Table of Contents
Iron (Fe) is a metal essential to almost all bacteria, plants, and animals. In humans, iron is a compnent of the red pigment hemoglobin that gives red blood cells their color and affects the transport of oxygen throughout the body, conversion of nutrients into energy, production of new deoxyribonucleic acid (DNA, genetic material), and regulation of cell growth and cell differentiation. Without iron, life on Earth would not exist. Humans must acquire all the iron they need from diet.
(Illustration by GGS Information Services/Thomson Gale.)
Most iron in the body is used to transport oxygen. Oxygen is carried in red blood cells through the circulatory system to all cells in the body. Hemoglobin is the protein within red blood cells that makes this possible, and iron is at the center of the hemoglobin molecule. An average-size adult man has about 4 grams of iron in his body, and an adult woman has about 3.5 grams. Approximately two-thirds of this iron is in hemoglobin. Myoglobin, a protein in muscle, also contains iron. Myoglobin provides short-term storage for oxygen. When muscles do work, this oxygen is released to meet the increased metabolic needs of muscle cells.
Iron is found in every cell in the body, including brain cells. It is needed to synthesize adenosine tri-phosphate (ATP), the compound that supplies most of the energy to drive cellular metabolism. Iron is also used in enzyme reactions that create new DNA, and in this way it affects cell division and differentiation. Iron is also essential to other enzyme reactions that break down potentially harmful molecules formed when immune system cells attack bacteria.
Plants absorb iron from the earth, and humans acquire iron through eating both plants and animals. In the stomach, acid in gastric juice acts on iron and changes it into a form that the body can absorb. Absorption takes place mainly in the first part of the small intestine (the duodenum). Once iron is absorbed into the bloodstream, it binds to a protein called trans-ferrin and is carried to all parts of the body, including the bone marrow where new red blood cells are made. Once in the cells, some iron is transferred to ferritin, a protein that holds the iron in reserve. When too much iron is absorbed, there is not enough transferrin to bind all of it. Free iron can build up in cells and trigger activities that cause damage and create health problems. Too little iron interferes with the body’s ability to get enough oxygen.
Sources of iron
The body has complex mechanisms to achieve iron balance by regulating iron absorption, reuse, and storage processes. Red blood cells live about 120 days. When they die, most of the iron in hemoglobin is recycled in the liver and sent to the bone marrow where it reused in new red blood cells. As a result, humans lose only a small amount of iron daily.
Only about 10–20% of the iron in food, or 1–2 mg for every 10 mg eaten, is absorbed into the bloodstream. Under normal conditions, when iron stores in the body are low, more iron is automatically absorbed. When they are high, less is absorbed. Iron that is not absorbed enters cells that line the intestine. As these cells fill up with iron, they fall into the intestine and leave the body in waste.
Both plant and animal foods provide humans with iron, but that iron comes in two forms, heme and nonheme, that are not equally available to the body. Heme iron comes from hemoglobin. It is found mainly in animal tissue. Red meat is an especially rich source of heme iron. Only trace amounts of heme iron are found in plants. Heme iron is in a form that is easier for humans to use. It is absorbed at a higher rate than
nonheme iron, and its rate of absorption is less influenced by other foods that simultaneously are present in the digestive system.
The following list gives the approximate iron content for some common sources of heme iron:
- chicken liver, cooked, 3 ounces: 12.8 mg
- beef, cooked, 3 ounces: 3.2 mg
- turkey light meat, cooked, 3 ounces: 2.3 mg
- chicken dark meat, cooked, 3 ounces: 1.13 mg
- pork loin, cooked, 3 ounces: 0.8 mg
- oysters, 6 medium: 5.04 mg
- shrimp, cooked, 8 large: 1.36 mg
- tuna, light, canned, 3 ounces: 1.3 mg
- halibut, cooked, 3 ounces: 0.9 mg
- crab, cooked, 3 ounces: 0.8 mg
About 40–45% of iron in animal tissue and functionally all the iron in plants is nonheme iron. Nonheme iron is also the type of iron found in dietary supplements and added to iron-fortified foods. Nonheme iron is less easily used by humans; it must be changed in the digestive system before it can be absorbed. Only about 2-10% of nonheme iron in food is absorbed compared to 20-25% of heme iron. In addition, the absorption of nonheme iron is strongly influenced by other substances present in the digestive system. The ability of the body to absorb nonheme iron is decreased by the simultaneous presence of tea, coffee, dairy products, phytic acid (a substance found in grains, dried beans and rice), eggs, soy protein, and some chocolates. Absorption of nonheme iron is increased by the simultaneous presence of vitamin C, certain organic acids, and a small amount of meat, fish, or poultry, which boosts the absorption of nonheme iron as well as providing heme iron. Vegetarians and vegans should take into consideration the influence of other foods on iron absorption when planning meals.
