Iron is a micronutrient necessary for human health, playing a central role in transporting oxygen throughout the body as a component of hemoglobin in red blood cells. It is also required for cellular respiration and DNA synthesis. Since the body has no regulated way to excrete excess iron, total iron levels are controlled almost entirely by efficiently absorbing the correct amount from the digestive tract. This careful management prevents both iron deficiency, which can lead to anemia, and iron overload, which can be toxic to tissues.
Identifying the Main Location of Iron Uptake
The primary location where the body extracts iron from ingested food is the proximal small intestine, specifically the duodenum and, to a lesser extent, the upper jejunum. This segment is uniquely structured to maximize mineral uptake. The duodenum’s proximity to the stomach ensures the food bolus remains highly acidic, a necessary condition for iron solubility and preparation for absorption.
The cells lining the duodenum, known as enterocytes, possess a high density of specialized transport proteins. These cells feature numerous folds and projections, creating a massive surface area for absorption. As contents move beyond the duodenum, the environment becomes progressively more alkaline, causing iron to become less soluble and significantly less available for uptake.
Heme Versus Non-Heme Iron
Dietary iron is categorized into two forms based on its chemical structure, which dictates how it is handled by the body. Heme iron is derived exclusively from animal sources, primarily found in the hemoglobin and myoglobin of meat, poultry, and seafood. This form is bound within a porphyrin ring structure, allowing it to be absorbed more readily and efficiently by intestinal cells.
Non-heme iron is the predominant form in most diets, originating from plant-based foods, fortified foods, and supplements. Unlike heme iron, non-heme absorption is highly susceptible to the presence of other foods and compounds in the meal. Its bioavailability is considerably lower, often requiring chemical changes to be absorbed.
How Iron Crosses the Intestinal Barrier
The absorption of iron involves a complex series of steps at the brush border of the duodenal enterocytes. Non-heme iron (\(\text{Fe}^{3+}\)) must first be converted into the ferrous state (\(\text{Fe}^{2+}\)) to be absorbed, a reduction catalyzed by the enzyme duodenal cytochrome b (Dcytb). Once reduced, the iron crosses the apical membrane into the enterocyte through the Divalent Metal Transporter 1 (DMT1). Heme iron is absorbed intact, likely via the Heme Carrier Protein 1 (HCP1), before being broken down inside the cell to release the iron. Whether absorbed as heme or non-heme, the iron joins a common pool inside the enterocyte.
Iron then exits the cell across the basolateral membrane and into the bloodstream via the sole known iron export protein, Ferroportin (FPN). As it exits, the ferrous iron is immediately oxidized back to the ferric state by the enzyme hephaestin, preparing it to bind to the transport protein transferrin in the blood. This cellular export process is tightly managed by the hormone hepcidin. Hepcidin is produced by the liver and binds to Ferroportin, causing its degradation and effectively shutting down iron export when the body’s iron stores are sufficient.
Dietary Enhancers and Inhibitors
The efficiency of iron absorption, particularly non-heme iron, is significantly influenced by co-consumed foods. Ascorbic acid (Vitamin C) is a potent enhancer because it helps maintain iron in the absorbable ferrous (\(\text{Fe}^{2+}\)) state, which is necessary for the DMT1 transporter. Consuming Vitamin C-rich foods with an iron source can increase absorption. Another enhancing factor is the presence of meat, poultry, or fish, often called the “meat factor.” Muscle tissue contains a component that stimulates the absorption of non-heme iron in the same meal.
Conversely, certain dietary compounds can significantly inhibit iron uptake. Phytates, found in whole grains, nuts, and legumes, bind to iron in the digestive tract, forming an insoluble compound that cannot be absorbed. Similarly, tannins and other polyphenols in beverages like tea and coffee reduce iron absorption by forming insoluble complexes. Calcium, primarily found in dairy products, is another inhibitor that can reduce the absorption of both heme and non-heme iron.