Iron is a trace mineral that serves as a cofactor for many biological processes required to sustain life. Every cell relies on this metal for proper functioning, making it one of the most tightly regulated elements we absorb. Its ability to easily switch between its ferrous (Fe²⁺) and ferric (Fe³⁺) states allows it to participate in the transfer of electrons, underpinning its numerous roles in metabolism and cellular machinery.
The Primary Function: Oxygen Transport
The primary function of iron is transporting oxygen throughout the body. About 70% of the body’s iron is found in hemoglobin and myoglobin. Hemoglobin, located within red blood cells, picks up oxygen in the lungs and releases it into peripheral tissues.
The iron atom resides at the core of the heme group, which is the site of oxygen binding. In the lungs, ferrous (Fe²⁺) iron reversibly binds to an oxygen molecule. This capacity allows red blood cells to carry millions of oxygen molecules.
In muscle cells, myoglobin, which also contains a heme-iron center, accepts and stores oxygen until it is needed for muscle contraction. This localized oxygen reservoir helps sustain aerobic metabolism during intense activity when blood flow may be temporarily limited.
Essential Roles in Cellular Metabolism
Beyond oxygen transport, iron is integrated into energy generation. It is a necessary component of the electron transport chain (ETC), the final stage of cellular respiration that produces adenosine triphosphate (ATP), the cell’s energy currency. Iron-sulfur (Fe-S) clusters and heme-containing proteins, such as cytochromes, are embedded within the ETC complexes in the mitochondria.
These iron components facilitate the sequential transfer of electrons, harnessing the energy released to pump protons and drive ATP synthesis. A lack of iron impairs the efficiency of this energy system.
Iron also functions as a cofactor for enzymes involved in the synthesis and repair of genetic material. The enzyme ribonucleotide reductase (RNR) requires iron to convert ribonucleotides into deoxyribonucleotides, which are the building blocks of DNA. Without RNR’s iron-dependent activity, cells cannot replicate their DNA, which is necessary for cell division and growth. Furthermore, many DNA repair enzymes contain Fe-S clusters that maintain genome stability.
Iron Regulation: Absorption, Storage, and Recycling
The body maintains iron balance through intake, storage, and reuse, as there is no regulated pathway for iron excretion. Dietary iron is absorbed primarily in the duodenum of the small intestine. Iron from animal sources (heme iron) is absorbed differently than non-heme iron from plant sources.
Once inside the intestinal cells, iron is either stored as a protein complex called ferritin or transferred into the bloodstream. When released, iron immediately binds to the transport protein transferrin, which carries it safely through the circulation to tissues that need it. Ferritin acts as the body’s primary iron reserve, releasing iron when systemic levels drop.
The majority of the iron required daily is obtained through efficient recycling, not the diet. Macrophages, specialized immune cells, break down old red blood cells at the end of their approximately 120-day lifespan. These cells recover the iron from the degraded hemoglobin and release it back into the blood for reuse in producing new red blood cells.
The Health Impact of Iron Imbalance
A sustained imbalance in iron levels can lead to significant health consequences, with deficiency being the most common nutritional disorder globally. Iron deficiency impairs oxygen transport and cellular energy production. The resulting lack of oxygen delivery manifests as iron deficiency anemia, causing fatigue, weakness, and pallor.
Impaired energy metabolism contributes to difficulty sustaining physical work and poor cognitive function. Deficiency also affects immune function and temperature regulation.
Conversely, excess iron is damaging because the body lacks a mechanism to increase iron excretion. Hereditary hemochromatosis is a genetic disorder causing the body to absorb too much iron, leading to chronic overload. The excess iron accumulates in major organs like the liver, heart, and pancreas, causing tissue damage through the generation of reactive oxygen species.
Symptoms of chronic iron overload, such as joint pain and liver disease, develop slowly. Acute iron toxicity, often seen in children who accidentally ingest high-dose supplements, is a medical emergency that can cause severe gastrointestinal distress and organ failure.