Iron is a mineral that our bodies need for many functions. For example, iron is part of hemoglobin, a protein which carries oxygen from our lungs throughout our bodies. It helps our muscles store and use oxygen. Iron is also part of many other proteins and enzymes.
Your body needs the right amount of iron. If you have too little iron, you may develop iron deficiency anemia. Causes of low iron levels include blood loss, poor diet, or an inability to absorb enough iron from foods. People at higher risk of having too little iron are young children and women who are pregnant or have periods.
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Too much iron can damage your body. Taking too many iron supplements can cause iron poisoning. Some people have an inherited disease called hemochromatosis. It causes too much iron to build up in the body.
Iron is a mineral that is naturally present in many foods, added to some food products, and available as a dietary supplement. Iron is an essential component of hemoglobin, an erythrocyte (red blood cell) protein that transfers oxygen from the lungs to the tissues [1]. As a component of myoglobin, another protein that provides oxygen, iron supports muscle metabolism and healthy connective tissue [2]. Iron is also necessary for physical growth, neurological development, cellular functioning, and synthesis of some hormones [2,3].
Dietary iron has two main forms: heme and nonheme [1]. Plants and iron-fortified foods contain nonheme iron only, whereas meat, seafood, and poultry contain both heme and nonheme iron [2]. Heme iron, which is formed when iron combines with protoporphyrin IX, contributes about 10% to 15% of total iron intakes in western populations [3-5].
Most of the 3 to 4 grams of elemental iron in adults is in hemoglobin [2]. Much of the remaining iron is stored in the form of ferritin or hemosiderin (a degradation product of ferritin) in the liver, spleen, and bone marrow or is located in myoglobin in muscle tissue [1,5]. Transferrin is the main protein in blood that binds to iron and transports it throughout the body. Humans typically lose only small amounts of iron in urine, feces, the gastrointestinal tract, and skin. Losses are greater in menstruating women because of blood loss. Hepcidin, a circulating peptide hormone, is the key regulator of both iron absorption and the distribution of iron throughout the body, including in plasma [1,2,6].
The assessment of iron status depends almost entirely on hematological indicators [7]. However, these indicators are not sensitive or specific enough to adequately describe the full spectrum of iron status, and this can complicate the diagnosis of iron deficiency. A complementary approach is to consider how iron intakes from the diet and dietary supplements compare with recommended intakes.
Iron deficiency progresses from depletion of iron stores (mild iron deficiency), to iron-deficiency erythropoiesis (erythrocyte production), and finally to iron deficiency anemia (IDA) [8,9]. With iron-deficiency erythropoiesis (also known as marginal iron deficiency), iron stores are depleted and transferrin saturation declines, but hemoglobin levels are usually within the normal range. IDA is characterized by low hemoglobin concentrations, and decreases in hematocrit (the proportion of red blood cells in blood by volume) and mean corpuscular volume (a measure of erythrocyte size) [2,10].
Serum ferritin concentration, a measure of the body's iron stores, is currently the most efficient and cost-effective test for diagnosing iron deficiency [11-13]. Because serum ferritin decreases during the first stage of iron depletion, it can identify low iron status before the onset of IDA [7,9,14]. A serum ferritin concentration lower than 30 mcg/L suggests iron deficiency, and a value lower than 10 mcg/L suggests IDA [15]. However, serum ferritin is subject to influence by inflammation (due, for example, to infectious disease), which elevates serum ferritin concentrations [16].
Hemoglobin and hematocrit tests are the most commonly used measures to screen patients for iron deficiency, even though they are neither sensitive nor specific [5,7,17]. Often, hemoglobin concentrations are combined with serum ferritin measurements to identify IDA [7]. Hemoglobin concentrations lower than 11 g/dL in children under 10 years of age, or lower than 12 g/dL in individuals age 10 years or older, suggest IDA [8]. Normal hematocrit values are approximately 41% to 50% in males and 36% to 44% in females [18].
Intake recommendations for iron and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine (IOM) of the National Academies (formerly National Academy of Sciences) [5]. DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and gender, include the following:
Table 1 lists the current iron RDAs for nonvegetarians. The RDAs for vegetarians are 1.8 times higher than for people who eat meat. This is because heme iron from meat is more bioavailable than nonheme iron from plant-based foods, and meat, poultry, and seafood increase the absorption of nonheme iron [5].
