Iron deficiency happens when your body loses more iron than it takes in, or when something prevents you from absorbing enough iron from food. It affects roughly half a billion women of reproductive age worldwide and is the most common nutritional deficiency leading to anemia. The causes fall into a few broad categories: not getting enough iron from your diet, losing iron through bleeding, having trouble absorbing iron, or needing more iron than usual during periods of high demand like pregnancy.
How Your Body Absorbs Iron
Understanding what goes wrong starts with understanding how iron normally gets into your bloodstream. Iron absorption happens in the upper part of your small intestine, where specialized cells lining the gut wall pull iron from digested food and transfer it into the blood. A protein called ferroportin acts as the exit door, shuttling iron out of those gut cells and into circulation. Your body regulates this process tightly using a hormone called hepcidin. When iron stores are adequate, hepcidin levels rise, causing ferroportin to break down so less iron gets through. When stores drop, hepcidin falls and the door opens wider.
This system means your body can only ramp up absorption so much. If you’re losing iron faster than your gut can replace it, or if something damages the absorptive lining, stores gradually empty.
Not Enough Iron in Your Diet
Iron in food comes in two forms. Heme iron, found in animal products like beef, poultry, shellfish, and organ meats, is absorbed significantly better than non-heme iron, which comes from plant sources like beans, lentils, spinach, fortified cereals, nuts, and dark chocolate. Most people in Western countries get the majority of their iron from non-heme sources, which makes the overall efficiency of their diet lower.
The recommended daily intake reflects this gap. Pre-menopausal women need 18 mg of iron per day, more than double the 8 mg recommended for adult men and post-menopausal women. That 18 mg target accounts for menstrual losses and the relatively low absorption rate of a mixed diet. Vegetarians and vegans face an additional challenge because they rely entirely on non-heme iron, which is more sensitive to compounds in food that block absorption.
What you eat alongside iron matters, too. Tannins, found in tea and coffee, can meaningfully reduce how much non-heme iron your body absorbs from a meal. In one controlled study, 100 mg of tannic acid reduced iron bioavailability from 25% down to about 17%. Vitamin C, on the other hand, enhances non-heme iron absorption, which is why pairing iron-rich plant foods with citrus fruits or bell peppers is a common recommendation.
Blood Loss Is the Most Common Cause
Every milliliter of blood contains about half a milligram of iron, so any source of chronic bleeding can drain your reserves. In pre-menopausal women, heavy menstrual periods are the leading cause of iron deficiency. Heavy menstrual bleeding is generally defined as losing more than 80 milliliters of blood per cycle, or bleeding that significantly interferes with daily life. Many women with heavy periods don’t realize their flow is above average because they have no baseline for comparison, which is one reason iron deficiency often goes unrecognized for years.
In men and post-menopausal women, the most likely source of blood loss is the gastrointestinal tract. Peptic ulcers are the most common culprit in the upper GI tract. These open sores develop on the stomach lining or the upper small intestine, often triggered by a bacterial infection or regular use of anti-inflammatory painkillers like ibuprofen or aspirin. Colon polyps, small growths on the lining of the large intestine, can also bleed slowly over time. Some polyps are harmless, but others can be cancerous or precancerous. This is why unexplained iron deficiency in older adults often prompts a colonoscopy.
The bleeding in these cases is frequently invisible. You may not notice blood in your stool because the amounts are small enough to be hidden, yet large enough, day after day, to outpace what your diet can replace.
Problems With Absorption
Even if you eat plenty of iron-rich food, certain conditions can prevent your gut from taking it in. Celiac disease is a classic example. The immune reaction triggered by gluten damages the lining of the upper small intestine, precisely where iron absorption occurs. This mucosal damage, particularly when it progresses to flattening of the tiny finger-like projections (villi) that increase the gut’s surface area, directly impairs the intestine’s ability to absorb iron. Iron deficiency is sometimes the first and only sign of celiac disease, appearing before digestive symptoms do. Once someone eliminates gluten, the inflammatory damage gradually reverses and iron-absorbing capacity returns, though this can take months.
Inflammatory bowel diseases like Crohn’s disease can cause similar problems when they affect the upper small intestine. Gastric bypass surgery, which reroutes food past parts of the stomach and duodenum, also reduces the surface area available for iron absorption.
Medications That Interfere
Your stomach acid plays an important role in iron absorption. It helps release iron from food particles and converts it into a form that’s easier for gut cells to take up. Proton pump inhibitors, commonly prescribed for acid reflux, suppress stomach acid production and can reduce non-heme iron absorption over time. If you’ve been on one of these medications for months or years, it may be contributing to low iron stores. Antacids and certain other acid-reducing medications can have a similar, though typically milder, effect.
Increased Iron Demand
Sometimes your body simply needs more iron than it normally would, and a previously adequate diet can’t keep up.
Pregnancy is the clearest example. In the first trimester, iron requirements are relatively modest at about 0.8 mg absorbed per day. By the second trimester, that jumps to 4 to 5 mg per day. In the third trimester, the requirement exceeds 6 mg daily, driven by the expanding blood volume, the growing placenta, and the developing baby’s own iron needs. That’s roughly a sevenfold increase from early to late pregnancy, which is why iron deficiency is so common among pregnant women. Globally, 37% of pregnant women are anemic, with iron deficiency being the leading nutritional cause.
Rapid growth during infancy, childhood, and adolescence also increases iron demand. Teenagers, especially girls who have begun menstruating, are at particular risk because growth and menstrual losses compound each other. Endurance athletes can develop iron deficiency through a combination of increased red blood cell production, iron lost in sweat, and, in runners, the mechanical destruction of red blood cells from repeated foot strikes.
How Iron Deficiency Gets Diagnosed
The most reliable marker is ferritin, a protein that reflects how much iron your body has in storage. A ferritin level below 30 ng/mL is widely used to diagnose iron deficiency, and some hematologists use a threshold of 50 ng/mL, recognizing that symptoms like fatigue and brain fog can appear well before stores are completely emptied. Many standard lab reports list the “normal” lower limit as low as 7 to 10 ng/mL based on population averages, which can be misleadingly reassuring. You can technically fall within the reference range and still be functionally iron deficient.
Ferritin can also be artificially elevated by inflammation, infection, or liver disease, which makes interpretation trickier in people with chronic conditions. In those cases, additional blood tests help clarify the picture.
Multiple Causes Often Overlap
Iron deficiency rarely has a single, neat explanation. A vegetarian woman with heavy periods and a daily tea habit has three factors working against her simultaneously: limited heme iron in her diet, regular blood loss, and tannins reducing absorption of the non-heme iron she does eat. Someone with celiac disease may also have poor dietary intake because they feel too unwell to eat much. Identifying all the contributing factors, not just the most obvious one, is what leads to lasting correction rather than a cycle of depletion, supplementation, and depletion again.