Iron 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 protein that transfers oxygen from the lungs to the tissues [1]. As a component of myoglobin, a protein that provides oxygen to muscles, iron supports metabolism [2]. Iron is also necessary for growth, development, normal cellular functioning, and synthesis of some hormones and connective tissue [2,3].

Dietary iron has two main forms: heme and nonheme [1]. Plants and iron-fortified foods contain nonheme iron only, whereas meat, seafood, and poultry contain both heme and nonheme iron [2]. Heme iron, which is formed when iron combines with protoporphyrin IX, contributes about 10% to 15% of total iron intakes in western populations [3-5].

Most of the 3 to 4 grams of elemental iron in adults is in hemoglobin [2]. 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 [1,5]. 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 [1,2,6].

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 [7]. 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 [2,8]. Hemoglobin and hematocrit tests are the most commonly used measures to screen patients for iron deficiency, even though they are neither sensitive nor specific [5,9]. Hemoglobin concentrations lower than 13 g/dL in men and 12 g/dL in women indicate the presence of IDA [5]. 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 [10].

References

1. Wessling-Resnick M. Iron. In: Ross AC, Caballero B, Cousins RJ, Tucker KL, Ziegler RG, eds. Modern Nutrition in Health and Disease. 11th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2014:176-88.
2. Aggett PJ. Iron. In: Erdman JW, Macdonald IA, Zeisel SH, eds. Present Knowledge in Nutrition. 10th ed. Washington, DC: Wiley-Blackwell; 2012:506-20.
3. Murray-Kolbe LE, Beard J. Iron. In: Coates PM, Betz JM, Blackman MR, et al., eds. Encyclopedia of Dietary Supplements. 2nd ed. London and New York: Informa Healthcare; 2010:432-8.
4. Hurrell R, Egli I. Iron bioavailability and dietary reference values. Am J Clin Nutr 2010;91:1461S-7S. [PubMed abstract]
5. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc : a Report of the Panel on Micronutrients . Washington, DC: National Academy Press; 2001.
6. Drakesmith H, Prentice AM. Hepcidin and the Iron-Infection Axis. Science 2012;338:768-72. [PubMed abstract]
7. Gibson RS. Assessment of Iron Status. In: Principles of Nutritional Assessment. 2nd ed. New York: Oxford University Press; 2005:443-76.
8. World Health Organization. Report: Priorities in the Assessment of Vitamin A and Iron Status in Populations, Panama City, Panama, 15-17 September 2010 . Geneva; 2012.
9. Centers for Disease Control and Prevention (CDC). Recommendations to prevent and control iron deficiency in the United States. MMWR Recomm Rep 1998;47:1-29. [PubMed abstract]
10. MedlinePlus [Internet]. Bethesda (MD): National Library of Medicine (US). Hematocrit .