Iron is one of the most essential minerals in your body, responsible for carrying oxygen to every tissue, powering cellular energy production, and supporting brain function, immune defense, and DNA replication. Adult men need about 8 mg per day, while women of reproductive age need 18 mg, and pregnant women need 27 mg. Even mild shortfalls can affect energy levels, mood, and immune function, because iron sits at the center of so many biological processes.
How Iron Carries Oxygen
The most well-known job of iron is oxygen transport. About two-thirds of the iron in your body is bound up in hemoglobin, the protein inside red blood cells that picks up oxygen in your lungs and delivers it throughout your body. Iron atoms sit at the center of hemoglobin’s structure, and oxygen physically bonds to them. When a red blood cell passes through the lungs, oxygen molecules enter a pocket in the hemoglobin protein and form a direct bond with the iron atom inside. That bond is strong enough to hold the oxygen during transit through your bloodstream but reversible enough to release it when your tissues need it.
A similar protein called myoglobin performs the same trick inside your muscles. Myoglobin stores oxygen locally so your muscles have a reserve to draw from during exertion. This is why iron deficiency often shows up first as fatigue and exercise intolerance: without enough iron, your blood carries less oxygen per trip, and your muscles run low faster.
Fueling Cellular Energy
Iron doesn’t just deliver oxygen. It also helps your cells use that oxygen to produce energy. Inside your mitochondria (the structures that generate energy in every cell), iron is a key component of the electron transport chain, the final step in converting food into usable fuel. Iron-containing proteins shuttle electrons through a series of reactions that ultimately produce ATP, the molecule your cells burn for energy.
Iron shows up in two forms in this process. Some mitochondrial proteins contain iron-sulfur clusters, small molecular structures that pass electrons between steps in the chain. Others contain heme, the same iron-carrying molecule found in hemoglobin. Together, these iron-dependent proteins are involved in nearly every stage of the energy production pathway, from breaking down fatty acids to running the final reactions that generate ATP. When iron is scarce, this entire system slows down, which is one reason iron-deficient people feel persistently tired even when they’re getting enough sleep.
Brain Function and Mood
Iron plays a surprisingly direct role in brain chemistry. It serves as a required helper molecule for the enzymes that produce dopamine and serotonin, two neurotransmitters that regulate mood, motivation, and emotional well-being. Without adequate iron, these enzymes can’t function at full capacity, and production of both neurotransmitters drops.
Iron is also essential for myelination, the process of insulating nerve fibers with a fatty coating that speeds up signal transmission. Iron deficiency during early brain development can cause lasting effects on myelination and behavior. In adults, low iron has been linked to increased anxiety and emotional dysregulation, likely because reduced serotonin and dopamine production in key brain regions disrupts normal mood signaling. This connection between iron and mental health is one reason clinicians sometimes check iron levels in patients with unexplained anxiety or depression.
Immune Defense
Your immune system depends on iron at multiple levels. Natural killer cells, which are part of your first-line defense against viruses and abnormal cells, increase their iron uptake when activated. Low systemic iron levels suppress their activation and reduce their ability to produce key signaling molecules that coordinate the immune response.
Iron also drives the proliferation of B cells, the immune cells responsible for producing antibodies. People with iron deficiency show significantly decreased antibody responses, which has practical implications: research has found that serum iron levels correlate with how well people respond to vaccinations. T cells similarly require iron for activation and differentiation, meaning that iron deficiency can weaken both the immediate and long-term branches of your immune system simultaneously.
DNA Replication and Cell Growth
Every time a cell divides, it needs to copy its entire DNA. The enzyme that makes this possible, ribonucleotide reductase, requires iron to function. This enzyme is the rate-limiting step in producing the raw building blocks of DNA, so when iron drops, DNA synthesis slows and cell division stalls. In mammals, a significant reduction in available iron leads to measurable decreases in DNA building blocks, DNA synthesis, and cell proliferation. This is why iron deficiency can affect rapidly dividing tissues first, including the lining of your gut, your skin, and your blood cell production in bone marrow.
The body even has a triage system for iron allocation. When supplies run low, cells prioritize iron delivery to ribonucleotide reductase over less critical iron-dependent processes, essentially sacrificing other functions to keep DNA repair and replication running.
How Your Body Absorbs Iron
Not all dietary iron is created equal. Iron from animal sources (meat, poultry, seafood) comes in a form called heme iron, which your gut absorbs at a rate of 25 to 30%. Iron from plant sources like grains, legumes, and vegetables is non-heme iron, absorbed at only about 3 to 5%. Heme iron accounts for just 10 to 15% of the iron most people consume, but it contributes a disproportionate share of what actually gets into the bloodstream.
Several common dietary compounds interfere with non-heme iron absorption. Phytates, found in whole grains and bran, can inhibit iron absorption by up to 82% at higher concentrations. Polyphenols in tea are particularly potent: drinking tea with a meal can reduce iron absorption by 56 to 85%, depending on the iron source. Calcium also competes with iron, reducing absorption by 18 to 27% when consumed together. Soy protein and egg white have similar inhibitory effects.
Vitamin C (ascorbic acid) is the most effective counterbalance. It can partially overcome the blocking effects of phytates and polyphenols. If you’re trying to maximize iron from plant-based foods, pairing them with citrus fruits, bell peppers, or other vitamin C-rich foods at the same meal makes a meaningful difference. Separating your tea or coffee from iron-rich meals by an hour or two also helps.
What Happens With Too Much Iron
Iron is unusual among essential minerals because your body has no efficient way to excrete excess amounts. This makes iron overload a real concern, particularly for people with genetic conditions like hemochromatosis or those receiving frequent blood transfusions.
The danger of excess iron comes down to chemistry. Free iron reacts with hydrogen peroxide (a normal byproduct of metabolism) to produce hydroxyl radicals, one of the most reactive and damaging molecules in biology. These radicals attack DNA, degrade cell membranes through lipid peroxidation, and can trigger a form of cell death called ferroptosis. Hydroxyl radicals react with biological molecules in less than a nanosecond, making them nearly impossible for the body’s antioxidant defenses to neutralize once formed.
This is why your body tightly regulates iron storage, locking most of it inside proteins like ferritin rather than letting it float freely. When ferritin releases its stored iron in an uncontrolled way, the resulting spike in free iron drives a surge in radical production and cellular damage. For most people eating a normal diet, this system works well. But high-dose iron supplements taken unnecessarily can overwhelm these safeguards, which is why iron supplementation should be guided by actual blood work rather than guesswork.
Recognizing Iron Deficiency
Iron deficiency is the most common nutritional deficiency worldwide. It develops gradually: your body first depletes its stored iron, then circulating iron drops, and finally hemoglobin falls low enough to cause anemia. The standard blood marker is serum ferritin, which reflects your iron stores. The traditional diagnostic threshold is below 15 micrograms per liter for women and below 12 for young children, but recent multinational research suggests that hemoglobin levels actually begin declining at ferritin levels around 23 to 25 micrograms per liter, meaning many people with technically “normal” ferritin may already be functionally low.
Common symptoms include persistent fatigue, weakness, pale skin, brittle nails, cold hands and feet, and difficulty concentrating. Because iron affects neurotransmitter production, some people notice irritability or low mood before the classic physical symptoms appear. Women with heavy menstrual periods, pregnant women, frequent blood donors, and people eating exclusively plant-based diets are at the highest risk and benefit most from periodic screening.