Iron is a mineral that is naturally present in many foods, added to some food products, and available as a dietary supplement. Iron is an essential component of hemoglobin, an erythrocyte protein that transfers oxygen from the lungs to the tissues. As a component of myoglobin, a protein that provides oxygen to muscles, iron supports metabolism. Iron is also necessary for growth, development, normal cellular functioning, and synthesis of some hormones and connective tissue.
Dietary iron has two main forms: heme and nonheme.
Plants and iron-fortified foods contain nonheme iron only,
whereas meat, seafood, and poultry contain both heme and nonheme iron.
Heme iron, which is formed when iron combines with protoporphyrin IX, contributes about 10% to 15% of total iron intakes in western populations.
Most of the 3 to 4 grams of elemental iron in adults is in hemoglobin. Much of the remaining iron is stored in the form of ferritin or hemosiderin (a degradation product of ferritin) in the liver, spleen, and bone marrow or is located in myoglobin in muscle tissue. Humans typically lose only small amounts of iron in urine, feces, the gastrointestinal tract, and skin. Losses are greater in menstruating women because of blood loss. Hepcidin, a circulating peptide hormone, is the key regulator of both iron absorption and the distribution of iron throughout the body, including in plasma.
Many different measures of iron status are available, and different measures are useful at different stages of iron depletion. Measures of serum ferritin can be used to identify iron depletion at an early stage. A reduced rate of delivery of stored and absorbed iron to meet cellular iron requirements represents a more advanced stage of iron depletion, which is associated with reduced serum iron, reticulocyte hemoglobin, and percentage transferrin saturation and with higher total iron binding capacity, red cell zinc protoporphyrin, and serum transferrin receptor concentration. The last stage of iron deficiency, characterized by iron-deficiency anemia (IDA), occurs when blood hemoglobin concentrations, hematocrit (the proportion of red blood cells in blood by volume), mean corpuscular volume, and mean cell hemoglobin are low. Hemoglobin and hematocrit tests are the most commonly used measures to screen patients for iron deficiency, even though they are neither sensitive nor specific. Hemoglobin concentrations lower than 13 g/dL in men and 12 g/dL in women indicate the presence of IDA. Normal hematocrit values, which are generally three times higher than hemoglobin levels, are approximately 41% to 50% in males and 36% to 44% in females .
Recommended Intakes
Table 1 lists the current iron RDAs for nonvegetarians.
The RDAs for vegetarians are 1.8 times higher than for people who eat meat. This is because heme iron from meat is more bioavailable than nonheme iron from plant-based foods, and meat, poultry, and seafood increase the absorption of nonheme iron.
| Age | Male | Female | Vegetarian Male | Female |
|---|---|---|---|---|
| Birth to 6 months | 0.27 mg | 0.27 mg | ||
| 7–12 months | 11 mg | 11 mg | ||
| 1–3 years | 7 mg | 7 mg | ||
| 4–8 years | 10 mg | 10 mg | ||
| 9–13 years | 8 mg | 8 mg | ||
| 14–18 years | 11 mg | 15 mg | ||
| 19–50 years | 8 mg | 18 mg | 14 mg | 32 mg |
| 51+ years | 8 mg | 8 mg | 14 mg | 14 mg |
Sources of Iron
Food
The richest sources of heme iron in the diet include lean meat and seafood.
Dietary sources of nonheme iron include nuts, beans, vegetables, and fortified grain products.
In the United States, about half of dietary iron comes from bread, cereal, and other grain products.
In the United States, Canada, and many other countries, wheat and other flours are fortified with iron.
Heme iron has higher bioavailability than nonheme iron, and other dietary components have less effect on the bioavailability of heme than nonheme iron.
The bioavailability of iron is approximately 14% to 18% from mixed diets that include substantial amounts of meat, seafood, and vitamin C (ascorbic acid, which enhances the bioavailability of nonheme iron) and 5% to 12% from vegetarian diets.
In addition to ascorbic acid, meat, poultry, and seafood can enhance nonheme iron absorption, whereas phytate (present in grains and beans) and certain polyphenols in some non-animal foods (such as cereals and legumes) have the opposite effect.
Unlike other inhibitors of iron absorption, calcium might reduce the bioavailability of both nonheme and heme iron. However, the effects of enhancers and inhibitors of iron absorption are attenuated by a typical mixed western diet, so they have little effect on most people’s iron status.
Some plant-based foods that are good sources of iron, such as spinach, have low iron bioavailability because they contain iron-absorption inhibitors, such as polyphenols.
| Food | Milligrams per serving |
Percent DV* |
|---|---|---|
| Breakfast cereals, fortified with 100% of the DV for iron, 1 serving | 18 | 100 |
| Oysters, eastern, cooked with moist heat, 3 ounces | 8 | 44 |
| White beans, canned, 1 cup | 8 | 44 |
| Chocolate, dark, 45%–69% cacao solids, 3 ounces | 7 | 39 |
| Beef liver, pan fried, 3 ounces | 5 | 28 |
| Lentils, boiled and drained, ½ cup | 3 | 17 |
| Spinach, boiled and drained, ½ cup | 3 | 17 |
| Tofu, firm, ½ cup | 3 | 17 |
| Kidney beans, canned, ½ cup | 2 | 11 |
| Sardines, Atlantic, canned in oil, drained solids with bone, 3 ounces | 2 | 11 |
| Chickpeas, boiled and drained, ½ cup | 2 | 11 |
| Tomatoes, canned, stewed, ½ cup | 2 | 11 |
| Beef, braised bottom round, trimmed to 1/8” fat, 3 ounces | 2 | 11 |
| Potato, baked, flesh and skin, 1 medium potato | 2 | 11 |
| Cashew nuts, oil roasted, 1 ounce (18 nuts) | 2 | 11 |
| Green peas, boiled, ½ cup | 1 | 6 |
| Chicken, roasted, meat and skin, 3 ounces | 1 | 6 |
| Rice, white, long grain, enriched, parboiled, drained, ½ cup | 1 | 6 |
| Bread, whole wheat, 1 slice | 1 | 6 |
| Bread, white, 1 slice | 1 | 6 |
| Raisins, seedless, ¼ cup | 1 | 6 |
| Spaghetti, whole wheat, cooked, 1 cup | 1 | 6 |
| Tuna, light, canned in water, 3 ounces | 1 | 6 |
| Turkey, roasted, breast meat and skin, 3 ounces | 1 | 6 |
| Nuts, pistachio, dry roasted, 1 ounce (49 nuts) | 1 | 6 |
| Broccoli, boiled and drained, ½ cup | 1 | 6 |
| Egg, hard boiled, 1 large | 1 | 6 |
| Rice, brown, long or medium grain, cooked, 1 cup | 1 | 6 |
| Cheese, cheddar, 1.5 ounces | 0 | 0 |
| Cantaloupe, diced, ½ cup | 0 | 0 |
| Mushrooms, white, sliced and stir-fried, ½ cup | 0 | 0 |
| Cheese, cottage, 2% milk fat, ½ cup | 0 | 0 |
| Milk, 1 cup | 0 | 0 |