Low iron happens when your body loses iron faster than it takes in, absorbs less than it needs, or faces a sudden spike in demand it can’t keep up with. It affects over 1.6 billion people worldwide, making it the most common nutritional deficiency on the planet. The causes fall into a few clear categories: blood loss, poor absorption, not enough iron in your diet, increased bodily demand, and chronic inflammation that locks iron away inside your cells.
Blood Loss Is the Most Common Cause
Every drop of blood contains iron packed inside red blood cells, so any source of ongoing bleeding steadily drains your reserves. In men and postmenopausal women, the most likely culprit is bleeding somewhere in the digestive tract. Ulcers, polyps, hernias, and colorectal cancers can all cause slow, invisible bleeding that goes unnoticed for months. An estimated 61% of patients with gastrointestinal bleeding have iron deficiency anemia, often before they notice any visible blood in their stool.
For premenopausal women, menstrual periods are the leading cause. A typical cycle results in roughly 1 mg of iron lost. That sounds small, but your body only absorbs 1 to 2 mg per day from food under ideal conditions, so even normal periods put women closer to the edge. Women with heavy periods (menorrhagia) lose about 5 to 6 times that amount per cycle, with some losing over 8 mg. At that rate, diet alone almost never keeps up.
Other sources of blood loss include frequent blood donations, surgery, and childbirth, where roughly 250 mg of iron is lost in blood at delivery alone.
Your Diet May Not Provide Enough
Iron from food comes in two forms. Heme iron, found in meat, poultry, and seafood, is absorbed at a rate of about 25%. Non-heme iron, found in plants, beans, fortified cereals, and eggs, is absorbed at 17% or less. That gap matters. If most of your iron comes from plant sources, you need to eat considerably more of it to absorb the same amount.
People following vegan or vegetarian diets are at higher risk for this reason, though the risk is manageable with intentional food choices. In the U.S. and Canada, about 10% of toddlers, young girls, and women of childbearing age are iron deficient despite living in a resource-rich setting. In lower-income regions, food insecurity makes the problem far more widespread.
What You Eat With Iron Matters Too
Certain compounds in food and drinks actively block iron absorption when consumed at the same meal. Tannins, found in tea, coffee, and some spices, are among the strongest inhibitors. Drinking tea or coffee with a meal can reduce iron absorption by 60 to 90% compared to drinking water. Even 150 mL of tea (a small cup) roughly cuts absorption in half. Oregano and spinach also contain compounds that reduce bioavailability by 30 to 60%.
Phytates, found in whole grains, legumes, nuts, and seeds, form complexes with iron in the gut that your body can’t break apart. Calcium has a similar blocking effect. None of this means you should avoid these foods entirely. Timing is what matters. Drinking your coffee between meals instead of with them, or pairing iron-rich foods with vitamin C (which boosts absorption), can make a real difference.
Your Gut May Not Be Absorbing Iron Properly
Iron is absorbed primarily in the upper part of the small intestine, so anything that damages or bypasses that stretch of gut can cause deficiency regardless of how much iron you eat.
Celiac disease is a classic example. It’s an immune reaction to gluten that inflames and flattens the lining of the upper small intestine, reducing the surface area where iron is taken up. The damage also disrupts the proteins your intestinal cells use to transport iron into the bloodstream. Some people are diagnosed with celiac disease only after unexplained iron deficiency leads to further testing.
Gastric bypass and other weight loss surgeries physically reroute food past the duodenum, the section of gut most responsible for iron absorption. Inflammatory bowel diseases like Crohn’s can damage the same area. Even long-term use of acid-reducing medications (proton pump inhibitors) can interfere with iron uptake, because stomach acid helps convert dietary non-heme iron into a form the gut can actually absorb. Without sufficient acid, the iron passes through unused.
Pregnancy and Growth Increase Iron Demand
Sometimes the problem isn’t that you’re losing iron or failing to absorb it. Your body simply needs more than usual. Pregnancy is the most dramatic example. A single pregnancy requires roughly 1,000 mg of iron total: about 350 mg goes to the fetus and placenta, 250 mg is lost in blood during delivery, 450 mg is needed to expand the mother’s own blood volume, and another 240 mg covers baseline daily losses over nine months.
Most of that demand is packed into the second and third trimesters, when a pregnant woman needs to absorb about 6 mg of iron per day. Since even a well-balanced diet delivers only 1 to 2 mg of absorbed iron daily, supplementation during pregnancy is standard practice.
Children going through rapid growth spurts and adolescents (especially girls who have begun menstruating) also face higher iron needs. Globally, 40% of children aged 6 months to 5 years are anemic, with iron deficiency as a primary driver.
Chronic Inflammation Traps Iron in Your Body
This one is counterintuitive. In chronic inflammatory conditions like rheumatoid arthritis, kidney disease, heart failure, or ongoing infections, your body may actually have adequate iron stores but refuse to release them. The mechanism involves a hormone called hepcidin, which your liver produces in higher amounts during inflammation.
Hepcidin blocks ferroportin, the only transport channel that moves iron out of your cells and into your blood. When hepcidin is high, iron gets trapped inside immune cells and intestinal cells. Your bone marrow, which needs iron to make red blood cells, is starved even though the iron technically exists in your body. This is sometimes called “anemia of chronic disease,” and it’s one reason iron levels can look confusing on lab tests. Ferritin (a marker of stored iron) may appear normal or even high because inflammation itself raises ferritin levels. The WHO recommends using a higher ferritin cutoff of 70 µg/L in adults with active inflammation, compared to the standard threshold, to account for this effect.
How These Causes Overlap
In practice, low iron rarely comes from a single cause. A woman with heavy periods who drinks tea with meals and follows a vegetarian diet is being hit from three directions at once. Someone with Crohn’s disease may have both malabsorption and intestinal blood loss. A pregnant woman with morning sickness who can’t tolerate iron-rich foods faces increased demand and decreased intake simultaneously.
This is why identifying the underlying cause matters more than simply taking a supplement. Iron supplements can refill your stores temporarily, but if the drain continues, levels will drop again. A blood test measuring serum ferritin is the most common first step. In healthy individuals, levels below 15 µg/L generally confirm iron deficiency. In people with inflammation, that threshold rises to 30 µg/L for children and 70 µg/L for adults. If the cause of iron loss isn’t obvious from your history, further testing to check for hidden bleeding or absorption problems is typically the next move.