M i cronutriente is the term used to refer to the minerals and essential vitamins diet obtained holding molecular functions and normal cell (1) . Iron (Fe), a mineral that is an essential component of hemoglobin (Hb), myoglobin and several enzymes, such as: cytochromes, catalases, peroxidases, oxidases and hydroxylases, is of capital importance. Its main functions are: reversibly fixing oxygen (O 2 ) for transport or storage and accepting and releasing electrons to generate immediate sources of energy (2) , but it also participates in other biochemical reactions of great importance, such as those related to O 2 metabolismand DNA synthesis. For these reasons, it is vital for the survival, proliferation and cellular differentiation of various tissues, including nervous tissue and the immune system (2,3) . Fe deficiency is the most common micronutrient deficiency in the world (1) and the most common cause of anemia.
Iron deficiency (FeP) consists of the deficiency of the systemic deposits of Fe, with potential harmful effect, especially in childhood. If this situation worsens or is maintained over time, iron deficiency anemia (AFe) will develop, with greater clinical repercussions. EFA, the most common hematological disease of childhood, is anemia caused by the failure of the marrow hematopoietic function in the synthesis of Hb due to a lack of Fe (4) .
Physiological recall of iron metabolism
The main mechanism for the regulation of Fe homeostasis is the degree of intestinal absorption. There is hardly any specific mechanism of excretion. Aspects that are not well known still persist (2-5) .
Fe, as a transition metal, is an excellent catalyst due to its ability to exchange electrons under aerobic conditions. These characteristics make it an essential element in essential cellular functions, such as: DNA synthesis, O 2 transport and cellular respiration. Its ability to coexist in two forms of oxidation, ferrous, reduced or divalent (Fe ++ ), and ferric, oxidized or trivalent (Fe +++ ), provides it with most of its properties, but at the same time, when its concentration exceeds the amount tolerated by the cell, makes it toxic; as it plays a decisive role in the genesis of highly reactive species (free radicals) from the O 2 molecule (6), which cause oxidative damage to important cellular components. It is not uncommon, then, that mechanisms have been developed that allow strict control of the levels of this mineral. Fe absorption is regulated by enterocytes, and complex mechanisms are involved in its management in which three proteins play a relevant role: transferrin (Tf), in relation to transport; ferritin (Ft), relative to the reserve; and the transferrin receptor (RTf), in relation to cell entry and use. In the body, Fe is transported and stored in the form of Fe +++ , while it acts in the form of Fe ++. In the last 2 decades, new proteins and new functions of those already known that participate in the homeostasis of Fe have been discovered, which have allowed progress towards a better understanding of its complex metabolism.
A wide variety of nuts, seeds, legumes, vegetables and fruits provide the so-called vegetable Fe or non-heme (90% of the intake). Red meat, liver and egg yolk, and, to a lesser extent, fish and other meats, provide the heme or animal Fe (the remaining 10%). Cow's milk (LV) and woman's milk (LM) are relatively low in Fe (0.2-1 mg / l) (3.5). The adapted formulas enriched or fortified in Fe (all those present in our country) have 5-13 mg / l: 5-8 mg / l the starting formulas (FI) and 8-13 mg / l the continuation formulas (FC ). The elemental Fe content recommendations of the European Society of Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) of 2005 are 0.3-1.3 mg / 100 Kcal for FI and 1-1.7 mg / 100 Kcal for FC, which are transferred to the European (and Spanish) regulations of 2006, although in FC it is extended to 0.6-2 mg / 100 Kcal (2.5.7) . In a more recent communication from the ESPGHAN Nutrition Committee (8), it is recommended that the formulas fortified in Fe should have a content of 4-8 mg / l for the FI and higher, but not optimally determined, for the FC. The Fe intake needs during infancy are expressed in table I.
Around 10% of the Fe present in food considered globally. In particular, around 3-8% in the form of ferric complexes (non-heme Fe: ferrous and ferric, the latter insoluble at pH> 3), and between 10-25% as part of the heme group (Hb and myoglobin) , which is much better absorbed. Fe from LM is absorbed 2-3 times better than that contained in LV (7,9) .