The following list gives the approximate iron content for some common foods that contain nonheme iron:
- cereal, 100% iron fortified, 1 cup: 18 mg
- soybeans, boiled, 1 cup: 8.8 mg
- tofu, firm, 1/2 cup: 6.22 mg
- kidney beans, cooked, 1 cup: 5.2 mg
- lima beans, cooked, 1 cup: 4.5
- pinto beans, cooked, 1 cup: 3.6 mg
- blackstrap molasses, 1 tablespoon: 3.5 mg
- raisins, small box, 1.5 ounces: .89 mg
- potato, medium with skin: 2.75 mg
- cashew nuts, 1 ounce: 1.70 mg
- whole wheat bread, 1 slice: 0.9 mg
Normal iron requirements
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for vitamins and minerals. The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
Iron requirements vary substantially at different ages. Periods of rapid growth in children increase the need for iron. Women who menstruate need more iron because of blood loss during menstruation. Pregnancy puts high demands on the iron supply in the body because of increased production of red blood cells to supply the developing fetus. In 2001, the IOM set RDAs for iron based on preventing iron deficiency at each age. Iron passes into breast milk, and infants can meet their iron needs through breast milk or iron-fortified formula. RDAs and ULs for iron are measured in milligrams (mg).
The following list gives the daily RDAs and IAs and ULs for vitamin C for healthy individuals as established by the IOM.
- children birth-6 months: RDA 0.27 mg; UL not established
- children 7-12 months: RDA 11 mg; UL not established
- children 1-3 years: RDA 7 mg; UL 40 mg
- children 4-8 years: RDA 10 mg; UL 40 mg
- children 9-13 years: RDA 9 mg; UL 40 mg
- boys 14-18 years: RDA 11 mg; UL 45 mg
- girls 14-18 years: RDA 55 mg; UL 45 mg
- men age 19-50: RDA 8 mg; UL 45 mg
- women age 19-50: RDA 18 mg; UL 45 mg
- men who smoke: RDA 125 mg; UL 45 mg
- pregnant women: RDA 27 mg; UL 45 mg
- breastfeeding women 18 years and younger: RDA 10 mg; UL 45 mg
- breastfeeding women 19 years and older: RDA 9 mg; 45 mg
Pregnant women should consult their healthcare provider before the fifteenth week of pregnancy about the need for iron supplementation. They should not start taking an iron supplement on their own.
Men and women over age 55 are not at risk for iron deficiency and should take a multivitamin containing iron only on instructions from their healthcare provider.
People with kidney disease, liver damage, alcoholism, or ulcers should consult a healthcare professional before taking a supplement containing iron.
Iron interacts with many drugs and nutritional supplements. General categories of substances that may increase or decrease the amount of iron that is absorbed include medications that decrease stomach acidity (e.g. antacids, Tagamet, Zantac), pancreatic enzyme supplements, calcium supplements and dairy products, vitamin C, citric, malic, tartaric, and lactic acids, and copper.
The presence of iron also increases or decreases the effectiveness of many prescription drugs. Individuals should review their medications with a doctor or pharmacist when they begin taking an iron supplement to see if their other medications need adjustment.
The World Health Organization (WHO) considers iron deficiency to be the most widespread dietary disorder in the world. WHO estimates that up to 80% of the world’s population is iron deficient and up to 30% have iron deficiency anemia. The two main causes of iron deficiency are low dietary intake and excessive blood loss. In the United States, women of childbearing age, young children, people with diseases that interfere with the absorption of iron (e.g. Crohn’s disease, celiac disease), and people receiving kidney dialysis are most likely to seriously be iron deficient. American men rarely have low levels of iron because the tend to eat more meat than women and do not lose blood through menstruation.
At first, the body is able to use stored iron to make up for an iron deficit, but over time, the amount of hemoglobin decreases and a condition called iron deficiency anemia develops. (This is only one type of anemia; other anemias have other causes.) Iron deficiency anemia decreases the amount of oxygen reaching cells in the body. Symptoms of iron deficiency anemia include:
- lack of energy
- feelings of weakness
- frequently feeling cold
- increased infections
- decreased work or school performance
- sore swollen tongue
- drive to eat dirt, clay or other non-food substances (pica)
The preferred way to treat mild iron deficiency is through changes in diet. If these changes are ineffective, iron supplements may be used. Dietary supplements contain different formulations such as ferrous fumarate, ferrous sulfate, and ferrous gluconate. Iron in these different formulations is absorbed at differing rates. Because too much iron can cause serious health problems, iron supplements should be taken under the supervision of a healthcare professional.
People who have many blood transfusions can also develop iron overload, but by far the most common cause of excess iron is accidental poisoning. Over 20,000 American children accidentally ingest iron— usually in the form of dietary supplements—each year. Iron poisoning is the leading cause of poisoning deaths in children under age 6 in the United States. Iron overdose is a medical emergency. Symptoms occurring within the first 12 hours include nausea, vomiting, abdominal pain, black stool, weakness, rapid pulse, low blood pressure, fever, difficulty breathing, and coma. If death does not occur within the first 12 hours, damage to the kidney liver damage, cardiovascular system and nervous system may appear within two days. Long-term damage to survivors of iron poisoning include cirrhosis (liver damage), permanent central nervous system damage, and stomach problems.
Parents should be aware that the RDA and UL for vitamins and minerals are much lower for children than for adults. Accidental overdose may occur if children are give adult vitamins or dietary supplements. Accidental iron overdose is a leading cause of poisoning deaths in young children. Parents should keep all dietary supplements away from children, just as they would other medicines.
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Tish Davidson, A.M.