Iron

What Is Iron?

Iron is a trace mineral and one of the most functionally important micronutrients in the body. An adult contains approximately 3-5 grams of iron, of which about 65 percent is incorporated into hemoglobin, the oxygen-carrying protein in red blood cells. Another 10 percent is found in myoglobin (muscle oxygen storage), and the remainder is stored as ferritin and hemosiderin primarily in the liver, spleen, and bone marrow.

Key Functions

• Oxygen transport: Iron is the central atom of heme in hemoglobin and myoglobin. It reversibly binds oxygen, enabling red blood cells to carry oxygen from the lungs to tissues.
• Energy metabolism: Iron-containing cytochromes are essential components of the mitochondrial electron transport chain, which generates ATP.
• DNA synthesis: Ribonucleotide reductase, the enzyme that converts ribonucleotides to deoxyribonucleotides, requires iron.
• Immune function: Iron-dependent enzymes in immune cells produce reactive oxygen species that kill pathogens.
• Neurotransmitter synthesis: Iron is a cofactor for enzymes that synthesize dopamine, serotonin, and norepinephrine.

Dietary Reference Values and Food Sources

The RDA for iron is 8 mg/day for adult men and postmenopausal women, and 18 mg/day for premenopausal women. The higher requirement for women reflects menstrual losses. Pregnant women need 27 mg/day. The UL is 45 mg/day.

Iron exists in two forms with very different bioavailability:

• Heme iron (15-35% absorbed): found in meat, poultry, and seafood. Liver (3 oz) provides approximately 5 mg; beef (3 oz) approximately 2 mg.
• Non-heme iron (2-20% absorbed): found in legumes, tofu, fortified cereals, spinach, and nuts. Absorption is enhanced by consuming vitamin C simultaneously and inhibited by calcium, tannins (tea/coffee), and phytates.

Iron Deficiency and Toxicity

Iron deficiency is the most common nutritional deficiency worldwide, affecting approximately 2 billion people. Progressive depletion leads to iron-deficiency anemia, with symptoms of fatigue, pallor, shortness of breath, impaired cognitive function, and reduced immune response. Excess iron is toxic — hereditary hemochromatosis causes iron overload that damages the liver, heart, and pancreas. Supplemental iron above the UL can cause gastrointestinal distress and, in children, accidental poisoning.