• The ferrous ions present in the intestinal lumen, or the ferric ions reduced to ferrous form after the action of dietary reducing agents, such as ascorbic acid, or the action of a ferrreductase present in the apical (luminal) pole of the enterocyte, the Duodenal cytochrome B, are absorbed by the enterocytes of the crests of the microvilli, predominantly at the duodenal level and to a lesser extent in the upper jejunal section, by means of the transmembrane protein transporter of divalent metal ions (DMT1, Divalent metal transporter ).
• Other ferric ions, solubilized in acidic pH and chelated by intestinal mucins, can also be absorbed directly by the enterocyte, although with much less efficiency, through a mechanism in which a membrane protein, b 3-integrin, intervenes. and a protein on the cytoplasmic side, mobilferrin, which are integrated into a cytoplasmic complex called paraferritin, in which a reductase, flavin-monooxygenase and b 2-microglobulin also participate, all resulting in the reduction of Fe ++ + absorbed.
• On the other hand, Fe heme has a specific transporter in the apical membrane, HCP1 ( Heme Carrier Protein ), not yet well characterized; part of this heme Fe, already inside the enterocyte, will release its Fe content through heme-oxygenases (2,3,5,6) (Fig. 1).
Figure 1. Metabolism of iron (Fe) in the enterocytes of the villous intestinal ridges: enterocyte absorption and blood flow.
The absorption mechanisms from the intestinal lumen of ferric ions (Fe +++ ) (1) , ferrous ions (Fe ++ ) (2) and heme iron (3) , as well as the three possible destinations of the Cytoplasmic Fe ++ : its integration into Fe regulatory proteins (4) , its passage into the blood through ferroportin (5) , with subsequent oxidation by hephaestin and binding with transferrin in its ferric form (6) , and its storage after conversion to Fe +++ as ferritin (7) .Vit. C: Vitamin C; CytbD: Duodenal cytochrome b; DMT1: divalent metal transporter 1; HCP1: Heme transporter protein; IRP: iron regulatory proteins; FO: ferrooxidase; Ft: ferritin; Tf: transferrin.
- Factors that increase absorption: increased Fe intake (up to a certain limit), Fe present in ferrous form, reducing substances in the diet, such as vitamin C, tissue hypoxia, increased erythropoiesis, and reduced systemic reserves of Faith.
- Factors that decrease absorption: presence in the diet of substances that form insoluble salts with it (phytates, oxalates, tannates, phosphates, carbonates, bile acids), of divalent metals that have the same absorption mechanism (zinc, copper, cadmium, cobalt, manganese, lead), the administration of gastric acid inhibitors or chelators, and iron overload (4) .
In the cytoplasm of the enterocyte, the ferrous Fe from any of the previous routes can be stored, after oxidation by a ferrioxidase and after binding in that ferric form to apoferritin, to form Ft. Another part of the ferrous cytoplasmic pool, according to its concentration , will be integrated into the Fe-regulated proteins, known as IRP ( Iron-regulatory proteins ) types 1 and 2, which act as Fe sensors and according to their affinity for specific loci called IRE ( Iron-responsive element ),at the proximal or terminal ends of the messenger RNA of different proteins related to Fe metabolism, they will prevent or favor its translation (Fig. 1). This constitutes another of the mechanisms for regulating iron metabolism, known as post-transcriptional: if there is iron deficiency, the expression of RTf (and other proteins that favor its absorption) increases and that of Ft decreases; in a situation of excess of Faith, the opposite happens.
In the basolateral membrane of the enterocytes located in the crypt of the microvilli, and as another mechanism of intestinal regulation of the uptake and fate of Fe, there are RTf1 that allow the reentry of iron after binding to its ligand, Tf. This binding is modulated by a dimeric protein similar to the molecules of the major class I histocompatibility complex, the HFE (H for histocompatibility and FE for iron) or hemochromatosis protein. The role of HFE is not yet fully clarified, but it seems to play a key role in the set of sensing mechanisms that determine the value of Fe in the immature crypt enterocyte and, therefore, in the amount of it that will be absorbed when this enterocyte matures and is on the ridge (6). If HFE does not exert its action, as in hemochromatosis, the enterocytic absorption of Fe will be stimulated, leading to iron overload.