The richest sources of heme iron in the diet include lean meat and seafood [19]. Dietary sources of nonheme iron include nuts, beans, vegetables, and fortified grain products. In the United States, about half of dietary iron comes from bread, cereal, and other grain products [2,3,5]. Breast milk contains highly bioavailable iron but in amounts that are not sufficient to meet the needs of infants older than 4 to 6 months [2,20].
Several food sources of iron are listed in Table 2. Some plant-based foods that are good sources of iron, such as spinach, have low iron bioavailability because they contain iron-absorption inhibitors, such as polyphenols [23,24].
Iron is available in many dietary supplements. Multivitamin/mineral supplements with iron, especially those designed for women, typically provide 18 mg iron (100% of the DV). Multivitamin/mineral supplements for men or seniors frequently contain less or no iron. Iron-only supplements usually deliver more than the DV, with many providing 65 mg iron (360% of the DV).
Frequently used forms of iron in supplements include ferrous and ferric iron salts, such as ferrous sulfate, ferrous gluconate, ferric citrate, and ferric sulfate [3,27]. Because of its higher solubility, ferrous iron in dietary supplements is more bioavailable than ferric iron [3]. High doses of supplemental iron (45 mg/day or more) may cause gastrointestinal side effects, such as nausea and constipation [5]. Other forms of supplemental iron, such as heme iron polypeptides, carbonyl iron, iron amino-acid chelates, and polysaccharide-iron complexes, might have fewer gastrointestinal side effects than ferrous or ferric salts [27].
The different forms of iron in supplements contain varying amounts of elemental iron. For example, ferrous fumarate is 33% elemental iron by weight, whereas ferrous sulfate is 20% and ferrous gluconate is 12% elemental iron [27]. Fortunately, elemental iron is listed in the Supplement Facts panel, so consumers do not need to calculate the amount of iron supplied by various forms of iron supplements.
Approximately 14% to 18% of Americans use a supplement containing iron [28,29]. Rates of use of supplements containing iron vary by age and gender, ranging from 6% of children age 12 to 19 years to 60% of women who are lactating and 72% of pregnant women [28,30].
Calcium might interfere with the absorption of iron, although this effect has not been definitively established [4,31]. For this reason, some experts suggest that people take individual calcium and iron supplements at different times of the day [32].
Some groups are at risk of obtaining excess iron. Individuals with hereditary hemochromatosis, which predisposes them to absorb excessive amounts of dietary iron, have an increased risk of iron overload [39]. One study suggests that elderly people are more likely to have chronic positive iron balance and elevated total body iron than iron deficiency. Among 1,106 elderly White adults age 67 to 96 years in the Framingham Heart Study, 13% had high iron stores (serum ferritin levels higher than 300 mcg/L in men and 200 mcg/L in women), of which only 1% was due to chronic disease [40]. The authors did not assess genotypes, so they could not determine whether these results were due to hemochromatosis [40].
Iron deficiency is not uncommon in the United States, especially among young children, women of reproductive age, and pregnant women. Because iron deficiency is associated with poor diet, malabsorptive disorders, and blood loss, people with iron deficiency usually have other nutrient deficiencies [2]. The World Health Organization (WHO) estimates that approximately half of the 1.62 billion cases of anemia worldwide are due to iron deficiency [41]. In developing countries, iron deficiency often results from enteropathies and blood loss associated with gastrointestinal parasites [2].
In 2002, the WHO characterized IDA as one of the 10 leading risk factors for disease around the world [42]. Although iron deficiency is the most common cause of anemia, deficiencies of other micronutrients (such as folate and vitamin B12) and other factors (such as chronic infection and inflammation) can cause different forms of anemia or contribute to their severity.
The functional deficits associated with IDA include gastrointestinal disturbances; weakness; fatigue; difficulty concentrating; and impaired cognitive function, immune function, exercise or work performance, and body temperature regulation [15,43]. In infants and children, IDA can result in psychomotor and cognitive abnormalities that, without treatment, can lead to learning difficulties [2,43]. Some evidence indicates that the effects of deficiencies early in life persist through adulthood [2]. Because iron deficiency is often accompanied by deficiencies of other nutrients, the signs and symptoms of iron deficiency can be difficult to isolate [2]. be457b7860
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