Human iron metabolism

Iron is an essential nutrient for human beings, although it can also be toxic. The most important role is in the heme protein of hemoglobin and cytochrome P-450, and secondarily for myoglobin, which transports oxygen into muscle cells. Iron also is a component of a number of enzymes.

Requirements

The actual requirement for iron in food is quite low, as most of the total iron in the body is recycled from ferritin, a protein carrier of the metal.

Digestion

Iron, in food, is processed differently depending if it is in the form of heme (e.g., in meats) or in other molecules. As food breaks down in the stomach, ferric (Fe⁺³) iron is reduced to ferrous (Fe⁺²) iron by the enzyme ferric reductase. Ascorbic acid, a reducing agent, is a cofactor for absorption.

Non-heme iron, however, may not be available to the digestive processes. While spinach is legendary as an iron source, the iron in raw spinach is bound into a non-absorbable oxalate. Cooking the spinach does make the iron bioavailable; the cartoon character Popeye the Sailor Man was correct in eating his spinach from a can.

Most actual absorption takes place in the duodenum. Heme is absorbed via the heme transporter protein. Non-heme iron enters the apical surface of the enterocytes lining the duodenum via divalent metal transporter 1 (DMT1), carrying iron into the interstitial fluid by a protein called ferroportin (FP).

Distribution

Ferrous iron, in plasma, is reconverted to ferric, and bound to a carrier protein, ferritin.

Ferritin in plasma stays in equilibrium with ferritin in bone marrow, and plasma ferritin is indicative of actual iron stores in the marrow.