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Iron Absorption: Mechanisms, Sites, and Influencing Factors
Introduction to Iron Absorption
Iron is a crucial element for various biological functions, including oxygen transport, energy metabolism, and cellular processes. The human body meticulously regulates iron absorption to maintain a balance between iron intake and loss, as excess iron can be harmful due to free radical formation. This article explores the mechanisms, primary sites, and factors influencing iron absorption.
Mechanisms of Iron Absorption
Heme and Non-Heme Iron
Iron in the diet exists in two forms: heme and non-heme iron. Heme iron, primarily found in animal products, is absorbed more efficiently because it binds fewer intestinal chelators that inhibit absorption. Non-heme iron, found in plant-based foods, is absorbed via different pathways and is more susceptible to dietary inhibitors .
Absorption Pathways
Heme iron is absorbed into enterocytes as a metalloporphyrin and released intracellularly by heme oxygenase. Non-heme iron absorption involves several proteins, including DMT-1 (divalent metal transporter 1) for ferrous iron and a β3 integrin-mobilferrin pathway for ferric iron. Other proteins like SFT (Stimulator of Iron Transport) and Hephaestin also play roles in iron uptake and transfer to the plasma.
Primary Sites of Iron Absorption
Small Intestine
Iron absorption occurs throughout the small intestine, with the duodenum and proximal jejunum being the most efficient sites. Studies have shown that iron absorption decreases significantly when bypassing these proximal segments, indicating their critical role in iron uptake. In iron-deficient individuals, enhanced absorption extends further into the mid-jejunum.
Gastrointestinal Mucosa
The gastrointestinal mucosal epithelium is responsible for iron absorption. Conditions like chronic anemia can significantly increase iron absorption, while acute anemia or anoxemia does not have the same effect. The mucosal cells adapt to iron needs, showing increased absorption when body iron stores are depleted.
Factors Influencing Iron Absorption
Dietary Enhancers and Inhibitors
Iron absorption is influenced by various dietary factors. Enhancers such as ascorbic acid (vitamin C) and proteins can significantly increase iron bioavailability. Conversely, inhibitors like calcium, phytates, and polyphenols can reduce iron absorption .
Iron Status and Hepcidin
The body's iron status, reflected by serum ferritin levels, plays a crucial role in regulating iron absorption. Hepcidin, a hormone produced by the liver, negatively regulates iron absorption by binding to ferroportin, the only known cellular iron exporter, causing its internalization and degradation. Elevated hepcidin levels can reduce iron absorption, as seen in studies where daily and split dosing of iron supplements increased serum hepcidin and decreased subsequent iron absorption.
Optimizing Iron Absorption
Dosing Regimens
Research suggests that providing iron supplements on alternate days and in single doses optimizes iron absorption. This regimen prevents the increase in serum hepcidin associated with daily and split dosing, thereby enhancing iron bioavailability.
Whole Diet Considerations
Iron absorption from whole diets varies significantly based on the presence of enhancers and inhibitors. Systematic reviews have shown that diets with enhancers result in higher iron absorption compared to low-bioavailability diets. Regression equations can predict iron absorption based on serum ferritin levels and dietary modifiers, aiding in dietary planning to meet iron requirements .
Conclusion
Iron absorption is a complex process influenced by the form of iron, absorption sites, and various dietary and physiological factors. Understanding these mechanisms and factors can help optimize iron intake, particularly in individuals at risk of iron deficiency. Future research should continue to refine these insights and explore their applications in different populations and dietary contexts.
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