Iron is interesting nutritionally for a number of reasons. Firstly, iron is the most prevalent worldwide nutritional deficiency, and even the well fed Western populations have a high prevalence of iron deficiency and iron insufficiency. Secondly, iron is available in the diet in a number of forms, designated harm and non-hem, being derived from animal and plant sources, respectively. The evidence from the nutritional literature suggests that plant sources of iron are not so well absorbed because they are in an oxidation state that makes them likely to leave solution at the high pH of the small intestine and this limits the ability of non-haem iron to interact with the transporters on enterocytes required for its absorption. Most dietary supplements are non-haem iron and as a result iron supplements are non efficient at raising tissue stores of iron, when compared to red meat. Studies investigating the effects of iron however have not all agreed, and some have suggested that non-haem iron is as well absorbed as haem iron.
For example, in one study1 the effects of iron absorption were investigated in healthy humans following administration of a test meal of hamburgers or cereal. The hamburger meal was chosen because of its high haem iron content (7.5 mg) and the cereal meal was supplemented with hydrogen reduced metallic iron or ferrous sulfate to provide a non-haem iron content that varied between 1.4, 5.3 and 4.6 mg. Iron was recovered in the chyme of the gut at a lower level to the amount in the initial food, suggesting that it had been absorbed to some extent in each case. In contrast, zinc was detected in the chyme of the gut in higher concentrations than was present in the original meals, suggesting that a certain amount of zinc was secreted into the lumen of the gut during the digestive process. This may be due to the presence of zinc ions present as cofactors in a number of digestive enzymes. It is likely this secreted zinc would be reabsorbed along with digested food components and other nutrients.
The bioavailability of the iron in the two meals, based on their luminal disappearance, was similar between the organic haem and inorganic non-haem forms of iron. In addition the non-haem iron in the form of ferrous sulphate or hydrogen reduced metallic iron did not differ in their disappearance rates from the chyme of the gut. Analysis of the data suggested that iron disappearance was centred on the duodenum of the small intestine, but zinc disappearance was centred further down the gut in more distal regions such as the ileum. This study was different methodologically from many other because it measured the disappearance of the iron from the gut, rather than its excretion or appearance in tissues. This may explain the similarities in the apparent absorption of the haem and non-haem iron, something that other studies, mainly using different methodologies, have not seen. These results are certainly interesting, and suggest that certain types of non-haem iron may be well absorbed under certain circumstances.
RdB
