Iron Absorption

Introduction

Introduction to Iron Metabolism and Absorption

Iron is an essential micronutrient critical for numerous biological processes, including oxygen transport, DNA synthesis, and cellular respiration. The body tightly regulates iron absorption, storage, and distribution to maintain homeostasis, as both deficiency and excess can lead to severe pathological conditions. Dietary iron exists primarily in two forms: heme iron (from animal sources) and non-heme iron (from plant sources), each absorbed via distinct mechanisms in the duodenum and proximal jejunum.

Physiological Importance of Iron

Iron serves as a cofactor for heme proteins such as hemoglobin and myoglobin, which are vital for oxygen delivery and storage. It is also a component of iron-sulfur clusters in enzymes involved in the electron transport chain, facilitating ATP production. Due to its redox-active nature, iron must be carefully managed to prevent oxidative damage, which can lead to cellular injury and disease.

Study

Dietary Iron Sources and Absorption Mechanisms

Heme iron, derived from hemoglobin and myoglobin in animal products, is absorbed more efficiently (15-35%) than non-heme iron (2-20%) due to its direct uptake via the heme carrier protein 1 (HCP1) in enterocytes. Non-heme iron, predominantly found in plant-based foods, must first be reduced from ferric (Fe³⁺) to ferrous (Fe²⁺) iron by duodenal cytochrome b (Dcytb) before transport into enterocytes via divalent metal transporter 1 (DMT1). Ascorbic acid enhances non-heme iron absorption by facilitating this reduction, while phytates and polyphenols inhibit it.

Regulation of Iron Absorption by Hepcidin

Hepcidin, a peptide hormone synthesized in the liver, is the master regulator of systemic iron homeostasis. It binds to ferroportin, the sole iron exporter on enterocytes, macrophages, and hepatocytes, inducing its internalization and degradation. This reduces iron release into the plasma, thereby lowering serum iron levels. Hepcidin expression is upregulated by high iron stores and inflammation (via IL-6) and downregulated by iron deficiency, hypoxia, and erythropoietic demand.

Intracellular Iron Storage and Transport

Within cells, iron is stored in ferritin, a multimeric protein that sequesters up to 4,500 iron atoms in a non-toxic, bioavailable form. Ferritin levels are regulated post-transcriptionally by iron regulatory proteins (IRPs), which bind to iron-responsive elements (IREs) in the 5' untranslated region of ferritin mRNA, inhibiting its translation under low iron conditions. In the plasma, iron is transported bound to transferrin, a glycoprotein that delivers iron to tissues via transferrin receptor-mediated endocytosis.

Iron Recycling and Erythropoiesis

Approximately 90% of the body's daily iron requirement is met through recycling from senescent erythrocytes by macrophages in the spleen, liver, and bone marrow. Heme oxygenase-1 degrades heme, releasing iron, which is either stored in ferritin or exported via ferroportin. During erythropoiesis, iron is delivered to developing red blood cells in the bone marrow, where it is incorporated into hemoglobin. Dysregulation of this process can lead to anemia of chronic disease or iron overload disorders.

Pathophysiology of Iron Imbalance

Iron deficiency, the most common nutritional disorder worldwide, results in microcytic hypochromic anemia due to impaired hemoglobin synthesis. Conversely, iron overload, as seen in hereditary hemochromatosis or transfusional siderosis, leads to tissue damage via oxidative stress, particularly in the liver, heart, and endocrine organs. Mutations in genes such as HFE, HJV, or hepcidin are associated with hereditary hemochromatosis, characterized by inappropriately low hepcidin levels and excessive iron absorption.

Summary

Key Takeaways

Iron absorption occurs primarily in the duodenum and is regulated by dietary factors, hepcidin, and systemic iron demand. Heme iron is absorbed more efficiently than non-heme iron, which requires reduction to ferrous iron prior to uptake. Hepcidin is the central regulator of iron homeostasis, modulating ferroportin activity to control iron release into the plasma. Intracellular iron is stored in ferritin and transported in the plasma bound to transferrin.

Clinical Correlate

Iron deficiency anemia is diagnosed by low serum ferritin, high transferrin saturation, and microcytic red blood cells, while iron overload disorders such as hemochromatosis present with elevated serum ferritin and transferrin saturation. Treatment of iron deficiency involves oral or intravenous iron supplementation, whereas iron overload may require phlebotomy or iron chelation therapy. Understanding iron metabolism is critical for managing anemia, chronic kidney disease, and inflammatory conditions.

Molecular Targets for Therapeutic Intervention

Emerging therapies target hepcidin and its regulatory pathways to treat iron-related disorders. Hepcidin agonists are being investigated for iron overload conditions, while hepcidin antagonists may benefit patients with anemia of chronic disease. Additionally, prolyl hydroxylase inhibitors, which stabilize hypoxia-inducible factors (HIFs), can upregulate erythropoietin and iron absorption, offering a novel approach to anemia management.