Iron Absorption and Factors Affecting It: The Complete Evidence-Based Guide

Iron absorption is a tightly regulated physiological process that determines how much dietary iron enters the body. Although many individuals consume adequate amounts of iron, poor absorption due to dietary inhibitors, gastrointestinal disorders, inflammation, medications, and altered iron regulation often contributes to iron deficiency and anemia. Understanding the mechanisms of iron absorption is essential for preventing and treating iron deficiency effectively.

Iron deficiency remains the most common nutritional deficiency worldwide and is a major public health problem in India. Despite apparently adequate iron intake, many individuals continue to have low hemoglobin and ferritin levels due to poor iron bioavailability and absorption.

This comprehensive review examines the physiology of iron metabolism, mechanisms of absorption, regulation by hepcidin, dietary sources, and factors that influence iron bioavailability.

Table of Contents

• Introduction to Iron
• Distribution of Iron in the Human Body
• Functions of Iron
• Dietary Sources of Iron
• Types of Dietary Iron
• Site of Iron Absorption
• Mechanism of Heme Iron Absorption
• Mechanism of Non-Heme Iron Absorption
• Iron Transport and Storage
• Role of Hepcidin
• Regulation of Iron Absorption
• Iron Requirements and RDA

Introduction to Iron

Iron (Fe) is a chemical element with atomic number 26 and atomic weight approximately 56. It is one of the most abundant elements on Earth and an essential micronutrient for virtually all forms of life.

The unique biological importance of iron arises from its ability to reversibly donate and accept electrons. This property allows iron to participate in oxygen transport, cellular respiration, energy production, DNA synthesis, immune function, and numerous enzymatic reactions.

Iron and life are inseparable. With very few exceptions among microorganisms, all living organisms require iron for survival, growth, and reproduction.

Iron is essential for oxygen transport, ATP production, immune defense, neurological development, and cellular metabolism.

Total Body Iron

The adult human body contains approximately 4–5 grams of iron.

Free ionic iron rarely exists within the body because of its potential to generate reactive oxygen species and oxidative damage. Therefore, iron is almost always bound to proteins.

Body iron is distributed into two major pools:

• Functional Iron – Hemoglobin, myoglobin, and enzymes.
• Storage Iron – Ferritin and hemosiderin.

Distribution of Iron in the Human Body

Functions of Iron

1. Formation of Hemoglobin

The primary biological role of iron is participation in hemoglobin synthesis. Hemoglobin contains iron within its heme component and is responsible for oxygen transport from the lungs to body tissues.

One gram of hemoglobin can transport approximately 1.34 mL of oxygen.

2. Red Blood Cell Development

Iron is required not only for hemoglobin synthesis but also for normal maturation and development of erythrocytes.

3. Oxygen Storage in Muscle

Myoglobin is an iron-containing protein found in skeletal and cardiac muscle. It stores oxygen and facilitates oxygen delivery during muscular activity.

4. Cellular Energy Production

Numerous iron-containing enzymes participate in oxidative phosphorylation and ATP production within mitochondria.

Cytochromes use reversible oxidation and reduction of iron (Fe²⁺ ↔ Fe³⁺) to transfer electrons through the respiratory chain.

5. Detoxification

Cytochrome P450 enzymes contain heme iron and play a central role in drug metabolism and detoxification within the liver.

6. Immune Function

Adequate iron status is required for normal cellular and humoral immunity.

• Iron deficiency reduces T-lymphocyte activity.
• Natural killer cell activity decreases.
• Cell-mediated immunity becomes impaired.
• Both iron deficiency and iron overload adversely affect immune function.

Two important iron-binding proteins that help limit microbial growth include:

• Transferrin in blood
• Lactoferrin in breast milk

7. Nervous System Function

Iron participates in neurotransmitter synthesis, neuronal metabolism, myelin formation, and normal cognitive development.

8. DNA Synthesis and Cell Growth

Iron-containing enzymes are involved in DNA replication and cellular proliferation, making iron indispensable for growth and tissue repair.

