Inadequate oxygen (hypoxia), such as that experienced by those who live at high altitudes or those with chronic lung disease, induces compensatory physiologic responses, including increased red blood cell formation (erythropoiesis), increased blood vessel growth (angiogenesis), and increased production of enzymes utilized in anaerobic metabolism. Hypoxia is also observed in pathological conditions like ischemia/stroke and inflammatory disorders. Under hypoxic conditions, transcription factors known as hypoxia-inducible factors (HIF) bind to response elements in genes that encode various proteins involved in compensatory responses to hypoxia and increase their synthesis. Iron-dependent enzymes of the dioxygenase family, HIF prolyl hydroxylases and asparaginyl hydroxylase (factor inhibiting HIF-1 [FIH-1]), have been implicated in HIF regulation. When cellular oxygen tension is adequate, newly synthesized HIF-α subunits (HIF-1α, HIF-2α, HIF-3α) are modified by HIF prolyl hydroxylases in an iron/2-oxoglutarate-dependent process that targets HIF-α for rapid degradation. FIH-1-induced asparaginyl hydroxylation of HIF-α impairs the recruitment of co-activators to HIF-α transcriptional complex and therefore prevents HIF-α transcriptional activity. When cellular oxygen tension drops below a critical threshold, prolyl hydroxylase can no longer target HIF-α for degradation, allowing HIF-α to bind to HIF-1β and form a transcription complex that enters the nucleus and binds to specific hypoxia response elements (HRE) on target genes like the erythropoietin gene (EPO) (5).
Ribonucleotide reductases (RNRs) are iron-dependent enzymes that catalyze the synthesis of deoxyribonucleotides required for DNA replication. RNRs also facilitate DNA repair in response to DNA damage. Other enzymes essential for DNA synthesis and repair, such as DNA polymerases and DNA helicases, are Fe-S cluster proteins. Although the underlying mechanisms are still unclear, depletion of intracellular iron was found to inhibit cell cycle progression, growth, and division. Inhibition of heme synthesis also induced cell cycle arrest in breast cancer cells (6).
Iron is required for a number of additional vital functions, including growth, reproduction, healing, and immune function.
While iron is an essential mineral, it is potentially toxic because free iron inside the cell can lead to the generation of free radicals causing oxidative stress and cellular damage. Thus, it is important for the body to systemically regulate iron homeostasis. The body tightly regulates the transport of iron throughout various body compartments, such as developing red blood cells (erythroblasts), circulating macrophages, liver cells (hepatocytes) that store iron, and other tissues (7). Intracellular iron concentrations are regulated according to the body’s iron needs (see below), but extracellular signals also regulate iron homeostasis in the body through the action of hepcidin.
Hepcidin, a peptide hormone primarily synthesized by liver cells, is the key regulator of systemic iron homeostasis. Hepcidin can induce the internalization and degradation of the iron-efflux protein, ferroportin-1; ferroportin-1 regulates the release of iron from certain cells, such as enterocytes, hepatocytes, and iron-recycling macrophages, into plasma (8). When body iron concentration is low and in situations of iron-deficiency anemia, hepcidin expression is minimal, allowing for iron absorption from the diet and iron mobilization from body stores. In contrast, when there are sufficient iron stores or in the case of iron overload, hepcidin inhibits dietary iron absorption, promotes cellular iron sequestration, and reduces iron bioavailability. Hepcidin expression is up-regulated in conditions of inflammation and endoplasmic reticulum stress and down-regulated in hypoxia (9). In Type 2B hemochromatosis, deficiency in hepcidin due to mutations in the hepcidin gene, HAMP, causes abnormal iron accumulation in tissues (see Iron Overload). Of note, hepcidin is also thought to have a major antimicrobial role in the innate immune response by limiting iron availability to invading microorganisms (see Iron withholding defense during infection) (10).
Iron-responsive elements (IREs) are short sequences of nucleotides found in the messenger RNAs (mRNAs) that code for key proteins in the regulation of iron storage, transport, and utilization. Iron regulatory proteins (IRPs: IRP-1, IRP-2) can bind to IREs and control mRNA stability and translation, thereby regulating the synthesis of specific proteins, such as ferritin (iron storage protein) and transferrin receptor-1 (TfR; controls cellular iron uptake) (1, 2).
