy protein on iron absorption, presumably due to inhibitory
121 peptides formed on digestion. Formulas based on phytate-free glycinin (1 IS) would be the least inhibitory to iron absorption. The soy isolates were next incorporated into an infant formula of similar composition to commercial infant formulas. First the effect of phytic acid removal on iron absorption in adults using the extrinsic tag radioactive isotope technique with Fe and Fe (16) was measured, making paired comparisons in the same subject. Iron absorption was 2.4% from a small meal of 217 g of formula containing 76 mg of phytic acid. When phytic acid was removed, this increased significantly to 6.0%. The same formulas were fed to infants and iron absorption was measured using stable isotopes (11). When phytate was 75% degraded iron absorption increased from 5.5 to 6.8% whereas, complete removal increased absorption from 3.9 to 8.7% (similar to the adult values). In both studies the differences were statistically significant (p
