Iron

Iron: What is it?

Iron, one of the most abundant metals on Earth, is essential to most life forms and to normal human physiology. Iron is an integral part of many proteins and enzymes that maintain good health. In humans, iron is an essential component of proteins involved in oxygen transport. It is also essential for the regulation of cell growth and differentiation. A deficiency of iron limits oxygen delivery to cells, resulting in fatigue, poor work performance, and decreased immunity. On the other hand, excess amounts of iron can result in toxicity and even death.

Almost two-thirds of iron in the body is found in hemoglobin, the protein in red blood cells that carries oxygen to tissues. Smaller amounts of iron are found in myoglobin, a protein that helps supply oxygen to muscle, and in enzymes that assist biochemical reactions. Iron is also found in proteins that store iron for future needs and that transport iron in blood. Iron stores are regulated by intestinal iron absorption.

What foods provide iron?

There are two forms of dietary iron: heme and nonheme. Heme iron is derived from hemoglobin, the protein in red blood cells that delivers oxygen to cells. Heme iron is found in animal foods that originally contained hemoglobin, such as red meats, fish, and poultry. Iron in plant foods such as lentils and beans is arranged in a chemical structure called nonheme iron. This is the form of iron added to iron-enriched and iron-fortified foods. Heme iron is absorbed better than nonheme iron, but most dietary iron is nonheme iron. A variety of heme and nonheme sources of iron are listed in Tables 1 and 2.

Table 1: Selected Food Sources of Heme Iron
Food Milligrams
per serving
% DV*
Chicken liver, pan-fried, 3 ounces 11.0 61
Oysters, canned, 3 ounces 5.7 32
Beef liver, pan-fried, 3 ounces 5.2 29
Beef, chuck, blade roast, lean only, braised, 3 ounces 3.1 17
Turkey, dark meat, roasted, 3 ounces 2.0 11
Beef, ground, 85% lean, patty, broiled, 3 ounces 2.2 12
Beef, top sirloin, steak, lean only, broiled, 3 ounces 1.6 9
Tuna, light, canned in water, 3 ounces 1.3 7
Turkey, light meat, roasted, 3 ounces 1.1 6
Chicken, dark meat, meat only, roasted, 3 ounces 1.1 6
Chicken, light meat, meat only, roasted, 3 ounces 0.9 5
Tuna, fresh, yellowfin, cooked, dry heat, 3 ounces 0.8 4
Crab, Alaskan king, cooked, moist heat, 3 ounces 0.7 4
Pork, loin chop, broiled, 3 ounces 0.7 4
Shrimp, mixed species, cooked, moist heat, 4 large 0.3 2
Halibut, cooked, dry heat, 3 ounces 0.2 1
Table 2: Selected Food Sources of Nonheme Iron
Food Milligrams
per serving
% DV*
Ready-to-eat cereal, 100% iron fortified, ¾ cup 18.0 100
Oatmeal, instant, fortified, prepared with water, 1 packet 11.0 61
Soybeans, mature, boiled, 1 cup 8.8 48
Lentils, boiled, 1 cup 6.6 37
Beans, kidney, mature, boiled, 1 cup 5.2 29
Beans, lima, large, mature, boiled, 1 cup 4.5 25
Ready-to-eat cereal, 25% iron fortified, ¾ cup 4.5 25
Blackeye peas, (cowpeas), mature, boiled, 1 cup 4.3 24
Beans, navy, mature, boiled, 1 cup 4.3 24
Beans, black, mature, boiled, 1 cup 3.6 20
Beans, pinto, mature, boiled, 1 cup 3.6 21
Tofu, raw, firm, ½ cup 3.4 19
Spinach, fresh, boiled, drained, ½ cup 3.2 18
Spinach, canned, drained solids ½ cup 2.5 14
Spinach, frozen, chopped or leaf, boiled ½ cup 1.9 11
Raisins, seedless, packed, ½ cup 1.6 9
Grits, white, enriched, quick, prepared with water, 1 cup 1.5 8
Molasses, 1 tablespoon 0.9 5
Bread, white, commercially prepared, 1 slice 0.9 5
Bread, whole-wheat, commercially prepared, 1 slice 0.7 4

*DV = Daily Value. DVs are reference numbers developed by the Food and Drug Administration (FDA) to help consumers determine if a food contains a lot or a little of a specific nutrient. The DV for iron is 18 milligrams (mg). A food providing 5% of the DV or less is a low source while a food that provides 10–19% of the DV is a good source. A food that provides 20% or more of the DV is high in that nutrient. It is important to remember that foods that provide lower percentages of the DV also contribute to a healthful diet.

