The Primary Site of Iron Absorption
The small intestine, specifically the duodenum and upper jejunum, is the primary location for iron absorption. This section of the digestive tract is optimized for this process due to its acidic environment, the presence of specialized enterocyte cells with transport proteins, and a large surface area.
The Mechanisms of Heme vs. Non-Heme Iron Absorption
Dietary iron comes in two main forms: heme iron, primarily from animal sources, and non-heme iron, found in plants and fortified foods. These two types are absorbed differently.
Non-Heme Iron Absorption
Non-heme iron absorption is more complex and easily influenced by dietary components. It involves the reduction of ferric iron ($Fe^{3+}$) to the more absorbable ferrous state ($Fe^{2+}$) by enzymes like duodenal cytochrome B (Dcytb). The ferrous iron is then transported into enterocytes via the divalent metal transporter 1 (DMT1). Once inside the cell, it can be stored or exported into the bloodstream by ferroportin.
Heme Iron Absorption
Heme iron, more efficiently absorbed, is released from proteins like hemoglobin and myoglobin by digestive enzymes. The intact heme molecule is then absorbed by intestinal cells using a specific transporter, Heme Carrier Protein 1 (HCP1). Inside the cell, heme oxygenase-1 releases the iron to join the intracellular iron pool.
Comparison of Iron Absorption Types
| Feature | Heme Iron | Non-Heme Iron |
|---|---|---|
| Source | Animal products (meat, poultry, fish) | Plant-based foods (grains, legumes, vegetables) and fortified foods |
| Absorption Efficiency | High (15-35% of intake) | Low (2-20% of intake) |
| Influence of Dietary Factors | Largely unaffected by other foods | Highly sensitive to enhancers and inhibitors |
| Absorption Pathway | Absorbed as intact heme, then released inside cell | Absorbed as ferrous ($Fe^{2+}$) ion after reduction |
Factors Influencing Iron Absorption
Numerous factors impact how well the body absorbs iron:
Enhancers of Iron Absorption
- Vitamin C: Improves non-heme iron absorption.
- Meat, Fish, and Poultry: Enhance non-heme iron absorption.
- Vitamin A and Beta-Carotene: Help mobilize stored iron.
Inhibitors of Iron Absorption
- Phytates: Found in whole grains and legumes, they reduce non-heme iron absorption.
- Polyphenols: In tea, coffee, and certain vegetables, these compounds hinder non-heme iron uptake.
- Calcium: Can inhibit the absorption of both heme and non-heme iron.
- Oxalates: In foods like spinach, they bind non-heme iron.
The Role of Hepcidin in Regulation
The body regulates iron absorption using the hormone hepcidin. Produced in the liver, hepcidin controls the release of iron into the bloodstream from enterocytes via the ferroportin protein. High iron levels increase hepcidin, which reduces absorption. Low levels decrease hepcidin, allowing more iron transport.
Conclusion
Iron is primarily absorbed in the duodenum and upper jejunum of the small intestine. The type of iron and dietary factors significantly affect absorption efficiency. Body iron status, regulated by hepcidin, is also crucial. Optimizing iron absorption involves mindful dietary choices.
For more information on iron deficiency and treatment, you can visit the {Link: NHLBI website https://www.nhlbi.nih.gov/health/anemia/iron-deficiency-anemia}.
