Neonatal iron homeostasis

Introduction

Iron is critical for various tissues throughout the body, including red blood cells and the brain. While iron can be stored, it cannot be synthesized de novo in the body and therefore is an essential micronutrient that must be provided in the diet. An important feature of iron is that it is not actively excreted from the body, so regulation of total body iron occurs at the level of dietary absorption. Based on the need to ensure adequate iron intake to meet the body’s demands coupled with the importance of avoiding iron overload, iron uptake is typically closely regulated to ensure iron homeostasis is optimized. Iron acquisition begins prior to birth, with active iron transfer from the mother to fetus. These accumulated fetal iron stores are used as the primary iron source for the first 4 to 6 months after term birth, when dietary iron intake usually increases. Dietary absorption of iron is also tightly regulated, although the timing of development of this homeostatic mechanism is not known.

There are various conditions under which iron homeostasis may be disrupted. Despite the body’s tight regulatory mechanisms, iron deficiency is prevalent worldwide, affecting 1.6 to 2 billion individuals. ^, In 2002 the World Health Organization estimated that anemia resulting from iron deficiency was one of the 10 most important factors contributing to the global burden of diseases, with children and pregnant women at particularly high risk. Although individuals from low- and middle-resource countries are at highest risk, it is not well recognized that pregnant women and children in high-resource countries, such as the United States, are also at high risk. For example, Auerbach et al. screened a cohort of 102 consecutive, nonselected, nonanemic pregnant women in the United States and found that 42% were iron deficient.

When an infant’s iron balance is disrupted, it is the role of the clinician to identify this disruption and provide needed supplementation, guided by iron status parameters. Failure to recognize and treat either iron deficiency or overload can have long-term consequences that persist throughout an individual’s life. These effects may be hematologic in nature with extreme iron deficiency resulting in anemia, or they may be developmental, with iron deficiency leading to long-term neurodevelopmental impacts. Because of the potential adverse long-term consequences, ensuring optimal iron status is an essential part of neurodevelopmental care of at-risk infants.

In utero iron accretion

Breast milk is low in iron; therefore an infant’s iron needs during the first 4 to 6 months of life are typically supported by stores accrued during the third trimester of pregnancy, until iron-rich sources of supplementary foods are introduced in a child’s diet. A young infant’s iron balance appears to be highly dependent on iron loading during pregnancy. ^, Because this period is critical to ensuring adequate iron stores for the first several months of a healthy child’s life, several mechanisms are in place to ensure adequate placental iron transfer from mother to fetus.

Although the exact mechanism by which iron crosses the placenta is not known, several iron transporters that are critical to postnatal iron transfer are recognized to play a role in this process in utero. Iron transfer occurs to a certain extent throughout pregnancy, but the majority occurs in the third trimester. This is one reason why preterm infants are at added risk for iron deficiency postnatally. The transfer of iron across the placenta is an active process carried out primarily by the syncytiotrophoblasts. In the primate placenta, maternal blood bathes fetal tissue, thus diminishing the distance that iron must travel to pass from mother to fetus. Iron is transported in the circulation bound to a protein called transferrin. Maternal transferrin-bound iron is actively transported across the placenta via transferrin receptor 1. While transferrin receptor 1 is ubiquitous in cells throughout the body (except in mature erythrocytes), there is a particularly high concentration of this receptor on placental syncytiotrophoblasts, thus facilitating fetal iron acquisition. Iron enters syncytiotrophoblast cells via transferrin receptor 1, then is exported to the fetal side by ferroportin. The presence of ferroportin on cell surfaces, which affects the ability of cells to absorb and release iron, is regulated by hepcidin. Hepcidin is a negative regulator of iron, as will be discussed subsequently, and thus elevated hepcidin levels lead to decreased ferroportin within cell membranes and therefore decreased iron absorption and availability. To promote fetal iron transfer, maternal hepcidin values are particularly low during pregnancy.

