The effect of reducing the phytate in soy-protein isolates on nonheme-iron absorption was examined in 32 human subjects. Iron absorption was measured by using an extrinsic radioiron label in liquid-formula meals containing hydrolyzed corn starch, corn oil, and either egg white or one of a series of soy-protein isolates with different phytate contents. Iron absorption increased four- to fivefold when phytic acid was reduced from its native amount of 4.9-8.4 to less than 0.01 mg/g of isolate. Even relatively small quantities of residual phytate were strongly inhibitory and phytic acid had to be reduced to less than 0.3 mg/g of isolate (corresponding to less than 10 mg phytic acid/meal) before a meaningful increase in iron absorption was observed. However, even after removal of virtually all the phytic acid, iron absorption from the soy-protein meal was still only half that of the egg white control. It is concluded that phytic acid is a major inhibitory factor of iron absorption in soy-protein isolates but that other factors contribute to the poor bioavailability of iron from these products.

The effects of different polyphenol-containing beverages on Fe absorption from a bread meal were estimated in adult human subjects from the erythrocyte incorporation of radio-Fe. The test beverages contained different polyphenol structures and were rich in either phenolic acids (chlorogenic acid in coffee), monomeric flavonoids (herb teas, camomile (Matricaria recutita L.)), vervain (Verbena officinalis L.), lime flower (Tilia cordata Mill.), pennyroyal (Mentha pulegium L.) and peppermint (Mentha piperita L.), or complex polyphenol polymerization products (black tea and cocoa). All beverages were potent inhibitors of Fe absorption and reduced absorption in a dose-dependent fashion depending on the content of total polyphenols. Compared with a water control meal, beverages containing 20–50 mg total polyphenols/serving reduced Fe absorption from the bread meal by 50–70 %, whereas beverages containing 100–400 mg total polyphenols/serving reduced Fe absorption by 60–90 %. Inhibition by black tea was 79–94 %, peppermint tea 84 %, pennyroyal 73 %, cocoa 71 %, vervain 59 %, lime flower 52 % and camomile 47 %. At an identical concentration of total polyphenols, black tea was more inhibitory than cocoa, and more inhibitory than herb teas camomile, vervain, lime flower and pennyroyal, but was of equal inhibition to peppermint tea. Adding milk to coffee and tea had little or no influence on their inhibitory nature. Our findings demonstrate that herb teas, as well as black tea, coffee and cocoa can be potent inhibitors of Fe absorption. This property should be considered when giving dietary advice in relation to Fe nutrition.

Phytate degradation improves iron absorption from cereal porridges prepared with water but not with milk, except from high-tannin sorghum.

For the 25 meals evaluated, only the contents of animal tissue, phytic acid, and ascorbic acid were useful for estimating nonheme-iron absorption.

The facilitating effect of vitamin C on iron absorption from a complete diet is far less pronounced than that from single meals. These findings may explain why several prior studies did not show a significant effect on iron status of prolonged supplementation with vitamin C.

Fe absorption was measured in adult human subjects consuming different cereal foods fortified with radiolabelled FeSO4, ferrous fumarate or NaFeEDTA, or with radiolabelled FeSO4or ferric pyrophosphate in combination with different concentrations of Na2EDTA. Mean Fe absorption from wheat, wheat–soyabean and quinoa (Chenopodium quinoa) infant cereals fortified with FeSO4or ferrous fumarate ranged from 0·6 to 2·2 %. For each infant cereal, mean Fe absorption from ferrous fumarate was similar to that from FeSO4(absorption ratio 0·91–1·28). Mean Fe absorption from FeSO4-fortified bread rolls was 1·0 % when made from high-extraction wheat flour and 5·7 % when made from low-extraction wheat flour. Fe absorption from infant cereals and bread rolls fortified with NaFeEDTA was 1·9–3·9 times greater than when the same product was fortified with FeSO4. Both high phytate content and consumption of tea decreased Fe absorption from the NaFeEDTA-fortified rolls. When Na2EDTA up to a 1:1 molar ratio (EDTA:Fe) was added to FeSO4-fortified wheat cereal and wheat–soyabean cereal mean Fe absorption from the wheat cereal increased from 1·0 % to a maximum of 5·7 % at a molar ratio of 0·67:1, and from the wheat–soyabean cereal from 0·7 % to a maximum of 2·9 % at a molar ratio of 1:1. Adding Na2EDTA to ferric pyrophosphate-fortified wheat cereal did not significantly increase absorption (P>0·05). We conclude that Fe absorption is higher from cereal foods fortified with NaFeEDTA than when fortified with FeSO4or ferrous fumarate, and that Na2EDTA can be added to cereal foods to enhance absorption of soluble Fe-fortification compounds such as FeSO4.

Prior investigations have shown that rats are less sensitive than humans to dietary factors that influence the absorption of nonheme iron. This investigation was undertaken to determine whether this disparity is due to differences in the methods used to measure absorption in the two species. By use of identical methodology and test meals, absorption studies were performed in rats and humans to compare the effect of known dietary enhancers (ascorbic acid and meat) and inhibitors (tea, bran, and soy protein) on nonheme-iron absorption. Meat and tea had a marked effect on absorption in humans but did not influence absorption in rats. Although the effect of ascorbic acid, soy protein, and bran on absorption was statistically significant in rats, the absorptive response was far less than it was in humans. Our studies indicate that rodents cannot be used to assess the quantitative importance of dietary factors in human iron nutrition.

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