Iron is an essential trace mineral required for the transport of oxygen in blood as well as acting as a cofactor to a number of enzymes, most notably tyrosine hydroxylase. Dietary iron originates from two main sources in man. Haem iron is the form found in animal tissue and this iron is derived mainly in the haemoglobin and myoglobin molecules in the tissues of the animals. Haem iron is very bioavailable and readily absorbed. Haem iron is bioavailable because it is in the Fe2+ state, a factor which solubilises the iron in the alkaline environment of the small intestine and increases the probability of transport across the endothelial cells of the gut. Non-haem iron is found in plant tissues and includes storage forms of the mineral that have been precipitated by the plant. Such iron stores generally are in the oxidised Fe3+ form of the mineral and this decreases the solubility of the iron and thus inhibits its absorption. In addition, non-haem iron can also be bound tightly to plant tissues which makes the bioavailability of such iron low.
Because most of the population of the World relies on plants for their protein intake, non-haem iron is the main source of iron for humans globally. In addition, iron deficient anaemia is the most prevalent deficiency disease in man. Therefore researchers are interested in the factors that can improve the bioavailability of non-haem iron from plants sources. Much of this sort of research has used in vitro models of the gut in order to chemically assess the changes to the oxidation state of the iron. In one such experiment, researchers tested the solubility of non-haem iron from pinto beans1, with a number of different agents and foods. Assessment of the beans showed that the iron was found in three main sources within the beans. Some iron was spontaneously solubilised in the gut. This is the fraction that would have likely been absorbed. Another fraction of iron could be solubilised with the addition of chelating agents. A third fraction was not able to be solubilised because it was firmly bound to insoluble plant tissues.
The fractions within beans that were spontaneous soluble, solubilised through chelation and non-soluble represented 25. 45 and 30 % of the total iron content of the beans at consecutive incubations at pH 2 and pH 6. The researchers reported that chelating agents ascorbic acid (vitamin C) and citric acid (a component of citrus fruit) were able maximally solubilise the non-haem iron from the pinto beans (excluding the tightly bound insoluble fraction). The combination of citric acid and ascorbic acid lead to 70 % of the iron becoming solubilised. Orange juice was also able to maximally solubilise the iron content of the pinto beans. However, in contrast, tea was able to decrease the solubilisation of iron, presumably because the tannins in tea bound the free iron. Ascorbic acid could only solubilise 3 % of the iron in spinach, and addition of spinach to the pinto beans lowered the soluble fraction of iron available for absorption by removing the iron from solution.
Dr Robert Barrington’s Nutritional Recommendation: These results suggest that orange juice, citric acid and ascorbic acid are beneficial at solubilising the non-haem iron from plants at physiologically relevant pHs. Therefore consuming orange or lemon juice with foods containing non-haem iron may increase the absorption of the iron in the foods through increasing its solubility. Taking a vitamin C tablet with non-haem iron foods would have a similar effect. Tea appears able to lower the amount of soluble iron in the gut from non-haem sources and this may relate to the presence of tannins in the tea. Tannins can bind to minerals and this takes them out of solution and unavailable for absorption. Spinach also appears to be able to bind minerals and thus reduce the solubilised fraction available for absorption. Therefore eating spinach and tea with non-haem sources of iron might be detrimental particularly in those with a borderline or low iron status. Such people should rely on haem iron as the main dietary source of iron.