Pass to blood and plasma transport
In addition to the two previous routes, another part of the poolFerrous cytoplasmic is conducted to the basolateral (or vascular) membrane of the enterocyte, where it passes into the blood by means of a transmembrane transporter protein, ferroportin, with subsequent conversion of the ferrous to ferric form by a coupled ferroxidase protein, analogous to ceruloplasmin, but of intestinal location, hephaestin (Fig. 1). There is an inhibitor of ferroportin, hepcidin, of hepatic origin, which would prevent the plasma release of Fe, and constitutes the main systemic regulator of Fe metabolism: it is stimulated in situations of inflammation and Fe overload, and is inhibited in situations of hypoxia and Fe deficiency. Already in the vascular lumen, and in its iron form, it binds for the most part to a plasma beta-globulin, Tf, which constitutes its main transport protein in blood. Each Tf molecule has two active binding sites for Fe (di-Tf, if it binds two Fe atoms, mono-Tf, if it binds only one, apo-Tf, if neither); under normal conditions, only about a third of the available sites are occupied. There is a small proportion of Fe bound to apoferritin, to constitute the serum Ft, and, in case of abundance, to other plasma components (albumin, low molecular weight compounds, citrate…). Proteins involved in the passage into blood of heme enterocyte Fe that have not suffered the action of hemoxygenases have been described, but their role is not yet clear. There is a small proportion of Fe bound to apoferritin, to constitute the serum Ft, and, in case of abundance, to other plasma components (albumin, low molecular weight compounds, citrate…). Proteins involved in the passage into blood of heme enterocyte Fe that have not suffered the action of hemoxygenases have been described, but their role is not yet clear. There is a small proportion of Fe bound to apoferritin, to constitute the serum Ft, and, in case of abundance, to other plasma components (albumin, low molecular weight compounds, citrate…). Proteins involved in the passage into blood of heme enterocyte Fe that have not suffered the action of hemoxygenases have been described, but their role is not yet clear.
The presence of an RTf, known as type 1 (RTf1), in the cell membranes of all nucleated cells, allows the incorporation of Fe into the cell interior. The Fe-Tf-RTF1 complex is internalized in endosomal acid vesicles, where Fe is released from Tf, reduced to Fe ++ and transferred by DMT1 to the cell's cytoplasm for use (6). There is greater expression of RTf in those cells with higher Fe requirements, as in erythroblasts, cells of the reticulo-endothelial system (SRE) and hepatocytes. In erythroblasts, the Fe released in the cytoplasm will be used in the mitochondria to form heme, or it will be stored in the form of Ft. There is another Tf receptor (RTf2), which differs from RTf1 and is predominantly expressed in hepatocytes. The hepatocyte exerts its regulatory role depending on the level of Fe in the blood, in which several regulatory proteins intervene, among which are: hemojuvelin, HFE and RTf2, it is thought that through the expression of hepcidin (3). Also, other mechanisms of cellular incorporation of Fe independent of Tf have been described, but they are less well known and less important.
• Functional or utilization compartment: a) 0.1-0.2% in plasma: serum Fe; b) 65-70% in mature red blood cells and erythroblasts: hemoglobin Fe (1 g of Hb = 3.5 mg of Fe); c) 5-10% in muscles: myoglobinic Fe; and d) 1-3% inside the cell: enzymatic Fe.
• Reserve or deposit compartment (22-30%): in the cells of the ERS, mainly liver and spleen, and medullary precursors: Deposit Fe, in the form of Ft (soluble spheroidal glycoprotein, inside which it can hold up to 4,500 atoms Fe) and hemosiderin (Ft added, insoluble, with release of Fe much slower than in the case of Ft).
Quantitatively unimportant, but qualitatively it constitutes the only way for the body to get rid of excess Fe. In feces, urine and skin, by cell desquamation, mainly of enterocytes, with their Ft deposits (they can be varied in a certain way depending on the post-transcriptional regulation previously referred to), also in bile and sweat. It is estimated at about 0.3-0.5 mg / day in children.