Dietary Iron

Dietary iron exists in two major forms:

Heme Iron

Heme iron consists of iron incorporated within a porphyrin ring. It is found almost exclusively in animal foods.

• Red meat
• Liver
• Poultry
• Fish
• Shellfish

Although heme iron contributes only about 5–10% of total dietary iron intake, it is highly bioavailable.

Absorption rate: approximately 15–25%

Non-Heme Iron

Non-heme iron is the predominant form of iron in plant foods and represents the major source of iron in vegetarian and Indian diets.

• Cereals
• Millets
• Pulses
• Legumes
• Green leafy vegetables
• Nuts and seeds

Absorption rate: approximately 2–10%

Non-heme iron absorption is strongly influenced by dietary enhancers and inhibitors.

Important Dietary Sources of Iron

Animal Sources

• Liver
• Red meat
• Spleen
• Poultry
• Fish

Plant Sources

• Garden cress seeds (Halim)
• Ragi
• Soybeans
• Poha
• Roasted chana
• Amaranth leaves
• Mustard greens
• Colocasia leaves
• Dried apricots
• Raisins
• Dates

Site of Iron Absorption

Iron absorption occurs primarily within the mature villus enterocytes of the:

• Duodenum
• Proximal jejunum

Because the body lacks a dedicated mechanism for iron excretion, regulation occurs mainly at the level of intestinal absorption.

Mechanism of Heme Iron Absorption

Heme iron is absorbed through a specialized carrier-mediated process.

• Heme enters enterocytes through transporter systems including PCFT/HCP1.
• Heme oxygenase releases iron from the porphyrin ring.
• Released iron enters the intracellular iron pool.
• Iron is exported via ferroportin into circulation.

Heme iron absorption is relatively resistant to dietary inhibitors.

Mechanism of Non-Heme Iron Absorption

Most dietary non-heme iron enters the intestine in the ferric (Fe³⁺) state, which is poorly soluble.

Before absorption can occur, ferric iron must be reduced to ferrous (Fe²⁺) iron.

• Dcytb converts Fe³⁺ to Fe²⁺.
• DMT1 transports Fe²⁺ into enterocytes.
• Iron may be stored temporarily as ferritin.
• Iron exits via ferroportin.
• Hephaestin oxidizes iron for transferrin binding.

Iron that remains bound to ferritin inside enterocytes is lost when intestinal cells are shed.

Iron Transport and Storage

Once absorbed, iron immediately binds to transferrin in the bloodstream.

• Transferrin transports iron to tissues.
• Bone marrow receives iron for erythropoiesis.
• Liver stores excess iron as ferritin.
• Macrophages recycle iron from senescent red blood cells.

Normal transferrin saturation ranges from approximately 20–45%.

Role of Hepcidin in Iron Regulation

The most important regulator of iron metabolism is hepcidin, a 25-amino-acid peptide hormone synthesized by hepatocytes.

Discovered in 2000, hepcidin acts as the master regulator of iron homeostasis.

• High hepcidin decreases iron absorption.
• Low hepcidin increases iron absorption.
• Hepcidin binds ferroportin and causes its degradation.
• Iron export from enterocytes is reduced.
• Iron release from macrophages is reduced.

Hepcidin is the body’s iron gatekeeper. When hepcidin rises, iron becomes trapped within cells and less iron reaches the bloodstream.

Conditions That Increase Hepcidin

• Inflammation
• Chronic infections
• Iron overload
• Chronic kidney disease

Conditions That Decrease Hepcidin

• Iron deficiency
• Pregnancy
• Blood loss
• Increased erythropoiesis
• Hypoxia

Recommended Dietary Allowance (ICMR)

Continued in Part 2: Factors Enhancing Iron Absorption, Factors Inhibiting Iron Absorption, Dietary Interactions, Medication Interactions, Clinical Applications, and Evidence-Based Dietary Strategies.