Iron: Reference and Dietary Sources
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Abstract
In this article, we describe:
- the major purposes of this specific nutrient in the human body,
- its experimentally confirmed health uses,
- conventional ways to estimate nutrient status,
- nutrient’s toxicities and deficiencies,
- experimentally confirmed and approved levels of the nutrient intake for different demographics,
- dietary sources of the nutrient.
Introduction
Iron is a mineral that is an essential component of hemoglobin, an erythrocyte (red blood cell) protein that transfers oxygen from the lungs to the tissues. As a component of myoglobin, another protein that provides oxygen, iron supports muscle metabolism and healthy connective tissue. Iron is also necessary for physical growth, neurological development, cellular functioning, and hormone synthesis.
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 (formed when iron combines with protoporphyrin IX) and nonheme iron.
Most of the 3 to 4 grams of elemental iron in adults is in hemoglobin. Much of the remaining iron is stored in the liver, spleen, and bone marrow or is located in myoglobin in muscle tissue. Transferrin is the main protein in blood that binds to iron and transports it throughout the body. 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.
The assessment of iron status depends almost entirely on hematological indicators. However, these indicators are not sensitive or specific enough to adequately describe the full spectrum of iron status, and this can complicate the diagnosis of iron deficiency. A complementary approach is to consider how iron intakes from the diet and dietary supplements compare with recommended intakes.
Serum ferritin concentration, a measure of the body’s iron stores, is currently the most efficient and cost-effective test for diagnosing iron deficiency. Because serum ferritin decreases during the first stage of iron depletion, it can identify low iron status before the onset of IDA. A serum ferritin concentration lower than 30 mcg/L suggests iron deficiency, and a value lower than 10 mcg/L suggests IDA. However, serum ferritin is subject to influence by inflammation (due, for example, to infectious disease), which elevates serum ferritin concentrations.
Hemoglobin and hematocrit tests are the most commonly used measures to screen patients for iron deficiency, even though they are neither sensitive nor specific. Often, hemoglobin concentrations are combined with serum ferritin measurements to identify IDA. Hemoglobin concentrations lower than 11 g/dL in children under 10 years of age, or lower than 12 g/dL in individuals aged 10 years or older, suggest IDA. Normal hematocrit values are approximately 41% to 50% in males and 36% to 44% in females.
Iron deficiency progresses from depletion of iron stores (mild iron deficiency), to iron-deficiency erythropoiesis (erythrocyte production), and finally to iron deficiency anemia (IDA). With iron-deficiency erythropoiesis, iron stores are depleted and transferrin saturation declines, but hemoglobin levels are usually within the normal range. IDA is characterized by low hemoglobin concentrations, and decreases in hematocrit (the proportion of red blood cells in blood by volume) and mean corpuscular volume (a measure of erythrocyte size).
Iron deficiency is not uncommon, especially among pregnant women; infants and young children; women with heavy menstrual bleeding; frequent blood donors; people with cancer; people who have gastrointestinal disorders or have had gastrointestinal surgery; people with heart failure. Iron deficiency is usually associated with poor diet, malabsorptive disorders, and blood loss.
Adults with normal intestinal function have very little risk of iron overload from dietary sources of iron. However, acute intakes of more than 20 mg/kg iron from supplements or medicines can lead to gastric upset, constipation, nausea, abdominal pain, vomiting, and faintness, especially if food is not taken at the same time. Taking supplements containing 25 mg elemental iron or more can also reduce zinc absorption and plasma zinc concentrations. In severe cases (e.g., one-time ingestions of 60 mg/kg), overdoses of iron can lead to multisystem organ failure, coma, convulsions, and even death.
Nutri-IQ tool provides easy and convenient way to analyze patient’s bothersome symptoms and relate them to nutrients inadequacy (deficiency or toxicity).
Recommended Dietary Allowances (RDAs) for Iron
| Age | Male | Female | Pregnancy | Lactation |
|---|---|---|---|---|
| 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 | 27 mg | 10 mg |
| 19–50 years | 8 mg | 18 mg | 27 mg | 9 mg |
| 51+ years | 8 mg | 8 mg |
* Adequate Intake (AI)
Dietary Sources of Iron
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. About half of dietary iron comes from bread, cereal, and other grain products. Breast milk contains highly bioavailable iron but in amounts that are not sufficient to meet the needs of infants older than 4 to 6 months .
In the United States, Canada, and many other countries, wheat and other flours are fortified with iron. Infant formulas are fortified with 12 mg iron per liter.
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.
| Food | Milligrams per serving | Percent DV* |
|---|---|---|
| Fortified breakfast cereals, 1 serving | 18 | 100 |
| Eastern oysters, cooked with moist heat, 3 ounces | 8 | 44 |
| Canned white beans, 1 cup | 8 | 44 |
| Dark chocolate, 45%–69% cacao solids, 3 ounces | 7 | 39 |
| Pan fried beef liver, 3 ounces | 5 | 28 |
| Boiled lentils, ½ cup | 3 | 17 |
| Boiled spinach, ½ cup | 3 | 17 |
| Firm tofu, ½ cup | 3 | 17 |
| Canned kidney beans, ½ cup | 2 | 11 |
| Canned in oil Atlantic sardines, with bone, 3 ounces | 2 | 11 |
| Chickpeas, boiled and drained, ½ cup | 2 | 11 |
| Canned tomatoes, ½ cup | 2 | 11 |
| Braised bottom round beef, 3 ounces | 2 | 11 |
| Baked potato, 1 medium | 2 | 11 |
| Oil roasted cashew nuts, 1 ounce (18 nuts) | 2 | 11 |
| Boiled green peas, ½ cup | 1 | 6 |
| Roasted chicken, meat and skin, 3 ounces | 1 | 6 |
| White enriched rice, ½ cup | 1 | 6 |
| Whole wheat bread, 1 slice | 1 | 6 |
| White bread, 1 slice | 1 | 6 |
| Seedless raisins, ¼ cup | 1 | 6 |
| Whole wheat spaghetti, 1 cup | 1 | 6 |
| Light Tuna, canned in water, 3 ounces | 1 | 6 |
| Roasted turkey breast meat and skin, 3 ounces | 1 | 6 |
| Dry roasted pistachio nuts, 1 ounce (49 nuts) | 1 | 6 |
| Boiled broccoli, ½ cup | 1 | 6 |
| Hard boiled egg, 1 large | 1 | 6 |
| Brown rice, 1 cup | 1 | 6 |
| Cheddar cheese, 1.5 ounces | 0 | 0 |
| Cantaloupe, diced, ½ cup | 0 | 0 |
| White mushrooms, stir-fried, ½ cup | 0 | 0 |
| Cottage cheese, ½ cup | 0 | 0 |
| Milk, 1 cup | 0 | 0 |
- * DV = Daily Value.
- The DV for iron is 18 mg for adults and children age 4 years and older.
- Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.
References
- Elson Haas. “Staying Healthy with Nutrition”
- U.S. Department of Health & Human Services: https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/