Red blood cells have a half-life of approximately 120 days, after which they are removed from the circulation by the cells of the ERS. About 1% of circulating erythrocytes are renewed each day. The released Fe is stored in the ferric deposits of the ERS or passes into plasma where it binds to transferrin, being from these two sources from where it is reused by erythroblasts.
Particularities in the pediatric age
The fetus actively receives Fe through the placenta, even in situations of maternal deficiency. The accumulated reserves in utero (80% in the third trimester) and in the first weeks of life, as a consequence of the decrease in the high levels of neonatal Hb, cover the requirements of the healthy, term newborn during the first 6 months. The newborn has approximately 0.5 g of Fe, while the adult around 4-5 g, which implies that the child's growth needs to absorb an approximate amount of 0.5-0.8 mg / day daily, which Together with that required to counteract the small losses caused by cell desquamation and hemorrhage, it means that the daily needs for Fe absorption are ˜ 0.8-1 mg. If the estimated absorption is 10%, the daily diet should provide about 10 mg of Fe.
The pediatric prevalence is higher in infants and adolescent women. The differences between populations will depend mainly on socioeconomic conditions related to the diet of pregnant women and childhood.
Lack of Faith is the most widespread and common nutritional disorder in the world, affecting more than 30% of the world's population (1,9) . Studies in developed countries have shown a considerable decrease in recent decades, attributed to nutritional improvements and the establishment of preventive programs (10) . In underdeveloped countries, the frequency is 2-4 times higher (2), as a consequence, above all, of poor nutrition secondary to poverty. Studies in Europe vary between 9-34% of FeP and 3-8% of AFe in children between 1-2 years of age, depending on the socioeconomic conditions of the populations studied, such as: age of introduction of VL, use of formulas supplemented in Fe and availability of foods rich in Fe. A substudy of the Euro-Growth (11) of 2001, regarding the iron status determined at the age of 12 months in 533 healthy children from 10 European countries, showed a 7.2 % FeP and 2.3% AFe. In Spain, a 2002 prevalence study, conducted in Navarra (12), with a sample of 94 infants, observed 9.6% FeP and 4.3% AFe. In later ages, the prevalence is lower, 2-5% FeP and <1% AFe, with an increase in adolescent women.
The causes include an insufficient contribution, high requirements and / or an excessive loss (5) .
Childhood, especially the first 2 years, has a high risk of iron deficiency, mainly due to its limited dietary sources of Fe and its increased needs for growth. Adolescence is another period of risk due to its higher growth rate and, in the case of girls, menstrual losses are added.
We distinguish three fundamental, non-exclusive groups: decrease in contribution, increase in needs, and increase in losses (Table II).
In all cases of eFA, and especially in older children, blood loss should be considered as a possible cause.
Iron deficiency passes through three progressive stages: latent FeP, FeP without anemia (or manifest FeP), and AFe.
There are three successive stages, of increasing symptomatic intensity, in Fe deficiency: 1) Latent FeP : the emptying of the iron deposits of the ERS begins, first in the liver and spleen, and later, in the bone marrow, of asymptomatic course; 2) FeP without anemia : Fe deficit increases, evidenced in its lower serum availability, with greater biochemical analytical involvement, but without involvement of the hemogram, and appearance of symptoms attributable to the deficiency of Fe-containing tissue enzymes; and 3) EFA : own hematological alterations, greater involvement of previous anomalies and symptoms of anemia.
The initial symptoms of Fe deficiency, largely related to its role in certain enzymatic reactions, fundamentally affect functions: brain, digestive and immune, all of which improve when FeP is corrected before anemia is corrected. Several of the long-term effects on the CNS would be related to alterations in neurometabolism, in the function of neurotransmitters and in myelination, synaptogenesis and dendritogenesis during the brain development stage, some persistent, even after correction of the Fe deficiency (13) . One of the consequences, among other neurobiological alterations, would be the decrease in the speed of visual and auditory conduction (2). The pathophysiology derived from the decrease in Hb is common to other anemias.
