Iron Metabolism, Bioavailability, and Deficiency States

Core Idea

Dietary iron exists in two forms: heme iron (animal sources, ~15-35% bioavailability) and non-heme iron (plant sources, ~2-20% bioavailability). Absorption is regulated by hepcidin, which increases with iron stores and inflammation. Enhancers (vitamin C, meat) and inhibitors (phytates, polyphenols, calcium) significantly affect non-heme iron absorption. Iron deficiency progresses through depletion, early functional deficiency, and iron-deficiency anemia, each with distinct biochemical markers.

Explainer

Iron is one of the most abundant elements on Earth yet one of the most common nutritional deficiencies globally. The paradox is explained by bioavailability — from your study of minerals and trace elements, you know that the amount of a mineral ingested tells only part of the story. Iron bioavailability is complicated by the fact that it comes in two chemically distinct forms with dramatically different absorption rates.

Heme iron, derived from hemoglobin and myoglobin in meat, fish, and poultry, is absorbed directly by enterocytes via a dedicated transporter and reaches the bloodstream at 15–35% efficiency. Non-heme iron, which makes up the majority of iron in plant foods and fortified products, must first be reduced from ferric (Fe³⁺) to ferrous (Fe²⁺) form by a brush-border enzyme before transport via DMT1 (divalent metal transporter 1). This extra reduction step makes non-heme absorption highly variable (2–20%) and sensitive to luminal chemistry. This is why vegetarians and vegans face higher deficiency risk despite adequate dietary iron intake on paper — the form of iron matters as much as the quantity.

Once inside the enterocyte, iron takes one of two paths: it either enters circulation via ferroportin or is sequestered in ferritin within the cell and lost when the cell sloughs off. The decision is regulated by hepcidin, a peptide hormone made by the liver. When iron stores are replete, hepcidin is high — it binds ferroportin and triggers its degradation, trapping iron in the enterocyte. When stores are low, hepcidin falls, ferroportin remains open, and absorption increases. This elegant feedback loop also explains the anemia of chronic disease: inflammation raises hepcidin independently of iron stores, so the body has iron in its depots but cannot release it into circulation.

Iron deficiency follows a predictable three-stage progression that maps to the body's iron compartments. First comes storage depletion: serum ferritin (the most sensitive early marker) falls as liver stores empty, but hemoglobin remains normal. Second, early functional deficiency: iron-deficient erythropoiesis begins, transferrin saturation drops, and the red cell distribution width (RDW) widens as new red cells become smaller and paler. Only in the third stage — iron-deficiency anemia — does hemoglobin fall below threshold, producing the classic microcytic, hypochromic picture with symptoms of fatigue, pallor, and reduced cognitive performance. Understanding this progression explains why ferritin is the preferred screening test: catching deficiency before anemia develops makes treatment far easier.

Dietary context profoundly shapes practical outcomes. Vitamin C (ascorbate) reduces Fe³⁺ to Fe²⁺ and chelates it in soluble form, enhancing non-heme iron absorption up to sixfold — this is why consuming citrus alongside iron-rich plant foods is clinically meaningful advice. Conversely, phytates (in whole grains and legumes), polyphenols (in tea and coffee), and calcium (in dairy) all inhibit non-heme iron absorption by competing for the transporter or forming insoluble complexes. This creates a practical paradox: healthy dietary patterns high in whole grains, legumes, and plants also contain the highest concentrations of absorption inhibitors. For at-risk populations — women of reproductive age, vegetarians, infants — strategic meal timing and food pairing become real clinical tools.