Iron is one of the most essential minerals in your body, involved in everything from carrying oxygen to your tissues to producing the energy your cells need to function. About 70% of your body’s iron sits in two proteins: hemoglobin in red blood cells and myoglobin in muscle cells. Without enough iron, your blood can’t deliver oxygen efficiently, your energy drops, and systems from your brain to your immune defenses start to falter. Iron deficiency remains the most common nutritional deficiency worldwide, affecting roughly half a billion women and 269 million young children.
Oxygen Delivery and Energy Production
Iron’s most critical job is oxygen transport. Hemoglobin, the protein inside red blood cells, uses iron atoms to grab oxygen molecules in the lungs and release them wherever your body needs fuel. Myoglobin does something similar inside muscle tissue, storing oxygen so your muscles have a ready supply during physical activity. Without adequate iron, these proteins can’t form properly, and your tissues essentially start to suffocate at a cellular level.
Iron also plays a central role in how your cells generate energy. Deep inside each cell, structures called mitochondria run a chain reaction that converts the food you eat into usable energy. The early steps of this process depend heavily on clusters of iron and sulfur atoms that shuttle electrons along the chain. Your mitochondria can contain up to ten of these iron-based clusters in a single protein complex. When iron is scarce, this energy-production machinery slows down, which is why fatigue is often the earliest and most noticeable symptom of low iron.
Brain Function and Mood
Iron is required to build several key chemical messengers in the brain, including dopamine, serotonin, and norepinephrine. These neurotransmitters regulate mood, attention, motivation, and learning. This is why iron deficiency often shows up as brain fog, difficulty concentrating, or low mood before any blood test looks abnormal. Children are particularly vulnerable: iron shortfalls during early development can impair cognitive function and learning ability in ways that may not fully reverse even after iron levels are restored.
Immune Defense
Your immune system depends on iron to mount an effective response to infections. Both T cells and B cells, the two main players in your adaptive immune system, need iron to multiply and mature when a threat is detected. Iron-dependent enzymes drive the signaling pathways that activate T cells, and iron is also necessary for building the DNA and RNA that immune cells need as they rapidly divide. People with chronically low iron tend to get sick more often and recover more slowly.
How Much Iron You Need
Daily iron requirements vary significantly by age, sex, and life stage. Adult men and women over 51 need about 8 mg per day. Women between 19 and 50 need 18 mg daily, more than double the male requirement, primarily because of monthly menstrual blood loss. During pregnancy, the requirement jumps to 27 mg per day to support the growing fetus and expanded blood volume. Infants aged 7 to 12 months need 11 mg daily, a surprisingly high amount relative to their size, because they’re building iron stores rapidly during a period of fast growth.
Heme vs. Non-Heme Iron
Not all dietary iron is absorbed equally. Iron from animal sources (meat, poultry, fish) comes in a form called heme iron, and your body absorbs 25 to 30% of it. Iron from plant sources like beans, lentils, spinach, and fortified cereals is non-heme iron, and absorption drops to just 1 to 10%. This difference matters a lot in practice. A bowl of lentils might contain a generous amount of iron on paper, but your body may only extract a fraction of it.
Several common substances further reduce how much non-heme iron you absorb. Phytates in whole grains and legumes, polyphenols in tea, coffee, and wine, and oxalic acid in spinach and chard all bind to iron and block uptake. Calcium is unique in that it inhibits absorption of both heme and non-heme iron. Even certain proteins, including those in eggs, dairy, and soy, can reduce iron absorption.
Vitamin C is the most effective counterbalance. Eating vitamin C-rich foods alongside iron-rich meals directly increases non-heme iron absorption in proportion to the amount of vitamin C present. Pairing a squeeze of lemon juice with a spinach salad or eating bell peppers with beans makes a real, measurable difference in how much iron your body takes in.
What Iron Deficiency Looks Like
Iron deficiency doesn’t happen overnight. It progresses through stages, starting with depleted iron stores long before you develop full-blown anemia. In early depletion, you may feel tired, cold, or mentally sluggish without any obvious changes on a standard blood count. A blood protein called ferritin reflects your stored iron: levels below 30 micrograms per liter indicate iron deficiency, even if your hemoglobin is still normal. The WHO uses a stricter cutoff of 15 micrograms per liter, but many clinicians start investigating at 30. In people with chronic inflammatory conditions like autoimmune disease, ferritin can appear falsely elevated, so the diagnostic threshold rises to 100 micrograms per liter.
This stage, iron deficiency without anemia, is widely underdiagnosed. Symptoms can include persistent fatigue, exercise intolerance, restless legs, hair thinning, difficulty concentrating, and frequent infections. Many people live with these symptoms for months or years without connecting them to iron. Once stores drop far enough that hemoglobin production is impaired, iron deficiency anemia develops, bringing more severe fatigue, pale skin, shortness of breath, dizziness, and a rapid heartbeat.
Globally, 40% of children aged 6 months to 5 years and 37% of pregnant women are affected by anemia, with dietary iron deficiency as the leading cause. Menstruating adolescents and women of reproductive age are also at high risk, with about 30% of women aged 15 to 49 affected worldwide.
When Iron Becomes Harmful
Iron is unusual among nutrients because your body has no efficient way to get rid of excess amounts. Under normal circumstances, absorption is tightly regulated, but certain conditions can lead to dangerous buildup. Hemochromatosis, a genetic condition, causes the body to absorb too much iron from food over time. Without treatment, the excess iron deposits in organs and causes progressive damage.
The liver takes the first hit, potentially developing scarring (cirrhosis) that can progress to liver failure or liver cancer. Iron overload can also damage the pancreas, leading to diabetes. The heart may weaken into heart failure. Joints develop a form of arthritis, and sex glands can be damaged, lowering hormone levels and causing loss of libido or erectile dysfunction. Because these complications develop slowly over years, many people with hemochromatosis don’t realize anything is wrong until significant organ damage has occurred.
This is also why iron supplements should be taken with some caution. If you’re not actually deficient, extra iron has nowhere useful to go and can contribute to oxidative stress in your tissues. Getting your ferritin checked before starting supplementation gives you a clear picture of whether you need more iron or already have enough.