When the iron supply is low, iron is not available for storage or release into plasma. Less iron binds to IRPs, allowing the binding of IRPs to IREs. The binding of IRPs to IREs located in the 5’end of mRNAs coding for ferritin and ferroportin-1 (iron efflux protein) inhibits mRNA translation and protein synthesis. Translation of mRNA that codes for the key regulatory enzyme of heme synthesis in immature red blood cells is also reduced to conserve iron. In contrast, IRP binding to IREs in the 3’ end of mRNAs that code for TfR and divalent metal transporter-1 (DMT1) stimulates the synthesis of iron transporters, thereby increasing iron uptake into cells (1, 2).
When the iron supply is high, more iron binds to IRPs, thereby preventing the binding of IRPs to IREs on mRNAs. This allows for an increased synthesis of proteins involved in iron storage (ferritin) and efflux (ferroportin-1) and a decreased synthesis of iron transporters (TfR and DMT1) such that iron uptake is limited (2). In the brain, IRPs are also prevented from binding to the 5’end of amyloid precursor protein (APP) mRNA, allowing for APP expression. APP stimulates iron efflux from neurons through stabilizing ferroportin-1. In Parkinson’s disease (PD), APP expression is inappropriately suppressed, leading to iron accumulation in dopaminergic neurons (11, 12).
Iron is required by most infectious agents to grow and spread, as well as by the infected host in order to mount an effective immune response. Sufficient iron is critical for the differentiation and proliferation of T lymphocytes and the generation of reactive oxygen species (ROS) required for killing pathogens (13). During infection and inflammation, hepcidin synthesis is up-regulated, serum iron concentrations decrease, and concentrations of ferritin (the iron storage protein) increase, supporting the idea that sequestering iron from pathogens is an important host defense mechanism (2).
Total body content of iron in adults is estimated to be 2.3 g in women and 3.8 g in men (2). The body excretes very little iron; basal losses, menstrual blood loss, and the need of iron for the synthesis of new tissue are compensated by the daily absorption of a small proportion of dietary iron (1 to 2 mg/day). Body iron is primarily found in red blood cells, which contain 3.5 mg of iron per g of hemoglobin. Senescent red blood cells are engulfed by macrophages in the spleen, and about 20 mg of iron can be recovered daily from heme recycling. The released iron is either deposited to the ferritin of spleen macrophages or exported by ferroportin-1 (iron efflux protein) to transferrin (the main iron carrier in blood) that delivers iron to other tissues. Iron recycling is very efficient, with about 35 mg being recycled daily (1).
Measurements of iron stores, circulating iron, and hematological parameters may be used to assess the iron status of healthy people in the absence of inflammatory disorders, parasitic infection, and obesity. Commonly used iron status biomarkers include serum ferritin (iron-storage protein), serum iron, total iron binding capacity (TIBC), and saturation of transferrin (the main iron carrier in blood; TSAT). Soluble transferrin receptor (sTfR) is also an indicator of iron status when iron stores are depleted. In iron deficiency and iron-deficiency anemia, the abundance of cell surface-bound transferrin receptors that bind diferric transferrin is increased in order to maximize the uptake of available iron. Therefore, the concentration of sTfR generated by the cleavage of cell-bound transferrin receptors is increased in iron deficiency. Hematological markers, including hemoglobin concentration, mean corpuscular hemoglobin concentration, mean corpuscular volume of red blood cells, and reticulocyte hemoglobin content can help detect abnormality if anemia is present (9, 14).
Of note, serum ferritin is an acute-phase reactant protein that is up-regulated by inflammation. Importantly, serum hepcidin concentration is also increased by inflammation to limit iron availability to pathogens. Therefore, it is important to include inflammation markers (e.g., C-reactive protein, fibrinogen) when assessing iron status to rule out inflammation (14).
Vitamin A deficiency often coexists with iron deficiency and may exacerbate iron-deficiency anemia by altering iron metabolism (15). Vitamin A supplementation has been shown to have beneficial effects on iron-deficiency anemia and improve iron nutritional status among children and pregnant women (15, 16). The combination of vitamin A and iron seems to reduce anemia more effectively than either supplemental iron or vitamin A alone (17). Vitamin A may facilitate the mobilization of iron from storage sites to developing red blood cells for incorporation into hemoglobin (15, 16). Moreover, studies in rats have shown that iron deficiency alters plasma and liver levels of vitamin A (18, 19).
