Women between 19 and 50 need 18 mg of iron per day, more than double the 8 mg recommended for men. That gap exists almost entirely because of menstrual blood loss, but periods are only one piece of a larger picture. Low iron in women typically results from some combination of losing too much iron, not absorbing enough, or facing higher-than-normal demands.
Menstrual Blood Loss Is the Leading Cause
Every menstrual cycle removes iron from the body. For most women, this loss is manageable with a balanced diet. But when periods are heavy, prolonged, or frequent, iron leaves the body faster than food can replace it. Clinically heavy periods, defined as soaking through a pad or tampon every hour for several hours or passing clots larger than a quarter, can quietly drain iron stores over months or years without an obvious crisis.
The tricky part is that many women normalize heavy bleeding. They assume their flow is typical because they’ve never had a reason to compare. Meanwhile, their ferritin (the protein that stores iron) drops steadily. Fatigue, brain fog, and shortness of breath creep in so gradually they get chalked up to stress or poor sleep.
Uterine Fibroids and Other Structural Problems
Heavy periods don’t always happen on their own. Uterine fibroids, which are noncancerous growths in or on the uterus, are one of the most common reasons women bleed excessively. Fibroids can make periods heavier, longer, and sometimes cause bleeding between cycles. That repeated blood loss leads directly to iron depletion and, eventually, anemia. Cleveland Clinic notes that fatigue from fibroids is usually not caused by the growths themselves but by the anemia that develops from chronic blood loss.
Adenomyosis, a condition where the tissue that normally lines the uterus grows into its muscular wall, causes similar problems. Endometriosis and polyps can also increase bleeding. Any structural issue that amplifies menstrual blood loss accelerates iron depletion.
Pregnancy and Breastfeeding
Pregnancy dramatically increases the body’s need for iron. Blood plasma volume expands by 40% to 50% during a single pregnancy, while the number of red blood cells grows only 15% to 25%. That imbalance means the body needs far more iron to keep up with red blood cell production, placental development, and fetal growth. The recommended daily iron intake during pregnancy jumps to 27 mg, triple what non-pregnant men need and 50% more than non-pregnant women of the same age.
Breastfeeding lowers the requirement to about 9 mg per day, but many women enter the postpartum period already depleted from pregnancy and delivery. Blood loss during childbirth compounds the problem, and iron stores can take months to rebuild without deliberate dietary changes or supplementation.
Diet and Absorption Barriers
Iron comes in two forms in food. Heme iron, found in meat, poultry, and fish, is absorbed relatively efficiently. Non-heme iron, found in plant foods like spinach, beans, and fortified grains, is absorbed at much lower rates, sometimes as little as 1% to 23% depending on what else you eat at the same meal.
Several common foods and drinks actively block iron absorption. Phytic acid, found in whole grains, seeds, legumes, and some nuts, binds to iron in the gut and prevents it from being absorbed. Tannins in tea and coffee do the same. Calcium-rich foods eaten alongside iron-rich foods also reduce uptake. These effects only matter when these inhibitors are consumed at the same meal as iron, so spacing them out can make a real difference.
Women who follow vegetarian or vegan diets face a compounded challenge: their iron sources are exclusively non-heme, and many staple plant foods (beans, whole grains, nuts) contain the very compounds that block absorption. Pairing iron-rich foods with vitamin C, which enhances non-heme iron absorption, helps counteract this.
Medications That Reduce Absorption
Long-term use of proton pump inhibitors, the acid-suppressing medications commonly taken for heartburn or reflux, can interfere with iron absorption. Stomach acid helps convert iron into a form the body can use, and suppressing that acid may make it harder to absorb dietary iron and to respond to iron supplements. A systematic review in the American Journal of Gastroenterology found that while healthy people on these medications rarely develop outright deficiency from the drugs alone, people who are already iron-deficient may struggle to rebuild their stores while taking them.
Gut Conditions That Block Iron Uptake
Iron is primarily absorbed in the upper part of the small intestine. Any condition that damages or inflames that area can cripple iron absorption regardless of how much iron you eat. Celiac disease is the classic example. The immune reaction triggered by gluten destroys the tiny fingerlike projections (villi) that line the intestine and absorb nutrients. Even after starting a gluten-free diet, it can take time for those structures to heal and for iron-absorbing function to return.
Inflammatory bowel diseases like Crohn’s disease, especially when it affects the upper intestine, cause similar absorption problems. Gastric bypass surgery, which reroutes food past much of the stomach and upper intestine, also reduces the body’s ability to take in iron. Women with any of these conditions face a double burden: they lose iron through menstruation like everyone else but can’t replace it efficiently through food.
Exercise and Iron Loss in Active Women
Female athletes, particularly runners, lose iron through a mechanism most people have never heard of. Every time your foot strikes the ground, red blood cells passing through the capillaries in the soles of your feet are physically crushed by the impact. This is called foot-strike hemolysis, and research published in the Journal of Applied Physiology confirmed it by comparing runners to cyclists. Runners showed significantly more red blood cell destruction after exercise, and since the only meaningful difference between the two activities is foot impact, the evidence points directly to that mechanical trauma.
A single run doesn’t destroy enough red blood cells to matter. But daily or twice-daily training sessions create a cumulative effect that can deplete iron stores over weeks and months. Athletes in other high-impact sports like basketball and tennis show lower iron stores than those in low-impact sports like cycling and rowing, even after adjusting for body size. Combined with menstrual losses and sometimes restrictive diets aimed at weight management, active women are especially vulnerable.
How Low Iron Gets Diagnosed
The standard screening test measures ferritin, which reflects how much iron your body has in reserve. In otherwise healthy people, a ferritin level below 30 ng/mL generally indicates iron deficiency. If you have an infection or inflammation (which artificially raises ferritin), the threshold for adults bumps up to 70 ng/mL to account for the distortion. Ferritin above 150 ng/mL in menstruating women is considered normal to high.
Iron deficiency progresses in stages. First, stored iron (ferritin) drops. Then the body’s ability to make new red blood cells slows. Finally, hemoglobin falls and full iron-deficiency anemia develops. Many women are symptomatic well before they reach the anemia stage. Fatigue, difficulty concentrating, pale skin, cold hands and feet, brittle nails, and unusual cravings for ice or non-food items can all appear when ferritin is low but hemoglobin is still technically normal. If a blood test only checks hemoglobin without measuring ferritin, early iron deficiency gets missed entirely.
Why Multiple Causes Often Overlap
Low iron in women is rarely caused by one factor alone. A woman with moderately heavy periods who also drinks tea with meals and takes an acid-suppressing medication may deplete her stores gradually over years. A runner who eats a plant-based diet and has undiagnosed celiac disease may crash much faster. The combination matters more than any single cause, which is why identifying and addressing all contributing factors, not just the most obvious one, is what actually resolves the problem long-term.