What affects iron absorption?

Iron absorption refers to the amount of dietary iron that the body obtains and uses from food. Healthy adults absorb about 10% to 15% of dietary iron, but individual absorption is influenced by several factors.

Storage levels of iron have the greatest influence on iron absorption. Iron absorption increases when body stores are low. When iron stores are high, absorption decreases to help protect against toxic effects of iron overload. Iron absorption is also influenced by the type of dietary iron consumed. Absorption of heme iron from meat proteins is efficient. Absorption of heme iron ranges from 15% to 35%, and is not significantly affected by diet. In contrast, 2% to 20% of nonheme iron in plant foods such as rice, maize, black beans, soybeans and wheat is absorbed. Nonheme iron absorption is significantly influenced by various food components.

Meat proteins and vitamin C will improve the absorption of nonheme iron. Tannins (found in tea), calcium, polyphenols, and phytates (found in legumes and whole grains) can decrease absorption of nonheme iron. Some proteins found in soybeans also inhibit nonheme iron absorption. It is most important to include foods that enhance nonheme iron absorption when daily iron intake is less than recommended, when iron losses are high (which may occur with heavy menstrual losses), when iron requirements are high (as in pregnancy), and when only vegetarian nonheme sources of iron are consumed.

What is the recommended intake for iron?

Recommended Dietary Allowances (RDA), Adequate Intakes (AI), andTolerable Upper Intake Levels (UL).

Recommended Dietary Allowances for Iron for Infants 
Age Males
(mg/day)
Females
(mg/day)
Pregnancy
(mg/day)
Lactation
(mg/day)
7 to 12 months 11 11 N/A N/A
1 to 3 years 7 7 N/A N/A
4 to 8 years 10 10 N/A N/A
9 to 13 years 8 8 N/A N/A
14 to 18 years 11 15 27 10
19 to 50 years 8 18 27 9
51+ years 8 8 N/A N/A

Healthy full term infants are born with a supply of iron that lasts for 4 to 6 months. There is not enough evidence available to establish a RDA for iron for infants from birth through 6 months of age. Recommended iron intake for this age group is based on an Adequate Intake (AI) that reflects the average iron intake of healthy infants fed breast milk.

Iron in human breast milk is well absorbed by infants. It is estimated that infants can use greater than 50% of the iron in breast milk as compared to less than 12% of the iron in infant formula. The amount of iron in cow’s milk is low, and infants poorly absorb it. Feeding cow’s milk to infants also may result in gastrointestinal bleeding. For these reasons, cow’s milk should not be fed to infants until they are at least 1 year old.

Iron intake is negatively influenced by low nutrient density foods, which are high in calories but low in vitamins and minerals. Sugar sweetened sodas and most desserts are examples of low nutrient density foods, as are snack foods such as potato chips.

When can iron deficiency occur?

The World Health Organization considers iron deficiency the number one nutritional disorder in the world. As many as 80% of the world’s population may be iron deficient, while 30% may have iron deficiency anemia.

Iron deficiency develops gradually and usually begins with a negative iron balance, when iron intake does not meet the daily need for dietary iron.

Iron deficiency anemia can be associated with low dietary intake of iron, inadequate absorption of iron, or excessive blood loss. Women of childbearing age, pregnant women, preterm and low birth weight infants, older infants and toddlers, and teenage girls are at greatest risk of developing iron deficiency anemia because they have the greatest need for iron. Women with heavy menstrual losses can lose a significant amount of iron and are at considerable risk for iron deficiency. Adult men and post-menopausal women lose very little iron, and have a low risk of iron deficiency.

