Iron
Description
Iron constitutes about 0.004% of the body weight. Iron is found primarily in four forms in the body. The majority of bodily iron exists in hemoglobin molecules. About 20% of total body iron is bound to the iron storage protein ferritin in the liver, spleen, and bone marrow. Iron is also found bound to the iron transport protein transferrin. About 5% of bodily iron is found in the respiratory pigment of the muscle, myoglobin, and various respiratory enzymes, which catalyze oxidation-reduction processes within the cell.
The oxidative-reductive properties of iron in hemoglobin and myoglobin allow it to be effective in oxygen and carbon dioxide transportation processes that occur during respiration.
Of all the dietary iron ingested, only about 10-30% is absorbed. Absorption occurs primarily in the duodenum, and excretion of excess, or unused iron, is via feces.
Method of Action
Within the mucosal cells of the intestine, iron is oxidized and bound to transferrin. Transferrin then transports the iron to body tissues (e.g., liver and spleen) and to the bone marrow for hemoglobin synthesis. Iron is stored in the liver as ferritin.
Iron plays an essential role in respiration as a constituent of hemoglobin and myoglobin.
Hemoglobin consists of four protein subunits and an iron-carrying heme group. Oxygen becomes loosely associated with the iron of the heme group changing the tertiary and quarternary structure of the subunits. This change of hemoglobin's three dimensional structure increases the binding affinity of surrounding hemoglobin molecules in the immediate vicinity.
Under various conditions where high oxygen releasing efficiency is necessary, such as at high altitudes, the body releases the chemical 2,3-diphosphoglycerate, which decreases the oxygen binding capacity of hemoglobin. This allows oxygen to be delivered with higher efficiency to body tissues for increased cellular respiration. At lower altitudes, with higher oxygen concentrations, levels of 2,3-diphosphoglycerate decrease and hemoglobin binds oxygen with increased efficiency. Hemoglobin releases carbon dioxide with greater efficiency under these conditions, hence oxygen uptake and carbon dioxide release in the lungs is greatly facilitated.
Myoglobin, another respiratory pigment, found primarily in muscle tissue and has properties and functions very similar to those of hemoglobin.
There are two dietary forms of iron: heme and nonheme. Nonheme iron is the predominant form of iron ingested by humans, but is very poorly absorbed by the intestine. It constitutes most of the iron found in grains and vegetables, and about three-fifths of the iron found in meats. Heme iron, found in meat and animal products, is less prevalent but more effectively absorbed by the small intestine. Vitamin C, animal proteins, calcium, and acidic conditions within the intestine tend to promote iron absorption. Alkalinity, bran, phosphates, and severe infection or malabsorption can impair iron absorption.
Properties & Uses
Iron supplementation is effective in correcting hypochromic microcytic anemia which characterized by unusually small, pale red blood cells. Preferred forms of supplements are ferrous sulfate and ferrous gluconate.
Iron supplements may be advised for growing adolescents, especially girls at the onset of menses, and after pregnancy, when blood loss during delivery causes a reduction in iron stores.
However, iron supplementation can result in gastrointestinal problems leading to diarrhea, constipation, and nausea. Vitamin C is recommended as a co-supplement to enhance absorption. Supplements should be taken before meals.
Consequence of Deficiency
Iron deficiencies can result in anemia. Anemia also tends to occur as a result of impaired absorption, blood loss, or repeated pregnancy.
Iron deficient anemia is categorized into three stages. In the first stage, ferritin and hemosiderin (storage forms of iron) are depleted, resulting in increased iron absorption from foodstuffs. Transferrin, the iron-binding plasma protein, undergoes increased iron-binding capacity. The second stage is characterized by a depletion of iron levels. In the third stage, hemoglobin levels decrease and anemia is provoked.
Anemia can be assessed by a simple red blood cell count. Nutritional anemia, caused by inadequate iron intake and/or absorption, can result in a lack of pigment hemoglobin (hypochromia) and the development of abnormally small red blood cells (microcytia). Symptoms include pale pigmentation and fatigue.
Causes of iron deficient anemia include deficiencies of copper, vitamin B-6 and vitamin B-12, inadequate protein intake, hookworms, antacids, and various gastrointestinal disorders (e.g., diarrhea, intestinal disease).
Latent iron deficiency, a disorder arising from inadequate iron supplies, can be a result of fever or chronic infection.
Toxicity Factors
Iron overload can be the result of excessive consumption of iron medication, parenterally administered iron, or unusually high gastrointestinal absorption, which can be caused by chronic alcoholism, liver disease, or a genetic disorder known as idiopathic hemochromatosis.
Idiopathic hemochromatosis is manifested as an increased absorption of iron. Symptoms include the development of portal cirrhosis, bronze pigmentation, diabetes mellitus, and myocardial failure.
Excess iron is initially deposited in the liver and, upon saturation, becomes redistributed to the pancreas and heart muscles. Liver saturation is referred to as hemosiderosis and, with prolonged excessive iron intake, can result in increased susceptibility to infection. This condition, if untreated, may result in tissue damage, especially in the liver.
Recommended Dietary Allowance
| age | RDA (mg) | RNI (mg) | ||
| infants/children | ||||
| 0-3 months | 6 | 1.7 | ||
| 3-6 months | 6 | 4.3 | ||
| 6-12 months | 10 | 7.8 | ||
| 1-3 years | 10 | 6.9 | ||
| 4-6 years | 10 | 6.1 | ||
| 7-10 years | 10 | 8.7 | ||
| males | ||||
| 11-18 years | 12 | 11.3 | ||
| 19-50 years | 10 | 8.7 | ||
| 51+ years | 10 | 8.7 | ||
| females | ||||
| 11-50 years | 15 | 14.3 | ||
| 50+ years | 10 | 8.7 | ||
| pregnant | 30 | - | ||
| lactating | 15 | - |
For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.
Food Sources
| Almond | Apricot |
| Beef liver | Bran flakes |
| Brazil nut | Brewer's yeast |
| Chocolate | Clams |
| Hot cocoa | Egg yolk |
| Green pea | Hazelnuts |
| Lentil | Oysters |
| Oatmeal | Parsley |
| Peach | Nuts |
| Raisins | Shellfish |
| Soybeans | Turkey |
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