Consumption of iron with a mixed meal creates a pool of iron available for absorption. The rate and total capacity of the gut to absorb this iron depends on a number of factors. Haem iron from animal tissues is well absorbed. However, the non-haem iron from plant tissues is poorly absorbed. The relative absorption of plant and animals sources of iron relates to the insoluble Fe3+ iron found in plant tissue compared to the more soluble Fe2+ iron in animals tissue. A number of enhancers and inhibitors of iron are known, and their presence within a mixed meal will also therefore have an influence on the absorption of iron. Inhibitors of iron may include tea and coffee, dietary fibre and phytate. Generally substances that inhibit iron absorption are able to bind with it, and in the process create insoluble complexes. Enhancers of iron absorption can be chelating agents such as EDTA and vitamin C that unbind iron from tissues, or reducing agents such as cysteine and vitamin C that maintain the iron in its more soluble Fe2+ form.
Whole wheat foods contain the bran layer from the grain. Wheat bran can also be taken as a supplement. Wheat bran may impede the absorption of iron significantly in humans and researchers are therefore interested in its effects when present with a mixed diet containing iron. For example, when 12 grams of wheat bran fibre were added to muffins and milk consumed by healthy subjects, iron absorption from the muffins fell from 2.39 % to 0.62 %. When milk and beef was given in combination with the muffins iron absorption was 3.46 %, but this fell to 1.69 % with addition of 12 grams of wheat bran. However, addition of 100 mg of ascorbic acid (vitamin C) increased iron absorption from muffins and milk to 9.29 % but this fell to 3.89 % with addition of 12 grams of wheat bran fibre. Therefore the addition of vitamin C and beef to a meal improves the absorption of iron, supporting previous studies showing that animals tissue and reducing agents are effective enhancers of non-haem iron absorption in man,
The researchers were interested if it might be the phytate component of the fibre that was causing inhibition of the iron. To test this they measured the absorption of iron from monoferric phytate (iron bound to phytic acid) and from ferrous chloride (FeCl3). However there was no statistical difference in the absorption of iron from these two sources. To confirm these findings the authors then used the enzyme phytase to remove the phytate from wheat bran fibre and compared it with the absorption from unhydrolysed wheat bran fibre with its phytate content intact. However the iron absorption was similar from both test meals indicating that phytate was not the cause of the inhibitory effects on iron absorption. Lastly the researchers separated the soluble and insoluble fractions of fibre from wheat bran and investigated their effects on iron absorption. Results from this experiment showed that the soluble fraction of fibre was more inhibitory than the insoluble fraction in terms of iron absorption.
RdB