Vitamin A helps mobilize iron from its storage sites, so a deficiency of vitamin A limits the body’s ability to use stored iron. This results in an “apparent” iron deficiency because hemoglobin levels are low even though the body can maintain normal amounts of stored iron.

Signs of iron deficiency anemia include :

  • feeling tired and weak
  • decreased work and school performance
  • slow cognitive and social development during childhood
  • difficulty maintaining body temperature
  • decreased immune function, which increases susceptibility to infection
  • glossitis (an inflamed tongue)

Eating nonnutritive substances such as dirt and clay, often referred to as pica or geophagia, is sometimes seen in persons with iron deficiency. There is disagreement about the cause of this association.

Who may need extra iron to prevent a deficiency?

Three groups of people are most likely to benefit from iron supplements: people with a greater need for iron, individuals who tend to lose more iron, and people who do not absorb iron normally. These individuals include:

  • pregnant women
  • preterm and low birth weight infants
  • older infants and toddlers
  • teenage girls
  • women of childbearing age, especially those with heavy menstrual losses
  • people with renal failure, especially those undergoing routine dialysis
  • people with gastrointestinal disorders who do not absorb iron normally

Total dietary iron intake in vegetarian diets may meet recommended levels; however that iron is less available for absorption than in diets that include meat. Vegetarians who exclude all animal products from their diet may need almost twice as much dietary iron each day as non-vegetarians because of the lower intestinal absorption of nonheme iron in plant foods. Vegetarians should consider consuming nonheme iron sources together with a good source of vitamin C, such as citrus fruits, to improve the absorption of nonheme iron.

Does pregnancy increase the need for iron?

Nutrient requirements increase during pregnancy to support fetal growth and maternal health. Iron requirements of pregnant women are approximately double that of non-pregnant women because of increased blood volume during pregnancy, increased needs of the fetus, and blood losses that occur during delivery.

Low levels of hemoglobin and hematocrit may indicate iron deficiency. Hemoglobin is the protein in red blood cells that carries oxygen to tissues. Hematocrit is the proportion of whole blood that is made up of red blood cells. Nutritionists estimate that over half of pregnant women in the world may have hemoglobin levels consistent with iron deficiency.

Some facts about iron supplements

Iron supplementation is indicated when diet alone cannot restore deficient iron levels to normal within an acceptable timeframe. Supplements are especially important when an individual is experiencing clinical symptoms of iron deficiency anemia. The goals of providing oral iron supplements are to supply sufficient iron to restore normal storage levels of iron and to replenish hemoglobin deficits. When hemoglobin levels are below normal, physicians often measure serum ferritin, the storage form of iron. A serum ferritin level less than or equal to 15 micrograms per liter confirms iron deficiency anemia in women, and suggests a possible need for iron supplementation.

Supplemental iron is available in two forms: ferrous and ferric. Ferrous iron salts (ferrous fumarate, ferrous sulfate, and ferrous gluconate) are the best absorbed forms of iron supplements .

The amount of iron absorbed decreases with increasing doses. For this reason, it is recommended that most people take their prescribed daily iron supplement in two or three equally spaced doses.

Therapeutic doses of iron supplements, which are prescribed for iron deficiency anemia, may cause gastrointestinal side effects such as nausea, vomiting, constipation, diarrhea, dark colored stools, and/or abdominal distress .

Who should be cautious about taking iron supplements?

Iron deficiency is uncommon among adult men and postmenopausal women. These individuals should only take iron supplements when prescribed by a physician because of their greater risk of iron overload. Iron overload is a condition in which excess iron is found in the blood and stored in organs such as the liver and heart. Iron overload is associated with several genetic diseases including hemochromatosis, which affects approximately 1 in 250 individuals of northern European descent. Individuals with hemochromatosis absorb iron very efficiently, which can result in a build up of excess iron and can cause organ damage such as cirrhosis of the liver and heart failure. Hemochromatosis is often not diagnosed until excess iron stores have damaged an organ. Iron supplementation may accelerate the effects of hemochromatosis, an important reason why adult men and postmenopausal women who are not iron deficient should avoid iron supplements. Individuals with blood disorders that require frequent blood transfusions are also at risk of iron overload and are usually advised to avoid iron supplements.

