Low iron usually comes down to one of three problems: you’re losing blood, you’re not absorbing enough iron from food, or your body’s demand for iron has outpaced your supply. Sometimes more than one of these is happening at once. Understanding the specific cause matters because the fix for each one looks different.
Your body has no active way to excrete iron, so it tightly controls how much gets absorbed from food. A hormone called hepcidin acts as the master regulator. When your iron stores are adequate, hepcidin rises and blocks absorption. When stores drop, hepcidin falls and your gut lets more iron through. Problems arise when something disrupts this system or when losses simply outpace what absorption can keep up with.
Blood Loss Is the Most Common Cause
Every milliliter of blood contains about half a milligram of iron. Lose blood regularly, even in small amounts, and your iron stores drain faster than your diet can replenish them. The tricky part is that the bleeding doesn’t have to be obvious.
Heavy menstrual periods are the leading cause of iron deficiency in premenopausal women. If you soak through a pad or tampon every hour or two, pass clots larger than a quarter, or bleed for more than seven days, you’re likely losing enough blood to deplete your iron over time. Many women assume their periods are normal simply because they’ve always been heavy, so this cause frequently goes unrecognized for years.
Internal bleeding in the digestive tract is the other major culprit, especially in men and postmenopausal women. Peptic ulcers, colon polyps, hiatal hernias, and colorectal cancer can all cause slow, steady blood loss you’d never notice. Inflammatory bowel conditions like Crohn’s disease and ulcerative colitis do the same. This is why doctors often order a colonoscopy or upper endoscopy when someone has unexplained iron deficiency, particularly after age 50.
Frequent blood donation, nosebleeds, and urinary tract bleeding are less common but real contributors.
Your Diet May Not Provide Enough
The recommended daily iron intake for women aged 19 to 50 is 18 mg. For men in the same age range, it’s 8 mg. During pregnancy, that number jumps to 27 mg. After menopause, women’s needs drop to 8 mg as well.
Iron from food comes in two forms. Heme iron, found in meat, poultry, and seafood, is absorbed efficiently by the gut. Non-heme iron, found in beans, lentils, spinach, fortified cereals, and other plant foods, is harder for the body to take up. This distinction has real consequences: people who follow vegetarian or vegan diets need roughly 1.8 times more iron than meat eaters to compensate for the lower absorption rate of plant-based iron.
What you eat alongside iron-rich foods also matters. Vitamin C (from citrus, bell peppers, or tomatoes) significantly improves absorption of non-heme iron when eaten at the same meal. On the other hand, bran fiber, large doses of calcium (especially from supplements), and compounds called phytates and tannins found in tea, coffee, whole grains, and legumes can block non-heme iron absorption. Someone eating an iron-rich meal with a cup of black tea is absorbing considerably less than someone pairing that same meal with orange juice.
Absorption Problems in the Gut
You could eat plenty of iron and still end up deficient if your body can’t absorb it properly. Most dietary iron is absorbed in the duodenum, the first stretch of the small intestine. Anything that damages or bypasses this area limits how much iron reaches your bloodstream.
Celiac disease is a classic example. The immune reaction triggered by gluten flattens the absorptive lining of the small intestine, and iron deficiency is sometimes the first and only sign of the condition. Crohn’s disease can cause similar damage when it affects the upper small intestine.
Gastric bypass surgery physically reroutes food around the duodenum, which is why iron deficiency is one of the most common nutritional problems after the procedure. People who’ve had this surgery typically need lifelong iron monitoring and supplementation.
Medications That Reduce Absorption
Proton pump inhibitors (like omeprazole and lansoprazole) and H2 blockers, both commonly used for acid reflux, reduce stomach acid production. That matters because the acidic environment of the stomach converts iron into a soluble form your intestines can absorb. When stomach pH rises from long-term acid suppression, iron stays in its insoluble form and passes right through. If you’ve been on one of these medications for months or years, it’s worth checking your iron levels.
Increased Demand During Pregnancy and Growth
Pregnancy creates an enormous spike in iron needs. In the first trimester, a pregnant person needs about 0.8 mg of absorbed iron per day, which is close to normal. By the second trimester, that requirement jumps to 4 to 5 mg per day. In the third trimester, it exceeds 6 mg daily. This iron supports the expanding blood volume, the growing placenta, and the developing baby. Most women can’t meet these demands through diet alone, which is why prenatal vitamins contain iron and why iron deficiency is extremely common during pregnancy even in well-nourished populations.
Adolescents, especially girls who have started menstruating, are another high-risk group. Rapid growth increases iron demand at the same time menstrual losses begin. Teen girls need 15 mg of iron per day, compared to 11 mg for teen boys.
Chronic Inflammation Locks Iron Away
This one surprises many people. You can have plenty of iron stored in your body and still have low iron circulating in your blood. Chronic inflammatory conditions, including rheumatoid arthritis, lupus, chronic kidney disease, chronic infections, and some cancers, trigger a process called iron sequestration.
Here’s what happens: inflammatory signals, particularly a molecule called interleukin-6, stimulate the liver to overproduce hepcidin. Elevated hepcidin traps iron inside storage cells (macrophages) and blocks new iron from being absorbed in the gut. From the body’s perspective, this is a defensive move: withholding iron from potential invaders that need it to grow. But the result is that your red blood cells can’t get enough iron to function normally, even though your total body iron may be adequate. This condition, sometimes called anemia of chronic disease, is the second most common form of anemia worldwide.
The distinction matters because simply taking iron supplements won’t fix this type of low iron. The underlying inflammation needs to be addressed first.
Exercise and Athletic Training
Endurance athletes, especially runners, are prone to a form of iron loss that doesn’t involve obvious bleeding. During running, red blood cells passing through the capillaries of the feet get mechanically crushed with each footstrike. This process, called footstrike hemolysis, releases hemoglobin into the bloodstream. Normally the body recycles this hemoglobin efficiently, but during intensive training the recycling system can be overwhelmed, and some iron gets lost through urine.
Compression of large muscle groups on capillaries during exercise can also damage older red blood cells. On top of that, the high oxygen demand of exercise increases oxidative stress on red blood cells, making them more fragile. Combine these mechanisms with iron lost through sweat and the increased blood volume that comes with endurance training, and it’s easy to see why athletes often run low.
How Iron Deficiency Gets Detected
A standard blood test measuring serum ferritin is the most reliable way to assess your iron stores. Ferritin below 30 ng/mL is widely used as the threshold for iron deficiency. Some experts use a cutoff of 50 ng/mL, particularly when symptoms like fatigue, brain fog, or hair loss are present even without anemia. Ferritin can be misleadingly normal during active inflammation, since it rises as part of the inflammatory response, so your doctor may check additional markers if an inflammatory condition is suspected.
Iron deficiency exists on a spectrum. You can be iron deficient without being anemic, meaning your stores are depleted but your hemoglobin hasn’t dropped yet. This stage still causes symptoms, including fatigue, difficulty concentrating, restless legs, cold hands and feet, and brittle nails. If the deficiency worsens, hemoglobin eventually falls, and you develop iron deficiency anemia, with more pronounced fatigue, shortness of breath, dizziness, and pale skin.
Because the causes of low iron range from dietary gaps to internal bleeding to chronic disease, identifying why your iron is low is just as important as knowing that it is. A ferritin level tells you the “what.” Finding the cause tells you what to do about it.