Introduction

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 (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 synthesis of some hormones.

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. 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. Hepcidin, a circulating peptide hormone, is the key regulator of both iron absorption and the distribution of iron throughout the body, including in plasma.

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.

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 (also known as marginal iron deficiency), 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).

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.

Recommended Intakes

Intake recommendations for iron and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine (IOM) of the National Academies (formerly National Academy of Sciences). DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people.

These values, which vary by age and gender, include:

  • Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals; often used to plan nutritionally adequate diets for individuals.
  • Adequate Intake (AI): Intake at this level is assumed to ensure nutritional adequacy; established when evidence is insufficient to develop an RDA.
  • Estimated Average Requirement (EAR): Average daily level of intake estimated to meet the requirements of 50% of healthy individuals; usually used to assess the nutrient intakes of groups of people and to plan nutritionally adequate diets for them; can also be used to assess the nutrient intakes of individuals.
  • Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects.

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.

For infants from birth to 6 months, the FNB established an AI for iron that is equivalent to the mean intake of iron in healthy, breastfed infants.

Table 1: 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)

Monday, July 20, 2020 - 09:44