Most of the iron your body uses each day doesn’t come from food. It comes from recycling your own old red blood cells. Your body contains about 3 to 4 grams of iron total, and it runs a remarkably efficient internal recycling system that recovers and reuses the vast majority of that supply. Only a small fraction, roughly 1 to 2 milligrams per day, needs to be replaced through your diet.
The Recycling System That Supplies Most of Your Iron
Red blood cells live for about 120 days. When they reach the end of their lifespan, specialized immune cells called macrophages (mostly in the spleen and liver) swallow them whole, break them apart, and extract the iron locked inside their hemoglobin. That recovered iron gets sent back into the bloodstream, where it’s picked up by a transport protein called transferrin and shuttled to the bone marrow to build brand-new red blood cells. An average macrophage ingests at least one aging red blood cell per day, and collectively, this recycling process provides the large majority of the 20 to 25 milligrams of iron your bone marrow needs daily to produce fresh blood cells.
This is why iron deficiency develops slowly. Your body isn’t depending on each meal to supply its iron needs in real time. It’s running a closed loop, losing only tiny amounts each day and topping off those losses from food.
How Iron Enters Your Body From Food
The average diet provides about 20 milligrams of iron per day, but only about 10% of that actually makes it into your bloodstream. Absorption happens almost entirely in the first section of the small intestine, the duodenum. Iron crosses the intestinal wall in a tightly controlled process: it’s pulled into the cells lining the gut, then exported out the other side into the blood through a single exit channel, a protein called ferroportin. This is the only protein in the body that exports iron from cells into the bloodstream, which makes it an important control point.
Two forms of dietary iron exist. Heme iron, found in meat, poultry, and fish, is absorbed more efficiently. Non-heme iron, found in plants, grains, and fortified foods, is absorbed at lower rates and is more sensitive to other compounds in your meal. Calcium is one of the strongest inhibitors: as little as 165 milligrams of calcium (about the amount in a small glass of milk) can reduce iron absorption by 50 to 60%. Phytates, found in whole grains and legumes, also interfere significantly. On the other hand, vitamin C enhances non-heme iron absorption by converting it into a form the gut can take up more easily.
How Your Body Controls Iron Levels
Your body has no active way to excrete excess iron. You lose about 1 milligram per day through the shedding of skin cells and the cells lining your gut, plus small losses in sweat. Menstruation adds to those losses in women of reproductive age. Because there’s no iron “off switch” through excretion, the body controls iron levels almost entirely by regulating how much gets absorbed and how much gets released from storage.
The master regulator is a hormone produced by the liver called hepcidin. Hepcidin works by binding to ferroportin, the iron export protein, and triggering its destruction. When hepcidin levels are high (signaling that the body has enough iron), ferroportin gets pulled off cell surfaces and broken down. This traps iron inside gut cells and macrophages, preventing it from entering the bloodstream. When hepcidin is low (signaling the body needs more iron), ferroportin stays active, and iron flows freely from the gut and from recycling macrophages into circulation.
This system explains several real-world patterns. Chronic inflammation raises hepcidin levels, which is why people with long-term inflammatory conditions often develop low circulating iron even when their total body stores are adequate. The iron is there, locked inside cells, but hepcidin won’t let it out.
Where Iron Gets Stored
Iron that isn’t immediately needed gets packed into a storage protein called ferritin, found mainly in the liver, spleen, and bone marrow. A blood test measuring serum ferritin gives a rough picture of your total iron reserves. Normal ferritin ranges are 15 to 300 ng/mL for men and 15 to 200 ng/mL for women, though these numbers can be misleading during illness or inflammation because ferritin also rises as part of the body’s stress response.
Meanwhile, iron traveling through the bloodstream rides on transferrin. In a healthy person, only about one-third of transferrin is actually carrying iron at any given time. The remaining two-thirds represent spare capacity, ready to pick up and deliver iron as needed. When transferrin saturation drops too low, it signals that the supply chain is running short.
How Iron Stores Are Built Before Birth
Newborns arrive with their own iron supply, accumulated during pregnancy. The total amount of iron in a full-term baby is roughly 270 milligrams, with about 210 milligrams of that accruing during the third trimester alone. That works out to about 1.35 milligrams per kilogram of body weight accumulating each day in those final months. These stores are critical because breast milk contains very little iron, so the reserves built before birth carry infants through their first several months of life. Premature babies, who miss part of that third-trimester loading period, are at higher risk for iron deficiency early on.
Why the System Breaks Down
Iron deficiency develops when losses consistently outpace what your recycling system and diet can replace. The most common causes are blood loss (heavy periods, GI bleeding, frequent blood donation), poor absorption (celiac disease, gastric bypass, or diets very high in inhibitors like calcium and phytates), and increased demand (pregnancy, rapid growth in children). Because the body recycles iron so efficiently, it can take months or even years of negative balance before stores are depleted enough to cause symptoms like fatigue, weakness, or pale skin.
Iron overload, on the other hand, happens when too much iron accumulates with no way to get rid of it. The most common genetic cause is hereditary hemochromatosis, where hepcidin production is abnormally low, allowing unregulated iron absorption from every meal. Over years, excess iron deposits in the liver, heart, and pancreas, causing organ damage. This is the flip side of having no excretion pathway: the same design that makes iron conservation so efficient also means the body is vulnerable when the intake controls fail.