Low iron usually comes down to one of three things: you’re losing blood somewhere, you’re not absorbing iron well, or you’re not getting enough from your diet. Sometimes it’s a combination. The specific cause matters because it determines whether the fix is as simple as changing what you eat or whether something deeper needs attention.
Iron deficiency is diagnosed when serum ferritin drops below 30 μg/L in adults or below 20 μg/L in children. Until recently, many labs flagged levels as low only at 12 to 15 μg/L, but that threshold was raised in 2024 to better catch deficiency before it progresses to anemia. So if you were told your iron was “normal” in the past but feel otherwise, it’s worth rechecking against the updated range.
Blood Loss You Might Not Notice
The most common reason for low iron in adults is chronic blood loss, and it doesn’t have to be dramatic. A slow, steady bleed inside the digestive tract can drain your iron stores over months without obvious symptoms. Peptic ulcers, hiatal hernias, colon polyps, and colorectal cancer all cause this kind of hidden bleeding. You may never see blood in your stool because the amounts are small enough to go undetected without testing.
This is why doctors often order stool tests or recommend a colonoscopy when iron is unexplainably low, especially in men or postmenopausal women. In these groups, there’s no expected source of regular blood loss, so the body should be able to maintain its iron stores relatively easily. When it can’t, something is usually taking iron out faster than it’s coming in.
Heavy Periods Are a Leading Cause
For women and teens who menstruate, monthly blood loss is the single biggest driver of iron deficiency. Every period depletes iron, and when bleeding is heavy, the body simply can’t replenish stores fast enough between cycles. This creates a pattern where iron levels slowly drop month after month.
Heavy menstrual bleeding doesn’t have a strict volume cutoff in practice. It’s defined as bleeding heavy enough to interfere with your physical or emotional quality of life. If you’re soaking through pads or tampons every hour, passing large clots, or bleeding for more than seven days, that level of loss can push you into deficiency. The daily iron requirement for premenopausal women is 18 mg, more than double the 8 mg that men and postmenopausal women need, precisely because of this monthly loss.
Your Gut Might Not Be Absorbing Iron
Iron is absorbed in the upper part of your small intestine. For that process to work, the intestinal lining needs to be healthy, and conditions that damage it can quietly block absorption even if your diet is iron-rich.
Celiac disease is one of the most overlooked culprits. The immune reaction triggered by gluten flattens the absorptive surface of the intestine, making it harder to pull iron (and other nutrients) from food. Crohn’s disease and ulcerative colitis cause similar problems through chronic inflammation. Gastric bypass or any surgery that removes or reroutes part of the small intestine also reduces the surface area available to absorb iron.
Even your stomach plays a role. Iron from food exists in a form that needs stomach acid to become absorbable. When acid levels drop, iron passes through without being properly taken up. This is why long-term use of proton pump inhibitors (like omeprazole) and other acid-reducing medications can lower iron over time. These drugs raise stomach pH, making it harder for iron to convert into its absorbable form. Research in mice has also shown that omeprazole directly increases levels of a hormone called hepcidin, which actively blocks iron from crossing the intestinal wall into the bloodstream. So acid-suppressing drugs may interfere with iron through two separate pathways at once.
Inflammation Can Lock Iron Away
This one surprises most people. You can have plenty of iron stored in your body and still be functionally iron-deficient because inflammation prevents it from reaching your blood.
When your immune system is activated by a chronic condition, whether that’s an autoimmune disease, chronic kidney disease, heart failure, a long-standing infection, or even advanced cancer, your body ramps up production of hepcidin. This hormone degrades the protein that exports iron out of your cells and into circulation. The result: iron gets trapped inside storage cells and can’t be used to make red blood cells. Your iron supply looks adequate on some lab tests but is effectively unavailable.
This type, called anemia of inflammation, produces a mild to moderate anemia that won’t respond to iron supplements alone because the problem isn’t a lack of iron. It’s a blockade. Treating the underlying condition is what eventually restores normal iron flow.
Not Enough Iron in Your Diet
Dietary deficiency is more common than many people realize, particularly in certain groups. The body absorbs heme iron (from meat, poultry, and seafood) far more efficiently than non-heme iron (from plants, beans, and fortified grains). Vegetarians and vegans need roughly twice the recommended daily intake because plant-based iron is harder to absorb.
Pregnancy dramatically increases iron needs. The recommended intake jumps to 27 mg per day, up from 18 mg for non-pregnant women, because of the expanding blood volume and the demands of a growing fetus. Many women enter pregnancy with already marginal iron stores from years of menstrual losses, and the added demand tips them into deficiency quickly. Treatment is typically recommended when ferritin falls below 50 μg/L during pregnancy, a higher threshold than for other adults.
Here’s a quick reference for daily iron needs across different life stages:
- Children 1 to 3 years: 7 mg
- Children 4 to 8 years: 10 mg
- Teen girls 14 to 18: 15 mg
- Teen boys 14 to 18: 11 mg
- Adult women 19 to 50: 18 mg
- Adult men 19 to 50: 8 mg
- Adults over 51: 8 mg
- Pregnant women: 27 mg
Exercise Can Deplete Iron Too
Endurance athletes, especially distance runners, face a less well-known cause of iron loss. The repeated impact of feet striking pavement actually ruptures red blood cells in the small blood vessels of the feet, a phenomenon called footstrike hemolysis. The body compensates by producing new red blood cells, but this cycle gradually depletes iron stores. Over time, it can evolve into full iron-deficiency anemia, particularly in athletes who aren’t deliberately eating iron-rich foods to offset the losses.
Runners also lose small amounts of iron through sweat and, in some cases, through microscopic GI bleeding triggered by the physical jostling of long runs. The combination of these losses means that competitive or high-volume runners need to monitor their iron levels more closely than sedentary adults.
Multiple Factors Often Overlap
In practice, low iron rarely has just one cause. A woman with heavy periods who also takes a proton pump inhibitor and eats a mostly plant-based diet is being hit from three directions at once. A runner with celiac disease faces absorption problems on top of exercise-related losses. Identifying all the contributing factors is what makes the difference between a fix that works and one that doesn’t.
If your iron has been low repeatedly despite supplements, that’s a signal to look beyond diet. Persistent deficiency that doesn’t respond to oral iron points toward an absorption problem, an ongoing source of blood loss, or an inflammatory condition keeping iron locked away from your bloodstream.