Iron is available in two dietary forms. Haem iron is present in red meat as it comprises part of the haemoglobin and myoglobin molecules and is therefore present in tissues containing blood. Non-haem iron is a storage form or iron found in plants. The absorption of haem iron is much higher than that of non-haem iron and so those that rely on non-haem sources of iron are at a much greater risk of developing iron deficiencies. A number of factors are known to promote the absorption of non-haem iron including the presence of reducing agents such as vitamin C that maintains the iron in its reduced state, and the presence of protein, which may form peptide complexes with the iron and these may them undergo preferential transport across the enterocytes. However a number of inhibitors of non-haem iron are also known including tea and coffee. Generally studies have shown that tea is a more potent inhibitor on non-haem iron compared to coffee, but some studies have shown significant inhibition from coffee.
For example, in one study1 the effects of coffee consumption on iron absorption before, during and after a meal containing a measured amount of iron were observed. The presence of coffee with the meal reduced the iron absorption from the hamburger style meal by 39 %, compared to 64 % by the presence of tea. In another experiment in the same study, the authors fed filter coffee with a meal of egg and corn ingredients and the reductions in iron absorption were between 1.64 to 5.88 %. Switching to instant coffee dropped the absorption of iron from a test meal to 0.97 % and doubling the strength of the instant coffee decreased iron absorption further to 0.53 %. Consuming coffee one hour before a meal did not affect iron absorption, but when the coffee was consumed one hour after a meal, the absorption of iron was similar to that seen when the coffee was drunk with the meal. Adding iron supplements to a cup of coffee diminished their absorption when compared to consuming the iron supplements without coffee.
Therefore the absorption of non-heam iron appear to be inhibited by coffee, although perhaps not to the same degree as with tea. Coffee does not inhibit iron absorption if consumed before a meal, presumably because the coffee has been digested and absorbed before the meal is present in the gastrointestinal tract. Consuming the coffee one hour after a meal inhibits absorption because the meal has not been fully digested by the time the coffee comes into contact with it in the upper gut. Tea may inhibit iron absorption because it forms insoluble iron tannin complexes, but coffee may affect absorption by a mechanisms different to that of tea. For example, experiments have shown that while tea can reduce the solubility of iron by 12 %, addition of coffee does not affect iron solubility. More likely the coffee oxidises the non-haem iron to its ferric state, a form that has diminished absorption compared to the reduced ferric state. All the iron in the tea and coffee meals was in the oxidised ferric state.
Dr Robert Barrington’s Nutritional Recommendation: Those with borderline iron status should take their iron containing foods or supplements away from meals containing tea or coffee, or should allow for a reduction in absorption by increasing the intake if tea and coffee is consumed concomitantly. Because one of the ways that tea and coffee may affect non-haem iron absorption is through oxidation to a ferric state, addition of vitamin C and other reducing agents with a meal may improve non-haem iron absorption in the presence of tea and coffee.
RdB
