Iron

Iron is found in the two forms within the body and food, ferrous (Fe2+) and ferric (Fe3+) iron.¹ In animals and plants, iron can be categorized as heme iron or non-heme iron.¹ Heme iron is predominantly found in animal foods (meat, poultry, fish, animal products) and non-heme iron is found in plants.¹ Heme iron is absorbed greater than non-heme iron at a rate of 25-35% compared to 2-20%.¹

Digestion and Absorption:  

Iron must be liberated from protein in heme or non-heme sources. Ferric iron is reduced to ferrous iron before absorption can occur. Non-heme iron is absorbed primarily in the duodenum of the small intestine, binding to divalent metal transporter 1 (DMT1 - only binds ferrous iron) and handed to either ferritin (storage form of iron that converts ferrous iron into ferric iron) or mobiloferrin proteins.¹ Mobiloferrin passes ferrous iron to protein ferroportin and hephaestin. Ferroportin absorbs the ferrous iron from the enterocyte to the blood.¹ Hephaestin converts it back to ferric iron so protein transferrin can bind to it.¹ Heme iron binds to heme carrier protein (hcp1) to enter the intestinal cell, and heme oxygenase converts it into its ferrous form in which it is stored as ferritin or passed to mobiloferrin proteins until it is converted to transferrin as ferric iron.¹

Once passed to transferrin, ferric iron is transferred to various tissues in the body. Within the liver, it is reduced by the enzyme Steap3 into ferrous iron and stored as ferritin after being reconverted into ferric iron.¹ There are many regulatory molecules involved in iron absorption, including HFE (thought to block transferrin), hepcidin (hormone blocking ferroportin when iron stores are high), and HIF (transcription factors upregulating DMT1 and downregulating hepcidin when oxygen levels are low). 

Iron is absorbed to the extent it is needed within the body. It can vary based on growth, pregnancy, menstruation, storage amounts, etc. Absorption of iron can be increased by certain factors, such as vitamin C intake, stomach acidity, and consumption of meat, fish, and poultry. Absorption can be decreased by calcium, zinc, tannins, phytates, oxalates, and antacid use.¹

Circulation: Iron is found mostly within red blood cells as a part of hemoglobin, a protein that transports oxygen to tissues. The heme in hemoglobin contains iron.¹

Functions:  

Iron is found in many proteins in the body and plays many roles in metabolism, transport, and structure.

Proteins

Transport: Hemoglobin - A protein that transports oxygen to tissues and contains iron. Specifically, hemoglobin has four sites for iron and red blood cells can contain 250 million molecules of hemoglobin - this means each red blood cell can transport 1 billion oxygen molecules! ¹

Transport: Transferrin - A protein produced in the liver that transports ferric iron but also chromium, copper, manganese, cadmium, zinc, and nickel.² Ferrous iron is oxidized by ceruloplasmin (containing copper) in order to bind to transferrin; therefore, copper deficiency can lead to anemia through decreased iron circulation.¹

Storage: Ferritin and Hemosiderin - The storage form of iron, specifically ferric iron, found in bone marrow, intestine, liver, spleen, and more.² Ferritin within serum is an efficient indicator of iron status.¹ Hemosiderin is thought to be the breakdown product of ferritin but also is another storage protein.¹

Storage: Myoglobin - Found in muscle, specifically cardiac and type I muscle fibers, myoglobin is a protein that stores oxygen-containing iron. 

Metabolic - Cytochromes - Proteins involved in several processes within the body containing iron. A major function includes redox reactions within the electron transport chain that metabolizes drugs, alcohol, etc, 

Enzymes - Iron is found in many enzymes, such as peroxidases, myeloperoxidases, and catalases. 

Dietary  Reference Intake (DRI) - The recommended dietary allowance (RDA) for adult men and women (19-50 ) is 8 and 18 milligrams (mg).² Lactating and pregnant women have an increased need for iron. 

Iron Deficiency  -  Iron deficiency is common among women compared to men. Stores less than 300 mg in adults are deemed to be considered iron deficient.¹ There are different types of anemia, such as hypochromic microcytic anemia (small, pale red blood cells) and iron deficiency anemia (< 7 grams per 100 milliliters of blood).¹ However, many types of anemia can cause an array of symptoms. 

Iron Toxicity - Errors within genes can cause several types of genetic and congenital issues resulting in iron overload. Iron overload can lead to lipid peroxidation, organelle dysfunction, fibrosis (hepatic fibrosis), alterations in DNA, and tumor formation.¹ It is also associated with an increased risk of cancers and neurodegenerative diseases.¹

Dietary Sources of Iron

Iron is predominantly found in meat, fish, and poultry products as well as animal products mostly in the form of heme iron. Nonheme iron is also found in plant foods but may be impacted by oxalates and phytates. Vitamin C, often found in citrus fruits, and consumption of meat/fish/poultry can enhance iron absorption while calcium can reduce absorption. Iron is fortified in certain products, such as breakfast cereals. 

Examples of foods containing iron include:

Meat/Seafood/Animal Products- liver, oysters, chicken, egg

Vegetables - spinach, peas, tomatoes

Legumes - beans, lentils, soybeans

Grains - cereals, oatmeal, bread

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

2. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/