The transfer of iron from mother to fetus is a tightly regulated process as both iron deficiency and overload during pregnancy are associated with adverse effects. The iron requirements for a woman during pregnancy are estimated to be 1 g over the duration of pregnancy. This includes expansion of the maternal red cell mass with expansion of the blood volume during pregnancy, placental needs (90 mg), and fetal transfer (270 mg iron). ^, Additionally, the fetus has significant iron demands throughout pregnancy but particularly during the period of rapid brain growth and expansion of the fetal red cell mass in the third trimester. Fetal iron transfer is prioritized over maternal needs when mothers have mild iron deficiency; however, more severe deficiency states cannot be overcome. Thus severe maternal iron deficiency puts both the mother and developing fetus at risk for adverse effects of iron deficiency. This is of significant concern as pregnant women represent one of the groups at highest risk for iron deficiency. The incidence of iron deficiency during gestation ranges from approximately 45% in well-resourced countries to up to 80% in low-income countries. ^, This has significant health implications for mother and fetus. Maternal iron deficiency has been associated with adverse perinatal outcomes including in utero growth restriction, preterm birth, low birth weight, peripartum hemorrhage, and even increased maternal mortality. ^{, ,} Maternal conditions that decrease fetal iron transfer include maternal obesity, diabetes, and preeclampsia. Maternal alcohol use and smoking are also associated with decreased maternal-to-fetus iron transfer. It is unclear whether this disruption in placental iron transfer is caused by impaired placental function or an inappropriate elevation of the proinflammatory hormone hepcidin in mothers with these conditions.

Adverse long-term outcomes are possible in infants born with inadequate prenatal iron transfer. The fetal origins hypothesis links in utero exposures and environment to adult-onset disease. ^, Fetal nutritional disruptions, including iron deficiency, have been associated with an increased incidence of abnormal brain structure and development as well as neuropsychiatric disruptions later in adulthood. This likely reflects the importance of iron in brain development. In addition to its erythropoietic effects, iron plays key roles in energy production in the brain, synaptogenesis, dendritic pruning, and neurotransmitter production. The hippocampus appears to be particularly sensitive to iron deficiency during pregnancy and early infancy. ^, This is perhaps due to its high energy demand and rapid growth during pregnancy and the first 2 years of life. Consequently, iron deficiency during fetal life can have adverse effects on hippocampal development leading to long-term disruptions in neurodevelopmental conditions predominantly controlled by the hippocampus, including recognition memory. In addition to hippocampal function in particular, iron deficiency can impact the fetal brain more globally. This is likely due to a combination of brain-specific functions of iron as well as the high oxygen consumption of the brain during fetal life, for which iron is a critical component. The brain uses approximately 60% of total fetal oxygen availability, emphasizing the importance of optimized oxygen delivery in the maternoplacental unit.

While much of the research examining fetal iron deficiency is focused on the impacts on the fetal brain, emerging research is showing additional long-term impacts of fetal iron deficiency. For example, Gambling et al. showed in an animal model that iron deficiency during fetal life was associated with long-term elevated blood pressure in offspring.

Fetal iron deficiency is associated with adverse outcomes, but iron overload also appears to be associated with adverse outcomes, emphasizing the importance of tight iron regulation. Chang et al. showed an increased risk of low birth weight and prematurity with an elevated hematocrit in a population of adolescent women, highlighting the U-shaped relationship between high and low iron status in the perinatal period. ^, Additionally, several studies have suggested an association between impaired glucose regulation in iron-replete pregnant women treated with iron. As we will summarize, this U-shape relationship persists in infancy, childhood, and adulthood, highlighting the critical nature of iron homeostasis and targeting supplementation to those with iron deficiency.

Iron regulation in neonates

Both iron deficiency and overload can have detrimental effects. Because of the importance of ensuring iron homeostasis, regulation mechanisms are in place. Notably, iron cannot be easily excreted from the body once absorbed, therefore much of the regulation process for iron takes place in the gut lumen, at the site of iron absorption. As shown in Fig. 6.1 , ingested ferric iron (Fe3+) is reduced to ferrous iron (Fe2+) in the gut lumen by ferric reductase. Fe2+ is absorbed into the intestinal epithelial cell from the gut lumen via divalent metal transporter-1. Human lactoferrin, from breast milk, is absorbed along with its bound iron by a specific receptor. Iron then binds to ferritin intracellularly. Iron exits the epithelial cell on the basolateral surface via ferroportin-1. Fe2+ is oxidized to Fe3+ by hephaestin as it exits the cell. Iron binds transferrin as it exits the cell and is transported through the circulation as transferrin-bound Fe2+.

