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. The causes fall into a few broad categories: you’re losing blood faster than you can replace the iron in it, you’re not absorbing enough iron from food, or your dietary intake is too low to meet your body’s demands. Often, more than one factor is at play.
How Your Body Regulates Iron
Your body has a tightly controlled system for managing iron levels, centered on a hormone called hepcidin. When iron levels are sufficient, hepcidin rises and blocks the only iron export channel on your cells (a protein called ferroportin). This prevents more iron from entering the bloodstream, both from food in your gut and from your body’s internal iron stores.
When iron levels drop, the opposite happens. Hepcidin falls, ferroportin stays active, and your intestines absorb more iron from your diet while stored iron gets released back into circulation. This feedback loop works well under normal conditions, but it can’t compensate when blood loss is heavy, absorption is impaired, or dietary iron is consistently too low.
Chronic Blood Loss
Slow, ongoing blood loss is the most common cause of iron deficiency anemia in adults. Every milliliter of blood you lose takes iron with it, and if losses outpace what you absorb from food, your stores gradually empty. What makes this dangerous is that the bleeding is often invisible.
Inside the gastrointestinal tract, peptic ulcers, hiatal hernias, colon polyps, and colorectal cancer can all cause small but steady bleeding that you might never notice in your stool. Regular use of aspirin or other anti-inflammatory painkillers can also irritate the stomach lining enough to cause chronic low-grade bleeding. In many cases, iron deficiency anemia is the first clue that something is wrong in the GI tract, which is why doctors often investigate the gut when anemia shows up without an obvious explanation.
Menstruation and Pregnancy
Menstrual blood loss is considered largely responsible for depleted iron stores in premenopausal women, and the numbers are striking. An estimated 20% to 65% of menstruating women in the U.S. have minimal to absent iron reserves. Heavy or prolonged periods, sometimes called menorrhagia, affect nearly a quarter of women between ages 40 and 50. Women who bleed more than about 80 mL (roughly 3 ounces) per cycle are at significantly higher risk.
Pregnancy creates a different kind of demand. The body expands its blood volume and ramps up red blood cell production to support the placenta and growing fetus. That’s why the recommended daily iron intake jumps from 18 mg for women of childbearing age to 27 mg during pregnancy. Without supplementation, many pregnant women simply can’t get enough iron from food alone to keep up.
Poor Iron Absorption
Even if you eat plenty of iron-rich foods, certain conditions can prevent your gut from absorbing it. Most dietary iron is absorbed in the duodenum, the first section of the small intestine. Celiac disease damages this part of the gut, reducing its ability to take in iron and other nutrients. Gastric bypass surgery physically reroutes food past the duodenum entirely, making iron deficiency one of the most common nutritional consequences of the procedure.
Stomach acid also plays a key role. Non-heme iron (the type found in plants and fortified foods) needs an acidic environment to be converted into a form the body can absorb. Long-term use of acid-reducing medications like proton pump inhibitors lowers stomach acid enough to significantly impair this process. If you’ve been on these medications for months or years, your iron absorption may be quietly declining.
Dietary Iron and What Blocks It
Not all iron in food is created equal. Heme iron, found in meat, poultry, and fish, is absorbed at a rate of roughly 15% to 35%. Non-heme iron, found in beans, grains, and leafy greens, is absorbed at less than 10% and sometimes far less. Organ meats lead the pack with absorption rates of 25% to 30%, while grains come in at about 4% and dried legumes at just 2%.
Heme iron makes up only about 10% of dietary iron overall, but because it’s absorbed so much more efficiently, it can account for more than 40% of the iron your body actually takes in. This is why the NIH recommends that vegetarians aim for 1.8 times the standard iron intake to compensate for the lower bioavailability of plant-based iron.
Several common food compounds actively block iron absorption. Polyphenols, found in tea, coffee, and beans, are among the most potent. In one study, polyphenol-rich tea reduced iron absorption from fortified bread by 56% to 72%. Traditional green tea reduced absorption from an iron supplement by more than 85%. Phytates in whole grains and legumes have a similar effect, as does calcium. On the other hand, vitamin C enhances iron absorption, which is why pairing iron-rich foods with citrus or peppers makes a real difference.
Daily Iron Requirements by Age and Sex
- Children 1 to 3 years: 7 mg
- Children 4 to 8 years: 10 mg
- Males 14 to 18: 11 mg
- Females 14 to 18: 15 mg
- Males 19 and older: 8 mg
- Females 19 to 50: 18 mg
- Females 51 and older: 8 mg
- Pregnancy: 27 mg
Rare Genetic Causes
In uncommon cases, iron deficiency anemia has a genetic origin. A condition called iron-refractory iron deficiency anemia is caused by mutations in the TMPRSS6 gene. At least 40 such mutations have been identified. They disable a protein that normally keeps hepcidin in check, so hepcidin stays inappropriately high. The result is that the body blocks iron absorption and locks away its iron stores even when levels are dangerously low. People with this condition don’t respond to oral iron supplements, which is a hallmark clue to the diagnosis.
How Long Recovery Takes
Once the underlying cause is addressed and iron supplementation begins, hemoglobin levels typically start rising within a few weeks. But normalizing hemoglobin is only half the job. Your body also needs to rebuild its deeper iron reserves (measured by a blood marker called ferritin), and that takes longer. Current guidelines recommend continuing iron supplementation for about three months after iron levels have corrected, specifically to replenish those stores and prevent a quick relapse.
Some people experience stomach upset, constipation, or nausea from iron supplements. Taking iron every other day rather than daily produces nearly the same improvement in iron levels with fewer side effects, which can make it easier to stick with the full course of treatment.