Secondary, both to iron deficiency and anemia, although most are asymptomatic (9). To highlight in early childhood, its effects on the child's brain maturation.
• Repercussion on the central nervous system: irritability, attention deficit, learning difficulties and decreased performance. If it happens in early stages, there is an alteration in its maturation, affecting cognitive, motor and behavioral function (13) ; Depending on the intensity and duration of iron deficiency and the age at which it occurs, some disorders may be irreversible, even after correction of the deficit (2,5) .
• Pica: an eating disorder of unknown pathogenesis, consisting of the ingestion of non-nutritive substances, such as soil (geophagia) or ice (pagophagia).
• Digestive disorders: anorexia (perhaps the earliest), angular cheilitis, glossitis, hypochlorhydria and villous atrophy.
• Dermatological disorders: xerosis, skin desquamation, thinning hair, brittle nails and koilonychia (spoon-shaped).
• Immune disorders: affect chemotaxis, the bactericidal function of neutrophils and other forms of immune response. Controversy continues as to whether it favors or hinders certain infections, since it affects immune function, but, on the other hand, pathogens also require Fe for their metabolism, as in the case of malaria.
• Alterations in thermoregulation: less adaptive response to cold.
• Relationship with attention deficit hyperactivity disorder, restless leg syndrome, sleep disturbances and apnea breaks.
• Paleness: the most classic sign, but it is usually not visible until Hb values <7-8 g / dl.
• With lower Hb values (generally <5-6 g / dl): tachycardia, systolic heart murmur, cardiac dilation, irritability, anorexia and lethargy.
• Asthenia and excessive fatigue.
• Predisposition to cerebrovascular accident ( stroke ) in childhood: AFe is 10 times more frequent in these children than in controls, and it is present in more than half of these children without another underlying disease.
A blood test will confirm the diagnosis. It can be after clinical suspicion, suggestive symptoms or belonging to a risk group, or accidental finding. The determination of serum ferritin constitutes the most reliable accessible isolated parameter to assess Fe deposits (14).
A succession of biochemical and hematological events occurs as Fe deficiency progresses, beginning with latent FeP, continuing with overt FeP, and culminating in AFe (4,15) (Fig. 2).
Figure 2. Succession of biochemical and hematological events as iron deficiency progresses.
Other findings present in eFA include:
• Alterations in the platelet series, with normal leukocyte count: occasional thrombocytosis due to a probable increase in erythropoietin (similarity with thrombopoietin); although, occasionally, mild thrombocytopenia may appear.
• Hypercellularity of the bone marrow due to erythroid hyperplasia, with normal white and platelet series; the iron stains in the medullary reticular cells are negative, as an exponent of the absence of deposits in these cells.
The diagnosis can be, at times, complex due to the suboptimal sensitivity and specificity of the evaluated parameters and the relative arbitrariness of the normality limits. The traditional diagnosis of FeP is based on a costly strategy, combining several determinations to increase the specificity, not without error, which includes a decrease in sideremia, the Tf saturation index (ISTf) and Ft, and a increased total Fe-binding capacity (CTFH), while the diagnosis of AFe adds hematological alterations. However, the assessment of each of these parameters must be done carefully. The sideremia is highly fluctuating, with a wide range of normality that, in addition, decreases in the infectious and inflammatory processes. Similarly, the ISTf will vary, since this is obtained from the quotient between the sideremia and the CTFH. Ft is a faithful reflection of Fe deposits, but with the drawbacks of its biological variability and its behavior as an acute phase reactant. Recently, an increase in free erythrocyte protoporphyrins (PEL) and serum receptor for Tf (RsTf) have been added, which are not altered in infectious or inflammatory processes, but their technical complexity and variable values between laboratories limit their use , as well as reticulocyte indices, especially the decrease in the content of reticulocyte Hb (CHr), although not available in all cell counters(10) . RsTf originates from proteolytic cleavage of the specific receptor located on the surface of all cells in the body except for mature erythrocytes, its concentration being proportional to that of the latter, whose expression is in direct relation to intracellular Fe requirements.