Iron

BACKGROUND

Iron is a component of a number of proteins including haemoglobin, myoglobin, cytochromes and enzymes involved in redox reactions. Haemoglobin is important for transport of oxygen to tissues throughout the body. Iron can exist in a range of oxidation states. The interconversion of these various oxidation states allows iron to bind reversibly to ligands such as oxygen, nitrogen and sulphur atoms. Almost two thirds of the body's iron is found in haemoglobin in circulating erythrocytes. About a quarter of the body's iron is found in readily metabolised stores as ferritin or haemosiderin in the liver and reticulo-endothelial system. The remaining iron is in the myoglobin of muscle tissue and a variety of enzymes necessary for oxidative metabolism and other cell functions.

The iron content of the body is highly conserved (Bothwell et al 1979). To achieve iron balance, adult men need to absorb about 1 mg/day and adult menstruating women about 1.5 mg/day, although this is highly variable. Towards the end of pregnancy, the absorption of 4–5 mg/day is necessary. Requirements are higher during periods of rapid growth in early childhood and adolescence.

Inadequate iron intake can lead to varying degrees of deficiency, from low iron stores (as indicated by low serum ferritin and a decrease in iron-binding capacity); to early iron deficiency (decreased serum transferrin saturation; increased erythrocyte protoporphyrin concentration and increased serum transferrin receptor) to iron-deficiency anaemia (low haemoglobin and haematocrit as well as reduced mean corpuscular haemoglobin and volume). These biochemical measures are used as the key indicators in setting the iron requirements.

Wholegrain cereals, meats, fish and poultry are the major contributors to iron intake in Australia and New Zealand, but the iron from plant sources is less bioavailable. The form in which iron is consumed will affect dietary intake requirements as not all dietary iron is equally available to the body. The factors that determine the proportion of iron absorbed from food are complex. They include the iron status of an individual, as well as the iron content and composition of a meal. Normal absorption may vary from 50% in breast milk to 10% or less in infant cereals. Iron in foods can come in two general forms – as haem or non-haem iron. Iron from animal food sources such as meat, fish and poultry may be either haem or non-haem whereas the iron in plant sources such grains and vegetables is non-haem. The haem form is more bioavailable to humans than the non-haem.

The presence of other nutrients such as vitamin C and organic acids such as citric, lactic or malic acid can increase the absorption of non-haem iron. Consumption of meat, fish and poultry can also increase non-haem iron absorption from plant foods consumed at the same time. In contrast, some other components of the food supply such as calcium, zinc or phytates (found in legumes, rice and other grains) can inhibit the absorption of both haem and non-haem iron, and polyphenols and vegetable protein can inhibit absorption of non-haem iron. High iron intakes can, in turn, affect the absorption of other nutrients such as zinc or calcium.

Functional indicators of iron deficiency may include reduced physical work capacity, delayed psychomotor development in infants, impaired cognitive function, impaired immunity and adverse pregnancy outcomes. However, as these are difficult to relate directly to a specific dietary intake, biochemical indices are generally used in estimating dietary requirements.

The distribution of iron requirements is skewed to the right and it is difficult to achieve a steady state with iron because it is highly conserved in the body. For these reasons, factorial modelling rather than the classical balance study method is used to determine the average requirements for the various age, gender and physiological states. This factorial modelling proposes daily physiological requirement for absorbed iron based on estimates of basal losses (obligatory losses through faeces, urine, sweat and exfoliation of skin) and, where relevant, menstrual losses and needs for iron accretion in periods of growth such as childhood, adolescence or pregnancy (FNB:IOM 2001). These accretion needs are estimated from known changes in blood volume, fetal and placental iron concentration and increases in total body erythrocyte mass. The EARs are based on the need to maintain a normal, functional iron concentration, but only a small store (serum ferritin concentration of 15 µg/L).