Iron absorption is highly complex. Generally iron from meat, termed haem iron, is more readily absorbed than iron from plants, termed non-haem iron. The reasons for this relates largely to the chemical form of the iron in the different sources, In meat the iron tends to be in a reduced Fe2+ state, and this increases the solubility of the iron in the alkaline environment of the small intestine, facilitating its absorption. This Fe2+ state may be maintained by the presence of cysteine residues in the protein that act as reducing agents. However, the Fe3+ iron in plant foods is much less soluble and tends to form iron hydroxides in the small intestine which decreases solubility and increases their excretion in the faeces. In addition, plants contain inhibitors of iron, such as phytate, that may bind to the iron and decrease its solubility further. The iron status of the individual can also dictate iron absorption rates. Those deficient in iron tend to absorb high percentages of consumed iron, as transport processes become more efficient iron status declines.
Relating the iron absorption to the nutrient density of the foods, the type of iron and the iron status of the individual is therefore the most effective way to predict iron absorption. In fact researchers have suggested this concept for many decades. For example, one study suggested that the bioavailability of iron should be defined as the the amount of iron in milligrams absorbed per unit energy in subjects who are borderline in iron status1. The energy unit they suggested was 1000 kcal. The authors also defined those with borderline iron status as individuals with depleted iron stores but without iron-deficient anaemia. Use of such criteria is more useful than the traditional use of recommended intakes based on a single nutrient dose because it incorporates factors that affect iron absorption. The reason that such recommendations are not common place is likely a result of the ease of use of a single recommended intake of iron and the difficulty in implementing variable nutrient intakes.
Iron deficiency is currently the most common nutritional deficiency Worldwide, and despite access to large quantities of food, the developed nations of the West still have high prevalence of iron deficiency. Generally developed nations have populations with a poor mineral status because the typical Western diet is low in minerals. However, the high prevalence of iron deficiency is difficult to explain because the typical Western diet is high in red meat and red meat is an excellent source of iron. Closer examination of the figure shows that the prevalence of iron deficiency and borderline iron status is skewed to particular subgroups of the population. In particular females and the elderly are at high risk of being iron deficient. This relates largely to the loss of blood through the menstrual cycle of females and to the low nutrient absorption rates and poor nutrient intakes of the elderly. More specific criteria for iron intakes may be a more effective strategy to help these at risk groups.
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