Low iron typically comes down to one of three problems: you’re losing blood somewhere, your body isn’t absorbing iron well, or you’re not getting enough from your diet. Often it’s a combination. Understanding which category fits your situation is the first step toward fixing it, because the solution depends entirely on the cause.
Blood Loss Is the Most Common Cause
Every time you lose blood, you lose iron. Red blood cells contain most of your body’s iron supply, so anything that causes bleeding, even slowly, can drain your stores over weeks and months.
Heavy menstrual periods are the single most frequent reason premenopausal women develop low iron. Women aged 19 to 50 need 18 mg of iron daily, more than double the 8 mg men need, precisely because of monthly blood loss. If your periods are heavy, long, or frequent, your dietary intake may simply not keep pace with what you’re losing each cycle. Pregnancy raises the requirement even further to 27 mg per day, since your blood volume expands significantly to support the baby.
Internal bleeding you can’t see is another major culprit, and it’s the reason doctors take low iron seriously in men and postmenopausal women. Peptic ulcers, which are open sores in the stomach or upper small intestine, are the most common source of upper gastrointestinal bleeding. Regular use of anti-inflammatory painkillers like ibuprofen and aspirin damages the stomach lining and can trigger these ulcers. Colon polyps, small growths on the lining of the colon, can also bleed slowly. Most polyps are harmless, but some can become cancerous, which is why unexplained iron deficiency sometimes prompts a colonoscopy.
Your Gut May Not Be Absorbing Iron Properly
You could eat plenty of iron-rich food and still end up deficient if your small intestine can’t absorb it. Iron enters your bloodstream through the lining of the duodenum and the first stretch of the small intestine. Anything that damages that lining reduces how much iron actually gets through.
Celiac disease is a prime example. When someone with celiac eats gluten, their immune system attacks the lining of the small intestine, flattening the tiny finger-like projections (called villi) that absorb nutrients. Because the damage happens in exactly the same spot where iron absorption takes place, iron deficiency is often the first sign of undiagnosed celiac disease. The intestinal damage can also cause small amounts of internal bleeding, compounding the problem. Inflammatory bowel diseases like Crohn’s can have a similar effect.
Gastric bypass and other weight-loss surgeries physically reroute food past the duodenum, which dramatically cuts iron absorption. If you’ve had this type of surgery, you’ll likely need supplementation long-term.
Chronic Inflammation Locks Iron Away
This one surprises many people. If you have a chronic inflammatory condition, such as rheumatoid arthritis, lupus, kidney disease, or a long-standing infection, your body may have plenty of iron stored but refuse to release it into your bloodstream.
Here’s what happens: inflammation triggers your liver to produce a hormone called hepcidin. Hepcidin’s job is to shut down iron export from cells. It blocks the protein that moves iron out of storage cells and into the blood, so iron stays trapped inside cells instead of reaching your bone marrow where it’s needed to make red blood cells. This is your body’s ancient defense mechanism. During infections, withholding iron starves bacteria that need it to grow. But when inflammation is chronic, this defense backfires and leaves you functionally iron-deficient even though your total iron stores may look normal on some tests.
This is why your doctor may order multiple blood tests rather than relying on a single number. A protein called ferritin, which reflects iron stores, is normally considered low below 30 ng/mL. But ferritin itself rises during inflammation, so someone with a chronic disease might have a ferritin of 60 or 70 and still be iron-deficient. In those cases, doctors often use a higher threshold of 100 ng/mL to account for the inflammatory effect.
Your Diet May Fall Short
If none of the above applies to you, the answer may simply be that you’re not eating enough iron or not eating it in a form your body can use easily. There are two types of dietary iron: heme iron from animal sources (red meat, poultry, fish) and non-heme iron from plants (beans, lentils, spinach, fortified cereals). Your body absorbs heme iron much more efficiently than non-heme iron, which is why vegetarians and vegans are at higher risk for deficiency.
What you eat alongside iron matters just as much as the iron itself. Vitamin C dramatically boosts absorption of plant-based iron. Research shows that adding vitamin C to a meal can increase non-heme iron absorption from less than 1% to over 7%, depending on the dose. Squeezing lemon over lentils or eating bell peppers with beans is a practical way to take advantage of this.
On the flip side, several common foods and drinks actively block iron absorption. Coffee can inhibit iron absorption by as much as 60% in a single cup. Certain teas and cocoa are even more potent, reducing absorption by up to 90%. Phytate compounds found in whole grains, legumes, and nuts can cut absorption by 50 to 65%. Calcium in amounts above 300 mg (roughly the amount in a glass of milk) also interferes with iron uptake. None of this means you should avoid these foods entirely, but drinking your coffee or tea between meals rather than with them can make a real difference.
Exercise Can Contribute
Endurance athletes, especially runners, face a unique form of iron loss. The repeated impact of feet hitting the ground damages red blood cells as they pass through capillaries in the soles of the feet. A single run won’t cause meaningful iron loss, but daily or twice-daily training sessions have a cumulative effect that can become significant. Marathon runners and other high-volume athletes are particularly at risk. Oxidative stress from sustained high-intensity effort and the physical compression of blood cells in working muscles add to the damage.
Athletes also lose small amounts of iron through sweat. Combined with the red blood cell destruction, this means serious runners and endurance athletes often need to pay closer attention to their iron intake than the general population.
Who’s at Highest Risk
- Women with heavy periods, especially those who also follow a plant-based diet
- Pregnant women, whose iron needs jump to 27 mg daily
- People with celiac disease or inflammatory bowel disease, due to malabsorption in the small intestine
- Anyone taking NSAIDs regularly, which can cause slow bleeding from stomach ulcers
- Endurance athletes, particularly runners training at high volume
- Vegetarians and vegans, since plant-based iron is absorbed less efficiently
- Infants and young children who drink too much cow’s milk, which is low in iron and can displace iron-rich foods from the diet
How Iron Deficiency Is Diagnosed
A standard blood count can reveal anemia, but catching low iron before it becomes full-blown anemia requires checking ferritin levels. Normal ferritin ranges from 30 to 300 ng/mL. Levels below 30 ng/mL are specific for iron deficiency, meaning if your ferritin is that low, depleted iron stores are almost certainly the reason. Your doctor may also check how saturated your blood’s iron-transport proteins are, which helps distinguish true iron deficiency from the inflammatory type where iron is present but locked away.
The cause of iron deficiency matters as much as the diagnosis itself. In men and postmenopausal women, low iron without an obvious dietary explanation typically warrants investigation for internal bleeding. In premenopausal women, heavy periods are the likely explanation, but other causes should still be considered if iron levels don’t improve with supplementation and dietary changes.