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Inhibition of Iron Absorption
Recent research for direct measurement of the average bio-availability of iron in different Western-type diets found that absorption of iron through diet can be inhibited to some degree by a number of chemical micro elements common in food. Below there is a list of some of main inhibitors which can contribute to development of iron deficiency.
In North American and European diets, about 90 percent of phytates ( inositol hexaphosphate salts) originate from cereals. Phytates have been shown to strongly inhibit iron absorption in a dose-dependent fashion and even small amounts of phytates have a marked effect . Chemically, phytates are found in all kinds of seeds, grains, nuts, vegetables, roots (e.g., potatoes), and fruits:
- Bran has a high content of phytate and strongly inhibits iron absorption.
- Whole-wheat flour has a much higher content of phytates than does white wheat flour.
- Oats strongly inhibit iron absorption.
Almost all plants contain phenolic compounds as part of their defence system against insects, animals and humans. Some of the phenolic compounds (mainly those containing galloyl groups) are responsible for the inhibition of iron absorption:
- Tea, coffee, and cocoa are common plant products that contain iron-binding polyphenols.
- Many vegetables, especially green leafy vegetables (e.g., spinach), and herbs and spices (e.g., oregano) contain compounds that strongly inhibit iron absorption.
Calcium, consumed as a salt or in dairy products interferes significantly with the absorption of both heme and non-heme iron. However, because calcium and iron are both essential nutrients, calcium cannot be considered to be an inhibitor in the same way as phytates or phenolic compounds. For example, no inhibition is seen from the first 40 mg of calcium in a meal. A sigmoid relationship is then seen, reaching a 60 percent maximal inhibition of iron absorption by 300-600 mg calcium. The practical solution for this competition is to:
- Increase iron intake
- Increase its bio-availability
- Avoid the intake of foods rich in calcium and foods rich in iron at the same meal.
For unknown reasons, the addition of soy protein to a meal reduces the fraction of iron absorbed . This inhibition is not solely explained by the high phytate content of soy protein. However, because of the high iron content of soy proteins, the net effect on iron absorption of an addition of soy products to a meal is usually positive. In infant foods containing soy proteins, the inhibiting effect can be overcome by the addition of sufficient amounts of ascorbic acid (Vitamin C). Some fermented soy sauces, however, have been found to enhance iron absorption.