What are some current issues and controversies about iron?

Iron and heart disease

Because known risk factors cannot explain all cases of heart disease, researchers continue to look for new causes. Some evidence suggests that iron can stimulate the activity of free radicals. Free radicals are natural by-products of oxygen metabolism that are associated with chronic diseases, including cardiovascular disease. Free radicals may inflame and damage coronary arteries, the blood vessels that supply the heart muscle. This inflammation may contribute to the development of atherosclerosis, a condition characterized by partial or complete blockage of one or more coronary arteries. Other researchers suggest that iron may contribute to the oxidation of LDL (“bad”) cholesterol, changing it to a form that is more damaging to coronary arteries.

As far back as the 1980s, some researchers suggested that the regular menstrual loss of iron, rather than a protective effect from estrogen, could better explain the lower incidence of heart disease seen in pre-menopausal women. After menopause, a woman’s risk of developing coronary heart disease increases along with her iron stores. Researchers have also observed lower rates of heart disease in populations with lower iron stores, such as those in developing countries. In those geographic areas, lower iron stores are attributed to low meat (and iron) intake, high fiber diets that inhibit iron absorption, and gastrointestinal (GI) blood (and iron) loss due to parasitic infections.

Conflicting results, and different methods to measure iron stores, make it difficult to reach a final conclusion on this issue. However, researchers know that it is feasible to decrease iron stores in healthy individual through phlebotomy (blood letting or donation). Using phlebotomy, researchers hope to learn more about iron levels and cardiovascular disease.

Iron and intense exercise

Many men and women who engage in regular, intense exercise such as jogging, competitive swimming, and cycling have marginal or inadequate iron status. Possible explanations include increased gastrointestinal blood loss after running and a greater turnover of red blood cells. Also, red blood cells within the foot can rupture while running. For these reasons, the need for iron may be 30% greater in those who engage in regular intense exercise.

Three groups of athletes may be at greatest risk of iron depletion and deficiency: female athletes, distance runners, and vegetarian athletes. It is particularly important for members of these groups to consume recommended amounts of iron and to pay attention to dietary factors that enhance iron absorption. If appropriate nutrition intervention does not promote normal iron status, iron supplementation may be indicated. In one study of female swimmers, researchers found that supplementation with 125 milligrams (mg) of ferrous sulfate per day prevented iron depletion. These swimmers maintained adequate iron stores, and did not experience the gastrointestinal side effects often seen with higher doses of iron supplementation.

Iron and mineral interactions

Some researchers have raised concerns about interactions between iron, zinc, and calcium. When iron and zinc supplements are given together in a water solution and without food, greater doses of iron may decrease zinc absorption. However, the effect of supplemental iron on zinc absorption does not appear to be significant when supplements are consumed with food.There is evidence that calcium from supplements and dairy foods may inhibit iron absorption, but it has been very difficult to distinguish between the effects of calcium on iron absorption versus other inhibitory factors such as phytate.

What is the risk of iron toxicity?

There is considerable potential for iron toxicity because very little iron is excreted from the body. Thus, iron can accumulate in body tissues and organs when normal storage sites are full. For example, people with hemachromatosis are at risk of developing iron toxicity because of their high iron stores.

Table 5: Tolerable Upper Intake Levels for Iron for Infants 7 to 12 months, Children, and Adults 
Age Males
(mg/day)
Females
(mg/day)
Pregnancy
(mg/day)
Lactation
(mg/day)
7 to 12 months 40 40 N/A N/A
1 to 13 years 40 40 N/A N/A
14 to 18 years 45 45 45 45
19+ years 45 45 45 45

Resources

http://ods.od.nih.gov/factsheets/Iron-HealthProfessional/