What Is Iron?

Highlights

• Iron is an essential mineral for transporting oxygen in our bodies and is also vital for growth and development, hormone synthesis, and muscle metabolism.
• Meat, seafood, poultry, iron-fortified breakfast cereals, white beans, and lentils are some examples of food sources of iron.
• When the levels of iron stored in the body become low, iron-deficiency anemia can occur, resulting in blood carrying less oxygen from our lungs to organs and tissues throughout the body.
• Certain groups of people, such as pregnant women and premature or low-birth weight infants, tend to have a higher risk for iron-deficiency anemia.

The Basics

Iron is a mineral that is necessary for our bodies’ growth and development. In particular, our bodies use iron to create certain hormones and to make hemoglobin and myoglobin. Hemoglobin is a protein in red blood cells that carries oxygen from our lungs to the body’s tissues, and myoglobin is a protein that provides oxygen to our muscles.

Iron is naturally found in a variety of foods, such as lean meat, seafood, lentils, and spinach. Iron can also be added to food products through the process of fortification and additionally is available as a dietary supplement. The iron in our food has two main forms—heme iron and nonheme iron. Plant-based and iron-fortified foods only contain nonheme iron, whereas animal-derived foods like meat, seafood, and poultry contain both heme and nonheme iron.

Iron and Health

Inadequate intake of iron does not cause obvious symptoms in the short-term because our bodies use stored iron from our muscles, liver, spleen, and bone marrow. However, when the levels of iron stored in the body become low, this can progress to iron-deficiency anemia (IDA). IDA is characterized by low hemoglobin concentrations, a decreased proportion of red blood cells in blood by volume, and a lower average red blood cell size. Symptoms of IDA include gastrointestinal upset, weakness, and problems with concentration and memory, and people with IDA are less able to fight off infections, to work and exercise efficiently, and to control their body temperature well.

The remainder of this section will specifically focus on the role of iron and IDA in pregnant women, infants, and toddlers, as well as people with anemia of chronic disease.

• Pregnant women: The amount of blood in a woman’s body increases during pregnancy, which increases the amount of iron needed for both the pregnant woman and the fetus. Insufficient iron intake during pregnancy not only raises a woman’s risk of IDA, but also increases her future infant’s risk of low birthweight, premature birth, low iron stores, and impaired cognitive and behavioral development. Randomized controlled trials have demonstrated that iron supplementation can prevent IDA in pregnant women and can also prevent associated adverse consequences in infants. People who are pregnant or who may wish to become pregnant should talk to their health care provider about dietary supplement use, including iron supplementation.
• Infants: IDA during infancy can lead to adverse effects on neurodevelopment and is also associated with higher lead concentrations in the blood. Full-term infants have adequate iron stores for the first four to six months of life, but low-birthweight and preterm infants are at risk of iron deficiency at birth due to their lower iron stores. By age 6 months, however, an infant exclusively fed human milk will require an external source of iron apart from human milk. A complementary food source of iron beginning at about 6 months is particularly important for infants fed human milk because the iron content of human milk is low and a mother’s intake of iron during breastfeeding does not increase its content. Caregivers of infants exclusively fed human milk should talk with their pediatric care provider about whether there may be a need for supplementation with iron before age 6 months. The American Academy of Pediatrics recommends giving exclusively-human milk fed babies 1 mg/kilogram/day of a liquid oral iron supplement until iron-containing foods are introduced at about 6 months of age. Babies who consume infant formula should use an iron-fortified formula from birth through 12 months.
• Anemia of chronic disease: Certain chronic diseases can cause anemia of chronic disease, which is the second most-common type of anemia after IDA. These diseases—including rheumatoid arthritis, inflammatory bowel disease, and some cancers—ultimately divert iron from blood circulation to storage sites, limiting the amount of iron available for red blood cell production. The main therapy for this type of anemia is treatment of the underlying disease; iron supplementation is sometimes used when treatment is not possible, but its usage in this case is controversial because it may increase the risk of infection and cardiovascular events, and could also lead to tissue damage.

Recommended Intakes

The National Academies of Sciences, Engineering and Medicine have set dietary reference intakes (DRIs) for iron. These recommended dietary allowances (RDAs) differ by age, gender, and life stage (e.g., pregnancy). Note that the RDAs listed in Table 1 are for nonvegetarians; the RDAs for vegetarians are 1.8 times higher than for people who eat meat. This difference is because the body does not absorb nonheme iron in plant foods as well as heme iron found in animal foods. Additionally, the Food and Nutrition Board (FNB) at the Institute of Medicine at the National Academies has established an adequate intake (AI) for iron for infants from birth to six months that is equivalent to the mean intake of iron in healthy, breastfed infants.

Table 1. Recommended Dietary Allowances (RDAs) for Iron

*Adequate Intake (AI)

Table Source: The National Academies of Sciences, Engineering and Medicine

Certain groups are more likely to have inadequate intakes of iron, including pregnant women, infants, and young children; women with heavy menstrual bleeding; frequent blood donors; people with cancer or gastrointestinal disorders; and people who have had gastrointestinal surgery or heart failure.

Consequences of Excessive Iron Intake

While iron is essential for our bodies, getting too much of it can also have adverse effects. It is unlikely that a healthy person could consume too much iron from foods alone. However, taking high doses of iron supplements can cause symptoms such as constipation, nausea, and diarrhea. High doses of iron may cause inflammation of the stomach lining and ulcers, and iron can also decrease zinc absorption. Extremely high doses of iron can lead to organ failure, coma, convulsions, and death. Dietary supplements that contain iron should also be stored away from young children; these supplements have a warning statement on the label that they should be kept out of the reach of children, as accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under age 6. Additionally, those who have an inherited condition called hemochromatosis should avoid using iron and vitamin C supplements, because this condition causes toxic levels of iron to build up in their bodies.

It is also important to keep in mind that iron can interact with several medications and other dietary supplements, which can reduce the effectiveness of a medication or cause interference with iron absorption. As such, it’s always beneficial to talk to your doctor or pharmacist about other supplements or medications you may be taking.

Tolerable Upper Intake Levels (ULs) for iron from food and supplements have been established by the FNB based on the amounts of iron associated with gastrointestinal effects following supplemental intakes of iron salts. Note that a doctor might prescribe more than the upper limit to treat iron deficiency.

Table 2. Tolerable Upper Intake Levels (ULs) for Iron

Table Source: The National Academies of Sciences, Engineering and Medicine

Sources of Iron

Iron can be found in a variety of foods, including lean meat, seafood, poultry, white beans, lentils, spinach, kidney beans, peas, nuts, and dried fruits like raisins. While iron can be found in a variety of animal and plant-based foods, note that lean meat and seafood are the richest dietary sources of heme iron; nuts, beans, vegetables and fortified grain products are dietary sources of nonheme iron. Additionally, wheat and other flours are fortified with iron in the U.S., making iron-fortified breakfast cereals and breads options for getting enough iron. Infant formula is also fortified with iron. Heme iron is more bioavailable – meaning it can be more efficiently digested, absorbed and metabolized – than nonheme iron. The bioavailability of nonheme iron can be enhanced by concurrently consuming ascorbic acid (vitamin C) or heme iron sources.

Iron is also available in various dietary supplements, including multivitamin or multimineral supplements, particularly those designed for women. Multivitamin or multimineral supplements for men and seniors typically contain less or no iron.

Table 3. Food Sources of Iron