This process is under tight regulatory control by hepcidin. Hepcidin is a polypeptide hormone primarily produced in the liver. It has been clearly shown to serve as the primary regulator of iron in older children and adults, ^, and there are emerging studies suggesting that its regulatory mechanisms are intact in neonates. Hepcidin acts as a negative regulator of iron absorption and availability (see Fig. 6.1 ). It causes the internalization and degradation of the ferroportin transmembrane channel. Ferroportin is the primary channel through which iron exits cells in the intestinal and in storage cells for iron such as macrophages. Therefore internalization of ferroportin in intestinal epithelium causes iron to be trapped in the epithelial cell. It is then lost in feces as the epithelial cell is naturally sloughed over time. In storage cells such as macrophages, hepcidin leads to sequestration of iron within these cells. Therefore the net effect of an increase in hepcidin is decreased iron absorption and availability. ^,

The production of hepcidin varies based on both the body’s total body iron status and other clinical conditions such as infection and, more broadly, inflammation. Additionally, erythroferrone, which promotes erythropoiesis, leads to a downregulation of hepcidin, thus making iron available for red blood cell production. When an individual’s iron needs exceed the availability of iron within the body, hepcidin production decreases, leading to increased absorption of iron. This is the case in pregnancy, when iron demands of the increasing maternal red cell mass, the placenta, and the fetus rapidly increase from a woman’s baseline iron demands. During a healthy pregnancy, hepcidin values therefore decrease to allow increased iron absorption from the gut to accommodate this increased demand.

Some bacteria, most notably gram-negative bacteria, have been found to be siderophilic (i.e., require iron for normal function and replication). As an evolutionary adaptation, humans have evolved to increase their hepcidin production with infection and inflammation. This leads to decreased iron absorption as well as sequestration of iron within storage cells such as macrophages and hepatocytes. This leads to less free iron available for bacterial replication. While this certainly has historic benefits, the more widespread upregulation of hepcidin with any form of inflammation, including inflammation of chronic disease, has detrimental effects. Chronic inflammatory conditions, such as inflammatory bowel disease, have been linked to inappropriate upregulation in hepcidin resulting in functional iron deficiency. This is one of the theorized mechanisms behind mothers with obesity, a potentially proinflammatory condition, having infants at higher risk of iron deficiency. ^,

As noted previously, several regulatory mechanisms are in place to ensure adequate fetal iron accretion during pregnancy. In healthy, term infants, iron stores accrued during fetal life provide an adequate iron source to support a period of low iron intake from breast milk (though what iron is present is highly bioavailable due to lactoferrin) during the first 4 to 6 months of age, after which complimentary foods are typically introduced. In fact, neonatal iron status in healthy term infants is highly dependent on this loading, and infants born with low stores are unlikely to catch up without supplementation.

Preterm infants and term infants born with inadequate prenatal stores due to placental dysfunction or other causes are at risk for iron deficiency. When supplemental iron is offered, it does appear that neonates can regulate their iron status through mechanisms that approach adult mechanisms, though results are somewhat conflicting in this population. Bahr et al. found that hepcidin levels were lower in neonates at risk for iron deficiency, including preterm infants, and studies by our group and others have shown hepcidin values to correlate with iron treatment and markers of iron status, including ferritin. ^{, ,} Similar to older children and adults, hepcidin values in neonates have been shown to increase with inflammation and sepsis, suggesting that infants may also be able to regulate hepcidin to sequester iron from siderophilic bacteria. ^{, ,} However, not all studies support the idea that neonates can regulate their iron status. Yapakçi et al. showed no difference in hepcidin values following transfusion, though an increase was seen by Lorenz et al. Domellöf et al. examined iron uptake via iron-isotopes. They found no difference in iron absorption rates in iron supplemented vs. unsupplemented infants at 6 months of age, but did see a difference at 9 months. They concluded that younger infants may not have the ability to regulate absorption based on need, though as this study was conducted in a population with presumed low incidence of iron deficiency, this may have confounded their results as many infants at 6 months may have remained iron sufficient through prenatal stores.

In sum, the available evidence suggests that iron is tightly regulated in children and adults through the actions of hepcidin, erythropoietin, and erythroferrone to avoid both overload and deficiency. While these mechanisms may be less robust in neonates, there is evidence that even preterm neonates have some regulatory control over their iron status through the action of these regulatory hormones.

Measures of iron status

A number of regulatory hormones and proteins are involved in iron regulation as noted previously, and many of these can be used to assess iron status. These measures can be more challenging to interpret in neonates due to the normal hematologic changes of infancy; therefore age-specific norms must be used to interpret these measures appropriately.