The common bean (Phaseolus vulgaris) is relatively rich in protein, starch, dietary fibre, as well as the minerals iron, selenium and molybdenum and the vitamins folic acid, thiamine and vitamin B6. The iron content of the common bean is particularly high at around 3.5 to 9 mg per 100g, which is consistent despite varied growing conditions. However, the human bioavailability of the iron in common beans is only around 1 to 3%, suggesting that other nutritive factors within the bean are able to hinder absorption. Polyphenols and phytic acid are known to prevent the absorption of certain minerals, and because beans are high in both phytic acid and polyphenols (in the form of polymerised flavans), it is possible they both contribute to the poor bioavailability of iron. This is because phytic acid and polyphenols form insoluble complexes with minerals in the lumen of the gut.
To test the effects of phytic acid and polyphenols on the absorption of iron from beans, researchers1 fed healthy women stable isotopes of iron with a number of bean test meals. Some meals had the phytic acid removed, some meals had the polyphenols removed, and some meals had both removed. One meal added 20, 50 or 200 mg bean polyphenols to whole bean bread. Iron absorption from whole bean porridge was 2.5%, but polyphenol and phytic acid removal increased absorption 2.6 fold. Removal of polyphenols doubled the absorption from dephytinised porridge. Iron absorption was lowered by 14% and 45% by the addition of 50 and 200 mg of polyphenols to bean bread, respectively. These results show that when polyphenols are present, removal of the phytic acid does not further increase absorption. However, when polyphenols are not present, removing the phytic acid can increase absorption by 3.5%.
These results support findings in previous studies that show that dietary phytic acid and polyphenols can alter the absorption of nutrients. Phytic acid has been investigated extensively for its mineral binding qualities in humans. For example, research published in the Journal of Nutrition in 20092, demonstrated that enzymatic dephytinisation of maize porridge increased zinc absorption in Nigerian children with rickets. However, calcium absorption was increased in the presence of maize porridge, but absorption did not increase further with the removal of the phytic acid. Less research is available on the mineral binding absorption of polyphenols, but this study suggests that the effect of polyphenols in beans is to reduce the absorption of iron. Whether this would be the case for other minerals is not know. Therefore, although the mineral content of some foods might be high, there is no guarantee that absorption will occur.
RdB
