Iron is a trace mineral required as a cofactor in the haemoglobin molecule. In this role iron is an important contributor to the oxygen carrying capacity of blood. However, as well as this well known role in oxygen transport, iron is also required for a number of other functions including the synthesis of the catecholamine neurotransmitters (adrenaline, noradrenaline and dopamine) because it acts as a cofactor in the rate limiting tyrosine hydroxylase enzyme. However, despite other roles, 80 % of the absorbed iron in a healthy individual is used in the formation of haemoglobin, which can therefore be defined as iron’s primary role in human nutrition. Anaemia can develop from a lack of dietary iron and this is thought to affect 25 % of the World’s population. The amount of iron absorbed by an individual is dependent on a number of variables including the type of iron in the food, the quantity of iron, the iron status of the individual, as well as modulating agents in the diet. These factors combine to make predictions of iron absorption very difficult.
Dietary iron can be derived from animal or plant sources. Haem iron is the form of iron found in haemoglobin and myoglobin, and is therefore present in animals tissue. In contrast, non-haem iron is the form of storage iron found in plants. Haem iron is considerably better absorbed than non-haem iron, and so increasing the amount of red meat in the diet can increase iron absorption substantially. The absorption of non-haem iron can also be improved by consumption of certain enhancers, the most well known being vitamin C. Vitamin C is a powerful reducing agent and it keeps dietary non-haem iron in a reduced state which is known to improve absorption. However, concomitant consumption of meat, fish and alcohol with non-haem iron will also enhance its absorption. Certain dietary constituents are able to inhibit the absorption of iron and these can include tannins in tea, certain micronutrients such as zinc, calcium and copper, polyphenols in fruits and phytate from cereal.
Many studies have investigated the absorption rates of iron in different populations eating different diets. In a recent meta analysis published in the American Journal of Clinical Nutrition1, data was collated to investigate iron absorption and the factors that affect it. The authors used a Medline search to identify relevant studies and then analysed the data from all of the studies they identified as fulfilling their criteria. The result showed that the presence of an enhancer improved iron absorption significantly, but that the presence of inhibitors reduced iron absorption. However, single inhibitors did not significantly reduce iron absorption in isolation. Only the presence of multiple inhibitors created low iron bioavailability diets. The authors then used multiple studies to predict an iron absorption range for a particular blood ferritin level. Those subjects with a plasma ferritin level of between 6 to 80 μg/L (low in the normal reference range) had iron absorption that ranged from 2.1 to 23 %.
One problem that is apparent from the literature is that many bioavailability studies are often performed using a single meal approach. This method tends to overestimate the amount of absorption of a particular nutrient. A large scale analysis using multiple studies might therefore be a more accurate way of assessing the overall absorption of iron under different conditions. In developing nations those who consume low amount of read meat and rely on vegetables as a source of iron may be particularly susceptible to iron deficiency. Female athletes are at particular risk of iron deficiency in developed countries. It is also worth pointing out that although many in the World do not obtain enough iron, those eating high intakes of red meat and who are male may actually be ingesting too much iron, and this may increase the susceptibility of cells and tissues to undergo oxidative damage. Lowering red meat intake is one solution to this, but donating blood is also a good way to maintain correct iron stores in those with high intakes.
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