Introduction
The human body needs iron for many vital functions. It is also the trace element which is present in the highest concentration in the human body. Iron deficiency is a widespread problem.
Tasks and function
The human body has an iron content of 3-5g. The daily iron requirement is about 12-15mg. Only a part of the iron supplied by food is absorbed in the gastrointestinal tract and made available to the organism.
Iron is present either as an ion with two positive (Fe2+) or three positive (Fe3+) charges. Only Fe2+ can be absorbed by the intestinal cells. Therefore, the simultaneous absorption of vitamin C, which converts iron into the doubly charged form, leads to better absorption.
Furthermore, iron can be absorbed very well in heme-bound form. Hem is a molecule that binds the iron in many proteins – such as the red blood pigment, hemoglobin. Therefore, animal iron, which is present in high amounts in this form, is well absorbed.
Once the iron is absorbed into the intestinal cell, there are two possibilities: the iron can either be released into the blood via transporters and fed into the circulation. If there is already a high concentration of iron in the blood, these transporters become less active and instead the iron accumulates more in stores in the cells (the ferritin). As the life span of the intestinal cells is limited, the iron stored in them is excreted in the stool as these cells are regularly scoured off.
Many cells release increased amounts of ferritin into the blood in a state of high iron reserves. For this reason, the ferritin level can be considered a rough measure of the iron content in the body. In the blood, iron binds to the iron transport protein transferrin.
Since unbound iron is harmful to kidney and liver cells, for example, Transferrin must always be present in the healthy body to bind the iron so that it is not free in the body. Normally about 15-45% of the iron binding sites of transferrin are occupied (transferrin saturation). This value can be used to determine the current iron requirement of the body.
Due to the high capacity of the transferrin, even larger amounts of released iron can be intercepted without fear of damage from free iron. A person loses about 1-2mg iron per day. This is mainly due to dying skin and intestinal cells.
Bleeding (and thus menstruation) has a major influence on iron loss, since with every milliliter of blood about 0.5mg of iron is lost. This is a possible explanation for the fact that it is predominantly women who suffer from iron deficiency. Apart from normal cell death, the body has no means of excreting iron.
Therefore, it is essential that iron absorption is strictly regulated. One mechanism to reduce absorption is the secretion of the protein hepcidin by the liver. Hepcidin binds to iron transporters in the intestine and leads to their degradation. A disease in which this mechanism no longer functions, hereditary hemochromatosis, leads to severe iron overload of the liver and, if left untreated, to liver failure.
