Iron

Iron

SOURCE: U.S. Office of Dietary Supplements. “Iron: Fact Sheet for Health Professionals.” National Institutes of Health. U.S. Department of Health and Human Services. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional (accessed April 4, 2018).

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 iron-bearing, oxygen-carrying protein within red blood cells that makes this possible, and iron is at the center of the hemoglobin molecule. An average-sized adult male has about 4 grams of iron in his body, and an adult female has about 3.5 grams. Approximately two-thirds of this iron is found in hemoglobin. Myoglobin, a protein in muscle, also contains iron. Myoglobin provides short-term storage for oxygen. When muscles expend energy to perform their work, the stored 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 triphosphate (ATP), the compound that supplies most of the energy for cellular metabolism. Iron is also used to facilitate the enzyme reactions that create new DNA, which also affects cell division and differentiation. Iron also is essential to other enzyme reactions that break down potentially harmful molecules formed when immune system cells attack bacteria or other toxins.

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 transferrin 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 changed 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, conversely, can interfere with the body's ability to get enough oxygen.

The body has complex mechanisms to achieve iron balance by regulating iron absorption, reuse, and storage processes. Red blood cells live for 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 is 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 stores 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.

The iron found in plant and animal foods comes in two forms, heme and nonheme, which are not assimilated equally by 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 are present in the digestive system simultaneously.

The approximate iron content for some common sources of heme iron is:

The following list gives the approximate iron content for some common foods that contain nonheme iron:

The Food and Nutrition Board (FNB) at the U.S. Institute of Medicine (IOM) of the National Academies (formerly the National Academy of Science) has developed dietary reference intakes (DRIs) for vitamins and minerals. DRIs are a standard set of reference values for intake of nutrients by healthy people, including: recommended dietary allowance (RDA), adequate intake (AI), and tolerable upper intake level (UL). RDAs define the average daily amount of the nutrient needed to meet the health needs of 97%– 98% of the population. AI is an estimate determined when not enough information is available to determine an RDA. UL is the average maximum daily intake unlikely to cause adverse effects. 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. Athletes, female athletes especially, who participate in endurance sports are at increased risk of iron deficiency because iron is lost through exercise-induced mechanisms associated with endurance sports, particularly during intensive training and competition.

The RDAs for iron are based on maintaining iron status and preventing iron deficiency in different age groups. Iron passes into breast milk, and the iron needs of infants can be met through breast milk or iron-fortified formula. RDAs and ULs for iron are measured in milligrams (mg).

The following list established by the FNB gives the daily RDAs, AIs, and ULs for iron for healthy individuals according to age groups.

Pregnant women are advised to consult their healthcare providers before the 15th week of pregnancy about the need for iron supplementation. Because iron overload can produce unwanted reactions, it is not advisable to take an iron supplement without understanding whether iron stores are low and supplementation is necessary.

Men and women over age 55 are not at increased risk for iron deficiency, and multivitamins containing iron are not always needed. Iron should only be taken on instruction from a healthcare provider when iron deficiency has been noted through laboratory tests.

People with kidney disease, liver damage, alcoholism, or ulcers are also advised to consult a healthcare professional before taking a supplement containing iron.

Iron interacts with many drugs and nutritional supplements. General categories of substances that may affect the amount of iron that is absorbed include:

The presence of iron also alters 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 to be adjusted.

The World Health Organization (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 iron intake and excessive blood loss. Deficiency is less common in the United States, but 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 have serious iron deficiency. Hemoglobin is reduced significantly as a result of dialysis. Men in the United States rarely have low levels of iron because they tend to eat more meat than women and do not lose blood through menstruation.

The body is able to use stored iron to compensate for low iron status, but over time, iron deficiency anemia may still develop in some individuals when not enough healthy red blood cells with adequate hemoglobin are able to carry oxygen throughout the body. The causes of iron deficiency anemia include blood loss, insufficient iron intake, or inadequate assimilation of iron from dietary sources. Symptoms of iron deficiency anemia include:

Mild to moderate iron deficiency can be corrected by increasing food sources of iron in diet or by taking supplements. Iron may be given by injection in some cases where the individual's response to dietary changes and supplements is poor. Dietary supplements are available in different formulations, such as ferrous fumarate, ferrous sulfate, and ferrous gluconate. Iron in these different formulations is absorbed at different rates. Vitamin C taken with iron supplements sometimes helps the iron assimilate more easily. Because too much iron can cause iron overload and serious health problems, individuals are advised to take iron supplements only under the supervision of a healthcare professional.

Iron overload can happen through excess iron intake when no deficiency is present, but its occurrence is rare. Iron overload is also caused by an inherited disorder called hereditary hemochromatosis. This disorder affects as many as one in every 200 people of northern European descent. People affected by hereditary hemochromatosis have a genetic mutation that causes them to absorb iron from the intestine at a rate far higher than normal. The condition is treated by avoiding ironrich foods and removing blood (usually through blood donation) from the individual on a regular basis.

People who have many blood transfusions may also develop iron overload, but by far the most common cause of excess iron is accidental poisoning. Over 20,000 children accidentally ingest iron each year in the United States, usually in the form of dietary supplements. 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, cardiovascular system, and nervous system may appear within two days. Examples of longterm damage to survivors of iron poisoning include cirrhosis (liver damage), permanent central nervous system damage, and stomach problems.

Parents are advised to be aware that the RDA and UL for vitamins and minerals are much lower for children than for adults. Accidental overdose or overload may occur if children take adult vitamins or dietary supplements. All dietary supplements must be kept out of reach of children, as with any other medications.

See also Adolescent nutrition ; Breastfeeding ; Celiac disease ; Childhood nutrition ; Crohn's disease ; Dietary reference intakes (DRIs) ; Infant nutrition .