Low iron levels develop when your body loses more iron than it takes in, or when something prevents you from absorbing the iron you eat. This imbalance can happen gradually over months or even years, and the cause is often a combination of factors rather than a single one. Understanding what drives iron depletion helps you identify what might be behind your own low levels and what changes could make a difference.
Ferritin, a protein that reflects your body’s iron reserves, is the most reliable marker. Levels below 30 μg/L indicate iron deficiency, even if your red blood cell counts still look normal. The World Health Organization uses a stricter cutoff of below 15 μg/L for adults, but many clinicians treat at the higher threshold because symptoms like fatigue, brain fog, and hair loss often begin well before full-blown anemia sets in.
Blood Loss Is the Most Common Cause
Iron lives in your red blood cells, so any ongoing blood loss directly drains your reserves. Heavy menstrual periods are the leading cause of iron deficiency in premenopausal women. Losing more than 80 mL of blood per cycle (roughly soaking through a pad or tampon every hour for several hours) can outpace what your diet replaces.
Chronic use of anti-inflammatory painkillers like ibuprofen or aspirin is another underappreciated cause. These medications can create shallow erosions along the lining of your stomach and small intestine, producing slow, invisible bleeding you’d never notice. NSAID use is suspected to be the cause in 10 to 15 percent of patients with iron deficiency anemia. In clinical trials, over half of people taking NSAIDs long-term became anemic without any visible signs of bleeding.
Blood donation also has a significant impact. A single 500 mL donation removes roughly 200 to 250 mg of iron from your body. Donating just once a year drops ferritin levels by about 44 percent. Frequent donors who don’t actively replenish iron stores can become deficient within a few donation cycles.
Foods and Drinks That Block Absorption
You might eat enough iron-containing foods and still end up deficient if your meals are paired with compounds that block absorption. Two major categories do this: phenolic compounds (found in tea, coffee, and wine) and phytic acid (found in grains, legumes, and nuts).
The tannins in tea and coffee are particularly potent. Just 5 mg of tannic acid reduces iron absorption by 20 percent, 25 mg cuts it by 67 percent, and 100 mg blocks 88 percent. A single cup of black tea easily delivers enough tannins to fall in that range. Coffee inhibits absorption through the same tannin compounds plus chlorogenic acid. If you’re drinking tea or coffee with meals or shortly after, you’re significantly reducing how much iron your body pulls from that food.
Phytic acid works differently but with a similar result. It binds to iron in the gut and makes it unavailable. Cereals, legumes, oilseeds, and nuts are all high in phytic acid, and in grain-heavy diets, iron bioavailability can drop to just 5 to 15 percent of what the food technically contains. Wheat bran is among the highest sources, with phytic acid content ranging from 2.1 to 7.3 grams per 100 grams of dry weight. Soaking, sprouting, or fermenting grains and legumes before cooking breaks down a significant portion of their phytic acid and improves mineral absorption.
Calcium Competes With Iron
Calcium interferes with iron absorption regardless of whether it comes from dairy products or supplements. Research in intestinal cells shows that calcium doesn’t block iron from entering gut cells initially, but it does reduce the amount that makes it through to the bloodstream by decreasing the transport protein on the exit side of the cell. The practical takeaway: if you take both an iron supplement and a calcium supplement (or drink milk with an iron-rich meal), you’ll absorb less iron.
The good news is that this effect appears to be short-lived. After about four hours, cells adapt by increasing the expression of iron transport proteins, creating a rebound effect. Separating calcium-rich foods or supplements from iron-rich meals by two to three hours is generally enough to avoid the interference.
Medical Conditions That Prevent Absorption
Even with a perfect diet, certain conditions make it physically difficult for your body to absorb iron. Iron is primarily absorbed in the duodenum and the first part of the small intestine, so anything that damages or bypasses that section creates problems.
Celiac disease inflames and flattens the villi (tiny finger-like projections) lining the small intestine, reducing the surface area available to absorb nutrients. Unexplained iron deficiency is one of the most common reasons celiac disease gets diagnosed in adults who don’t have obvious digestive symptoms.
Gastric bypass surgery, particularly the Roux-en-Y procedure, physically reroutes food around the duodenum and upper small intestine. Since those are exactly the sections where iron absorption happens, post-surgical iron deficiency is extremely common and often requires long-term supplementation. Other inflammatory bowel conditions like Crohn’s disease can similarly damage the absorptive lining and impair iron uptake.
Intense Exercise Suppresses Iron Uptake
Endurance athletes and people who train at high intensity are at higher risk for iron deficiency through several overlapping mechanisms. Hard exercise triggers a temporary inflammatory response, which raises levels of a hormone called hepcidin. Hepcidin acts as the body’s iron gatekeeper: when it rises, iron absorption from the gut drops and iron already stored in cells gets locked in place. Hepcidin levels climb after exercise-induced inflammation and also follow a natural daily rhythm, peaking later in the day.
On top of the absorption problem, athletes lose iron through sweat, small amounts of blood in urine after intense efforts, and minor gastrointestinal bleeding during prolonged endurance exercise. The combination of reduced absorption and increased losses explains why distance runners, cyclists, and other endurance athletes frequently show depleted iron stores even when their diets look adequate on paper.
Dietary Patterns That Fall Short
Iron from animal sources (heme iron) is absorbed at roughly two to three times the rate of iron from plant sources (non-heme iron). Vegetarian and vegan diets rely entirely on non-heme iron, which is more vulnerable to all the inhibitors described above. This doesn’t mean plant-based diets inevitably lead to deficiency, but they require more deliberate planning.
Vitamin C dramatically improves non-heme iron absorption by converting it into a form that resists binding by phytates and tannins. Pairing iron-rich foods with citrus fruits, bell peppers, or tomatoes at the same meal is one of the simplest ways to boost uptake. Conversely, people who eat iron-rich foods but consistently pair them with tea, coffee, or large amounts of whole grains without any vitamin C source are working against themselves.
When Iron Is Intentionally Lowered
Not everyone with changing iron levels is trying to raise them. People with hemochromatosis, a genetic condition that causes the body to absorb too much iron, need their levels brought down. The standard treatment is therapeutic phlebotomy, which is essentially the same process as donating blood. A pint of blood is drawn once or twice a week for several months until iron and ferritin levels normalize. After that, maintenance sessions drop to once every one to three months, and eventually two to three times a year. The National Institute of Diabetes and Digestive and Kidney Diseases describes phlebotomy as the most direct and safe way to lower body iron stores, and early treatment can prevent the organ damage that excess iron causes over time.
Putting the Pieces Together
Iron deficiency rarely has a single dramatic cause. More often, it’s the accumulation of several smaller factors: a diet that’s moderate in iron but heavy in absorption inhibitors, a daily coffee habit timed with meals, a monthly period that’s heavier than average, or a regular NSAID for joint pain. Each one shaves off a little iron, and over months the deficit grows large enough to produce symptoms.
If your iron levels are low, it’s worth looking at the full picture rather than focusing on just one factor. Timing matters as much as quantity: when you eat iron relative to tea, coffee, calcium, and vitamin C can shift absorption dramatically. And if dietary adjustments don’t move the needle, a medical cause like celiac disease, gut inflammation, or hidden blood loss is worth investigating with your doctor.