Iron deficiency is the most common case of deficiency (lack, scarcity) of a chemical element, worldwide. It is economically significant because it reduces the ability of individuals to perform physical work and decreases both growth and learning in children.
Iron is found in plant and animal foods. A good diet contains 10-20 mg iron/day (daily physiological dose). In the USA, the diet contains 6 mg iron per 1000 kcal.
The iron found in animal foods (dietary iron) is mainly in the form of heme iron (iron in the composition of heme). The iron found in plant foods (dietary iron) and mineral supplements and a part of it in animal foods is in the form of nonheme iron (thus not in the composition of heme) or inorganic iron (iron of ferritin, hemosiderin, and iron-containing enzymes of these cells) and is mainly in the ferric form (Fe 3+).
Both forms are absorbed, without competing with each other, by the mucosal cells (enterocytes) of the duodenum and jejunum. Many factors that alter the absorption of non-heme iron have a small effect on the absorption of heme iron due to their chemical structure. Although non-heme iron from plant and animal foods constitutes 2/3 of all dietary iron, it is less absorbed and provides only 1/3 of the iron that enters the body from the diet.
In North America and Europe, only 1/3 of dietary iron (food iron) is heme iron (iron obtained through diet in the form of hemoglobin and myoglobin found in red meat, hemoglobin that remains in the numerous blood vessels found in the liver, spleen, but it should be noted that these are consumed less frequently than meat. Thus, red meat with Hb/Mb provides only 1/3 of the iron obtained from food, but it provides the largest amount of iron absorbed into the body (2/3 of all iron that enters the body).
Aside from the liver, which is richest in iron (but as we emphasized not massively consumed), red meat, spleen, and eggs, fish, cereals, beans, peas, beetroot, red wine, etc.
Spinach is very rich in iron, but only 5% of it can be absorbed. This is because spinach is also very rich in oxalates that bind with iron and eliminate it through excretion. Similarly, oxalate binds with calcium (which is found in abundance in spinach) making it also non-absorbable. This explains why when consuming spinach, feces will be dark in color. It is very rich in Vit. A, C, E, K, Folic Acid, Magnesium, and some important antioxidants. Thus, the iron in spinach is almost non-absorbable for the human body.
When foods are processed in pots, a quantity of iron is added from the pot.
The Recommended Dietary Allowance (RDA), for adolescents and adult females, which is 18 mg/day. RDA, the total amount of iron that should be found in food to provide the physiological dose of absorbed iron, varies based on age, sex, the dietary source of iron (heme iron is much better absorbed than iron in non-heme protein composition).
Children may require iron supplements if they are not breastfed. But a breastfeeding mother provides iron to the child when she has iron in her body. Blood donors, pregnant women are often at special risk for low iron levels and are often advised to take iron supplements.
The daily physiological needs for iron (the iron that needs to be absorbed and that under normal conditions is equal to the daily lost amount) in men are: 13 µg/kg/day or 1 mg/day. The daily physiological needs for iron in women are: 21 µg/kg/day or 1.4 mg/day. The daily physiological needs for iron in pregnant women in the last 6 months are: 80 µg/kg/day or 5-6 mg/day. The daily physiological needs for iron in children are: 22 µg/kg/day or 1.4-1.5 mg/day..
The normal homeostasis of iron is such that the amount of absorbed iron is equal to the amount of eliminated iron. Iron metabolism occurs in a virtually closed system. Only a small amount of iron is excreted and the majority of the iron released from the breakdown of Hb is reused by the body.
As we emphasized above, most of the iron (2/3 of the iron obtained through food) is in the form of non-heme iron and most of it is in the oxidized ferric form (Fe3+), but it is better absorbed when taken in the reduced ferrous form (Fe2+). Redox reaction Fe3+—Fe2+.
The transition from ferric to ferrous form is ensured in the stomach by the acidity of gastric juice and the presence of reducing agents in the diet (food components) such as Vit.C (ascorbic acid, which also increases the acidity of gastric juice) and proteins with -SH groups (found in meat composition). Some food substances form with iron non-soluble complexes (its soluble ones) by binding it (chelating substances) and preventing its absorption (phosphates, phytates, tannates).
The absorption of iron is carried out throughout the length of the G-I Tract, but is absorbed more in the upper intestine (duodenum and jejunum and less distally) which is a perfect limiting barrier. Normally only 10-15% (for some 5-10% up to 20%) of dietary iron is absorbed (1-2 mg/day) and about 60% in patients with iron deficiency. The total amount of iron in a person weighing 70 kg is about 4 g. This amount is maintained by a perfect balance between absorption and body losses.
The absorption of iron is determined by a series of factors: such as the administered dose, iron stores, the rate of erythropoiesis, dietary iron and its administered form (in heme composition or non-heme). Iron ions enter the mucosal cells of the intestine (enterocytes) through active transport. It is bound in the cytoplasm of the enterocyte with a non-ferritin transport protein (ferroportin and hephaestin) and then passes into the blood where it will bind with transferrin and be distributed to different parts of the body. A portion of the iron remains inside the enterocyte bound with intracellular proteins, apoferritin, to form ferritin.
The absorption is determined by the amount of iron reserves in the body and by the rate of erythropoiesis (in the case of iron deficiency anemia or pregnancy a larger amount is absorbed). The absorption of iron is usually inversely proportional to the iron reserves in the body (so the more reserves, the less iron is absorbed and vice versa).
When absorbed and reaches plasma, again iron transitions to Fe3+. In this state, it is bound with transferrin. Transferrin is a globular protein (beta 1-globulin) and under normal conditions only 30-40% of it is saturated with iron. The remaining 60-70% forms the unsaturated iron-binding capacity. The amount of Fe circulating in plasma bound with transferrin represents sideremia (50-168 µg/dl in M and 60-160 µg/dl in F and for some 70-150 µg/100 ml or γ/dl). Normally there is no free Fe in plasma.
When transferrin is fully loaded with iron sideremia can reach up to 300 µg/100 ml. This is also called the total plasma iron-binding capacity (TIBC). Transferrin transports the iron to the bone marrow. Transferrin also captures the iron released from the macrophages of the spleen, liver, etc. to reuse it in the bone marrow for Hb synthesis. The total amount of iron circulating in the blood bound with transferrin is about 3-4 mg. Free iron in plasma is toxic.
Hepcidin is a hormone produced by the liver which plays a key role in iron metabolism. Hepcidin normally prevents the release of iron from SFMN (Mononuclear Phagocyte System) cells and its absorption from the intestine. Thus, hepcidin regulates the perfect absorption of iron in the intestine (duodenum) mucosa depending on the body's needs. When there is little iron in the body, absorption increases and vice versa.
Normal reference ranges are:
The percent transferrin saturation (i.e., the result of the formula of serum iron/TIBC x 100 or SI/SI+UIBC x 100).
A small amount of iron is lost because the body does not have an active excretory mechanism for it. In patients with iron overload, the loss of iron can increase through epithelial cells up to 5 mg/24 hours.