Low iron typically comes down to one of three things: you’re not absorbing enough from food, you’re losing blood somewhere, or your body’s demand for iron has outpaced your intake. Sometimes it’s a combination. The tricky part is that iron deficiency can make you feel exhausted, short of breath, and mentally foggy well before it shows up as full-blown anemia on a blood test. Understanding the specific reason your iron is low matters because the fix depends entirely on the cause.
Your Diet May Not Deliver as Much Iron as You Think
Even if you eat iron-rich foods regularly, your body only absorbs a fraction of what’s on your plate. Iron from animal sources (called heme iron) is absorbed at a rate of about 25%. Iron from plants, grains, and fortified foods is absorbed at 17% or less. If your diet is mostly plant-based, your overall iron bioavailability drops to somewhere between 5% and 12%, compared to 14% to 18% for people who eat meat regularly.
What you eat alongside iron-rich foods also makes a big difference. Vitamin C boosts absorption significantly, which is why pairing spinach with citrus or peppers helps. On the other hand, several common foods and drinks actively block iron absorption: the tannins in tea, coffee, and red wine, phytates in whole grains and legumes, and calcium in dairy products all interfere with uptake. If your morning routine involves coffee with breakfast and tea in the afternoon, you could be undermining your iron intake without realizing it.
Blood Loss You Might Not Notice
Slow, chronic blood loss is one of the most common causes of iron deficiency, and it often happens without obvious symptoms. A peptic ulcer, hiatal hernia, colon polyp, or colorectal cancer can all cause small but steady bleeding in the digestive tract. This blood loss is sometimes so gradual that you wouldn’t see it, but over weeks and months it drains your iron stores.
Regular use of over-the-counter pain relievers like aspirin, ibuprofen, and naproxen can also cause internal bleeding in the stomach or intestines. If you take these frequently for headaches, joint pain, or other chronic issues, they could be a hidden contributor to your low iron.
Heavy Periods Are a Leading Cause in Women
Menstrual blood loss is the single biggest reason premenopausal women develop iron deficiency. A normal period doesn’t usually cause problems, but heavy menstrual bleeding, defined as losing more than 80 mL of blood per cycle, affects an estimated 18% to 38% of women of reproductive age. Women with heavy periods lose five to six times more iron per cycle than women with normal flow.
The consequences are significant. About 30% of women and adolescents with heavy periods have depleted iron stores, and 60% develop iron-deficiency anemia. The prevalence of heavy bleeding also tends to increase as women approach menopause, which means iron deficiency can worsen during the very years many women assume it should be improving.
Pregnancy Demands Dramatically More Iron
Pregnancy roughly doubles or triples your daily iron needs. In the first trimester, requirements are actually lower than usual because menstruation stops, sitting at about 0.8 mg per day. But they climb steeply after that: 4 to 5 mg daily in the second trimester and over 6 mg in the third. Over the full pregnancy, a woman needs roughly 1,000 mg of additional iron to support the growing placenta, expanded blood volume, and fetal development. Without supplementation or a very iron-dense diet, most women can’t keep up with that demand.
Your Gut Might Not Be Absorbing Iron Properly
Iron absorption happens in the upper part of the small intestine, and it depends on a surprisingly complex chain of events. Your stomach first needs to produce enough acid to convert iron into a form your intestinal cells can take up. Anything that disrupts this process, from the stomach acid stage to the intestinal lining itself, can leave you deficient regardless of how much iron you eat.
Celiac disease is one of the most significant culprits. It damages the lining of the small intestine, and roughly 46% of people with celiac disease develop iron-deficiency anemia. Crohn’s disease and other inflammatory bowel conditions can cause similar damage. An infection with H. pylori, a common stomach bacterium, is another underrecognized cause. It can trigger ulcers and interfere with iron uptake through mechanisms that aren’t fully understood.
Surgical procedures that remove or bypass portions of the stomach or small intestine, including gastric bypass for weight loss, also reduce your absorptive surface area. People who’ve had these surgeries often need lifelong iron monitoring.
Chronic Inflammation Can Trap Iron in Your Body
This one is counterintuitive: you can have plenty of iron stored in your body and still be functionally iron-deficient. When your immune system is fighting a chronic illness, whether it’s an autoimmune condition, a long-term infection, kidney disease, or heart failure, the resulting inflammation triggers your body to lock iron away inside immune cells called macrophages. At the same time, your intestines absorb less iron from food.
The net effect is that iron gets sequestered where your bone marrow can’t use it to make red blood cells. Blood tests in this situation look different from straightforward iron deficiency: serum iron is low, but ferritin (the storage protein) is normal or even elevated. This pattern, sometimes called anemia of inflammation, requires a different treatment approach than simple dietary iron deficiency, which is why identifying it correctly matters.
Intense Exercise Can Deplete Iron
Endurance athletes and people who train intensely are at elevated risk for low iron through several overlapping mechanisms. During high-impact activities like running, repeated foot strikes physically damage older, less elastic red blood cells, releasing their iron in a process called intravascular hemolysis. The body also loses small amounts of iron through sweat.
But the bigger issue is what happens after exercise. Intense physical activity triggers an inflammatory response that raises levels of a hormone called hepcidin. This hormone acts like a gatekeeper, blocking iron absorption from your gut and preventing stored iron from being released into your bloodstream. The effect is temporary, but if you train hard day after day, those repeated spikes in hepcidin can keep your iron levels chronically suppressed.
Rare Genetic Causes
In uncommon cases, low iron has a genetic origin. A condition called iron-refractory iron deficiency anemia, or IRIDA, is caused by inherited mutations that prevent your body from properly regulating hepcidin. People with IRIDA produce too much hepcidin even when their iron is critically low, which blocks absorption so effectively that oral iron supplements simply don’t work. The hallmarks are a lifelong pattern of anemia that doesn’t respond to iron pills and only partially improves with intravenous iron. If you’ve been supplementing for months without your levels budging, this is worth discussing with your doctor.
You Can Feel It Before It Shows on Standard Tests
Iron deficiency doesn’t follow a neat progression from “fine” to “anemic.” Many people experience real symptoms, particularly fatigue and reduced exercise tolerance, while their hemoglobin levels are still technically normal. This stage, sometimes called iron deficiency without anemia, is diagnosed by checking ferritin levels. The World Health Organization defines iron deficiency in adults as a ferritin level below 15 micrograms per liter in healthy individuals, or below 70 in people with active infection or inflammation (since inflammation artificially raises ferritin).
If your ferritin is low but your hemoglobin looks fine, your iron stores are depleted even if you haven’t crossed the threshold into anemia. Research consistently shows that replenishing iron at this stage improves fatigue. The muscle weakness, breathlessness during exercise, and brain fog that come with low iron happen because your muscles, heart, and brain all depend on iron-containing proteins to function, not just your red blood cells.