Iron deficiency anemia happens when your body doesn’t have enough iron to produce adequate hemoglobin, the protein in red blood cells that carries oxygen. It’s the most common nutritional deficiency worldwide, affecting roughly 40% of children under five, 37% of pregnant women, and 30% of non-pregnant women of reproductive age globally. The causes fall into a few clear categories: losing blood, not absorbing enough iron, not eating enough iron, or needing more iron than usual.
How Iron Builds Red Blood Cells
Iron sits at the center of hemoglobin. Without it, your bone marrow can’t assemble the molecule properly. The process works like this: developing red blood cells take up iron and use it to build heme, the oxygen-carrying component of hemoglobin. Heme then signals the cell to start producing the globin proteins that complete the hemoglobin molecule. When iron is scarce, that signal never fires. The cell suppresses protein production entirely rather than building defective hemoglobin.
The result is red blood cells that are smaller and paler than normal, carrying less oxygen per cell. This is why iron deficiency anemia produces that characteristic fatigue, shortness of breath, and pale skin. Your tissues are literally getting less oxygen with every heartbeat.
Chronic Blood Loss
Every milliliter of blood that leaves your body takes iron with it. Slow, ongoing blood loss is the single most common cause of iron deficiency anemia in adults in developed countries, and the bleeding often happens where you can’t see it.
Gastrointestinal bleeding is the leading culprit. Peptic ulcers, gastric polyps, inflammatory bowel disease, and colorectal cancers can all cause low-grade bleeding that goes unnoticed for months. You might lose just a small amount of blood daily, but over weeks and months, your iron stores drain steadily. This is why doctors investigating unexplained iron deficiency in men or postmenopausal women will typically order tests to look at the GI tract. A new diagnosis of iron deficiency anemia in these groups is sometimes the first sign of colon cancer.
Heavy menstrual periods are the other major source of chronic blood loss. Women who regularly soak through pads or tampons every hour, pass large clots, or bleed for more than seven days per cycle are at high risk. Over years of heavy periods, iron stores can deplete completely, especially if dietary intake doesn’t keep pace with losses. Regular use of aspirin or other anti-inflammatory medications can also contribute by promoting small amounts of stomach bleeding.
Poor Iron Absorption
You could eat plenty of iron and still become deficient if your gut can’t absorb it. Iron absorption happens primarily in the duodenum, the first stretch of the small intestine. Any condition that damages or bypasses this section creates a problem.
Celiac disease is a common example. The immune reaction to gluten flattens the absorptive lining of the small intestine, reducing its ability to pull iron (and many other nutrients) from food. Some people discover they have celiac disease only after being worked up for unexplained iron deficiency. Inflammatory bowel disease, particularly Crohn’s disease affecting the upper small intestine, can have a similar effect.
Gastric bypass surgery is another well-recognized cause. The procedure reroutes food past the duodenum entirely, so iron never reaches the tissue designed to absorb it. Johns Hopkins Medicine notes that this bypass of the duodenum directly leads to iron deficiency and other nutritional problems. People who’ve had this surgery typically need lifelong iron monitoring and supplementation.
Certain infections, like H. pylori in the stomach, can also impair iron absorption by reducing stomach acid. Iron needs an acidic environment to convert into a form the intestine can take up. Long-term use of acid-reducing medications (proton pump inhibitors) can have a similar effect.
What You Eat (and Drink) With Iron Matters
Iron in food comes in two forms. Heme iron, found in meat, poultry, and seafood, is absorbed relatively efficiently by the gut. Non-heme iron, found in plants, beans, fortified cereals, and supplements, is absorbed at a lower rate and is far more sensitive to what else you eat at the same meal.
Several common compounds interfere with non-heme iron absorption. Tannins, found in tea, are among the strongest inhibitors. Research has shown that the amount of tannic acid in a single cup of tea (about 250 mL brewed with 5 grams of leaves) significantly reduces non-heme iron absorption from that meal. Phytates, found in whole grains and legumes, and calcium from supplements also compete with iron uptake. Proteins from soy, milk, and egg yolk can further reduce absorption.
On the other side, vitamin C enhances non-heme iron absorption. Eating vitamin C-rich foods (citrus, bell peppers, strawberries, tomatoes) alongside iron-rich plant foods helps your gut absorb more of it. The effect is strongest when the vitamin C and iron are eaten together in the same meal rather than spread throughout the day. Heme iron from animal sources also boosts absorption of non-heme iron when both are eaten together.
For people who eat little or no meat, these interactions become especially important. A vegetarian or vegan diet can provide adequate iron, but it requires more deliberate pairing of foods. Drinking tea or coffee with meals, relying heavily on dairy alongside iron-rich foods, or eating a diet low in vitamin C can all tip the balance toward deficiency over time.
Increased Iron Demands
Sometimes the cause isn’t losing iron or failing to absorb it. Your body simply needs more than it did before.
Pregnancy is the clearest example. Blood volume increases substantially during pregnancy, and the developing baby and placenta draw heavily on maternal iron stores. The daily iron requirement jumps to 27 milligrams during pregnancy, roughly 50% more than a non-pregnant woman needs. Without supplementation, many women can’t meet this demand through diet alone. The WHO estimates that 37% of pregnant women worldwide are anemic, and iron deficiency is the primary driver.
Rapid growth in infancy and adolescence also raises iron needs. Babies are born with iron stores that last roughly four to six months. After that, they depend on dietary sources. Toddlers transitioning to solid foods and teenagers going through growth spurts are both at elevated risk if their diets are low in iron-rich foods. Globally, 40% of children aged six months to five years are anemic.
Endurance athletes, particularly runners, sometimes develop iron deficiency from a combination of factors: increased red blood cell production, minor GI bleeding during intense exercise, and iron loss through sweat. Frequent blood donors can also deplete their stores faster than diet replenishes them.
How Iron Deficiency Is Identified
Iron deficiency develops in stages. First, your stored iron drops. Then circulating iron decreases. Only in the final stage does hemoglobin fall low enough to qualify as anemia. This means you can be iron deficient and experiencing symptoms like fatigue and brain fog well before a standard blood count shows anything abnormal.
Ferritin, a protein that reflects your iron stores, is the most useful early marker. The WHO recommends interpreting ferritin levels carefully based on context. In otherwise healthy people, low ferritin reliably indicates depleted iron. But ferritin also rises during infection or inflammation, which can mask true deficiency. For people with active inflammation, the WHO uses higher thresholds: below 30 µg/L in children and below 70 µg/L in adults may still indicate iron deficiency even when the number looks “normal” by standard lab ranges.
Because so many different conditions can cause iron deficiency anemia, identifying the underlying reason is just as important as treating the low iron itself. Restoring iron levels without addressing ongoing blood loss or malabsorption means the problem will return. This is why a diagnosis of iron deficiency anemia, especially when it’s unexpected, often prompts further investigation into what’s driving it.