Adequate copper nutritional status is necessary for normal iron metabolism and red blood cell formation. Anemia is a clinical sign of copper deficiency, and iron has been found to accumulate in the livers of copper-deficient animals, indicating that copper (via copper-containing ceruloplasmin) is required for iron transport to the bone marrow for red blood cell formation (20). The connection between copper availability and iron metabolism has also been established in humans; copper deficiency can lead to secondary ceruloplasmin deficiency and hepatic iron overload and/or cirrhosis (21). Oral copper supplementation restored normal ceruloplasmin levels and plasma ferroxidase activity and corrected the iron metabolism disorder in a copper-deficient subject (22). Moreover, infants fed a high-iron formula absorbed less copper than infants fed a low-iron formula, suggesting that high iron intakes may interfere with copper absorption in infants (23).
Zinc is essential to maintain adequate erythropoiesis. When zinc deficiency coexists with iron deficiency, it may exacerbate iron-deficiency anemia (24). On the other hand, high doses of iron supplements, taken together with zinc supplements on an empty stomach, may inhibit the absorption of zinc. When taken with food, supplemental iron does not appear to inhibit zinc absorption. Iron-fortified foods have not been found to impair zinc absorption (25, 26).
The presence of calcium decreases iron absorption from both nonheme (i.e., most supplements and food sources other than meat, poultry, and seafood) and heme sources (27). However, calcium supplementation up to 12 weeks has not been found to change iron nutritional status, probably due to a compensatory increase in iron absorption (28). Individuals taking iron supplements should take them two hours apart from calcium-rich food or supplements to maximize iron absorption.
Severe iron-deficiency anemia can impair thyroid metabolism in the following ways: (1) by altering the thyroid-stimulating hormone response of the pituitary gland; (2) by reducing the activity of thyroid peroxidase that catalyzes the iodination of thyroglobulin for the production of thyroid hormones; and (3) in the liver by limiting the conversion of T4 to T3, increasing T3 turnover, and decreasing T3 binding to nuclear receptors (29). It is estimated that goiter and iron-deficiency anemia coexist in up to 25% of school-age children in west and north Africa (30). A randomized controlled study in iron-deficient children with goiter showed a greater reduction in thyroid size following the consumption of iodized salt together with 60 mg/day of iron four times per week compared to placebo (31). Additional interventions have confirmed that correcting iron-deficiency anemia improved the efficacy of iodine supplementation to mitigate thyroid disorders (reviewed in 29, 30).
Iron deficiency is the most common nutrient deficiency in the US and the world. Levels of iron deficiency are listed below from least to most severe.
Iron stores are depleted, but the functional iron supply is not limited.
Before the development of frank anemia, the supply of functional iron to tissues, including bone marrow, is inadequate such as to impair erythropoiesis.
By definition, anemia is present when individual hemoglobin concentrations fall below two standard deviations of the distribution mean for hemoglobin in a healthy population of the same gender and age and living at the same altitude (32). In 2013, iron-deficiency anemia was the leading cause of years lived with disability in children and adolescents in the 50 most populous countries. The countries with the highest prevalence of iron-deficiency anemia in individuals younger than 19 years were Afghanistan (41%) and Yemen (39.8%); India contributed the largest number of cases of anemia (147.9 million). The prevalence in the US was estimated to be 19.3% with nearly 16 million cases of iron-deficiency anemia in children and adolescents (33).
Iron-deficiency anemia occurs when there is inadequate iron to support normal red blood cell formation. The anemia of iron deficiency is usually characterized as microcytic and hypochromic, i.e., red blood cells are measurably smaller than normal and their hemoglobin content is decreased such that they are paler than normal. At this stage of iron deficiency, symptoms may be a result of inadequate oxygen delivery due to anemia and/or suboptimal function of iron-dependent enzymes. Changes in hematological parameters are used in the clinical diagnosis of iron-deficiency anemia (see Assessment of iron status). It is important to remember that iron deficiency is not the only cause of anemia, and that the diagnosis or treatment of iron deficiency solely on the basis of anemia may lead to misdiagnosis or inappropriate treatment of the underlying cause (34). See also the articles on Folate and Vitamin B12 for information on other nutritional causes of anemia.
Most of the symptoms of iron deficiency are a result of the associated anemia and may include fatigue, rapid heart rate, palpitations, and rapid breathing on exertion. Iron deficiency impairs athletic performance and physical work capacity in several ways. In iron-deficiency anemia, the reduced hemoglobin content of red blood cells results in decreased oxygen delivery to active tissues. Decreased myoglobin levels in muscle cells limit the amount of oxygen that can be delivered to mitochondria for oxidative metabolism. Iron depletion also decreases the oxidative capacity of muscle by diminishing the mitochondrial content of cytochromes and other iron-dependent enzymes required for electron transport and ATP synthesis (see Function) (35).