Your body does not produce iron. Unlike many substances your cells can build from scratch, iron is an element that must come from food or supplements. What your body does extraordinarily well, though, is recycle the iron it already has, reusing the same atoms over and over again so efficiently that you only need to absorb a tiny amount each day to stay in balance.
Why Your Body Can’t Make Iron
Iron is a chemical element, not a molecule your cells can assemble from other ingredients. Your body can synthesize complex proteins, hormones, and even cholesterol, but it cannot create an iron atom any more than it can create a gold atom. Every bit of iron inside you originally entered through your digestive tract, either from food or a supplement. A healthy adult carries about 3 to 4 grams of iron total, roughly the weight of a small nail, and that supply has been built up gradually over a lifetime of meals.
How Your Body Recycles Iron Instead
About two-thirds of your body’s iron sits inside hemoglobin, the protein in red blood cells that carries oxygen. Red blood cells live roughly 120 days before they wear out. When they do, specialized immune cells called macrophages, concentrated in the spleen, liver, and bone marrow, swallow the old cells whole. Inside the macrophage, hemoglobin is broken apart and the iron is extracted. That recovered iron is either stored for later or sent back into the bloodstream to be used again.
This recycling system is remarkably productive. It returns about 20 milligrams of iron to the bone marrow every day for the creation of new red blood cells. By comparison, you absorb only 1 to 2 milligrams of new iron from food on a typical day. So your body runs almost entirely on recycled iron, topping off the supply with small amounts from your diet.
How Iron Gets Into Your Body
The small intestine, particularly the first section called the duodenum, is where iron absorption happens. Specialized cells lining the gut wall use a transporter protein to pull iron from digested food into the cell. A second protein on the opposite side of the cell then pushes that iron out into the bloodstream. These two proteins work in tandem, and their levels adjust based on how much iron your body needs. When stores are low, the gut ramps up production of both transporters. When stores are full, it dials them back.
Iron from animal sources (red meat, poultry, fish) is bound within a structure that your gut absorbs more readily. Iron from plant sources (beans, spinach, fortified cereals) arrives in a different chemical form that competes with other minerals for absorption, making it harder to take up. Eating vitamin C alongside plant-based iron improves absorption significantly, which is why pairing lentils with tomato sauce or bell peppers is a practical strategy.
The Hormone That Controls It All
Your body regulates its iron levels with a hormone called hepcidin, produced by the liver. Hepcidin acts like a gatekeeper. When iron levels are adequate, the liver releases more hepcidin, which locks down the export channels on gut cells and macrophages. Iron gets trapped inside those cells instead of entering the bloodstream. When iron is low, hepcidin drops, the export channels stay open, and more iron flows into circulation.
This system explains why taking iron supplements when your stores are already full doesn’t help and can cause problems. High hepcidin blocks absorption, so the extra iron simply passes through your gut (often causing constipation or nausea) without being absorbed. It also explains why chronic inflammation can cause iron deficiency even when dietary intake is fine: inflammation raises hepcidin, which sequesters iron inside cells where it can’t be used to make new red blood cells.
Where Iron Lives in Your Body
Most iron is in hemoglobin, doing the active work of oxygen transport. The remainder is split between a few locations. Your liver, spleen, and bone marrow store iron inside a protein called ferritin, which acts as a vault that can release iron when demand rises. A smaller amount lives in myoglobin, a related protein in muscle tissue that holds oxygen locally for use during exertion. And a transport protein called transferrin shuttles iron through the bloodstream, delivering it wherever it’s needed. Most transferrin-bound iron goes straight to the bone marrow, which consumes iron at a high rate to churn out roughly 200 billion new red blood cells every day.
How Iron Leaves Your Body
There is no dedicated system for excreting excess iron. Your body loses small amounts passively through shed skin cells, the lining of the intestinal tract sloughing off, and trace amounts in sweat. For men and postmenopausal women, these losses add up to roughly 1 milligram per day. For premenopausal women, menstrual bleeding is the largest single source of iron loss, which is why the recommended daily intake for women aged 19 to 50 is 18 milligrams, more than double the 8 milligrams recommended for men of the same age. During pregnancy, the recommendation jumps to 27 milligrams per day because the body must supply iron to a growing fetus and expanding blood volume.
Athletes face additional losses. Sweat, minor gastrointestinal bleeding from high-impact exercise, and the mechanical destruction of red blood cells in the feet during running all contribute to higher iron turnover. Female athletes who train heavily and menstruate regularly are at particularly high risk of depletion.
What This Means in Practice
Because your body cannot manufacture iron, your long-term iron status depends on a simple equation: what you absorb from food minus what you lose. The recycling system handles day-to-day needs brilliantly, but it can’t replace iron that leaves the body entirely. If losses outpace intake for long enough, ferritin stores drop first, then transferrin saturation falls, and eventually hemoglobin production slows, leading to iron-deficiency anemia.
Building iron stores back up takes time. Even with supplementation, it typically takes three to six months to fully replenish depleted ferritin. The body absorbs iron in small increments, and hepcidin rises after each dose, temporarily reducing absorption of the next one. This is why many clinicians now recommend taking iron supplements every other day rather than daily, since the lower hepcidin levels on the off day allow better absorption overall.