Iron is an essential mineral required for various bodily functions, including the formation of red blood cells that transport oxygen throughout the body. Dietary iron exists in two primary forms: heme iron and non-heme iron. Heme iron is found in animal-based foods like meat, poultry, and fish. Non-heme iron is present in plant-based foods such as grains, beans, vegetables, fruits, nuts, and seeds, as well as fortified products. Understanding how non-heme iron is absorbed is relevant for individuals following vegetarian or vegan diets, or anyone optimizing their iron intake, as its absorption is less efficient.
How Non-Heme Iron Enters the Body
The journey of non-heme iron into the body begins in the digestive tract. Unlike heme iron, which is absorbed intact, non-heme iron requires conversion to a more absorbable form. This process starts in the stomach, where hydrochloric acid helps solubilize the iron and convert ferric iron (Fe3+) into its ferrous state (Fe2+), which is more readily absorbed.
From the stomach, the ferrous iron moves into the duodenum, the first section of the small intestine, where most iron absorption occurs. Specialized cells lining the intestinal wall, called enterocytes, take up the ferrous iron through specific transporters like Divalent Metal Transporter 1 (DMT1). Once inside the enterocyte, the iron can either be stored within the cell in a protein called ferritin or transported out of the cell into the bloodstream via ferroportin 1 (FPN1).
Dietary Influences on Absorption
The absorption of non-heme iron is significantly influenced by other components present in a meal. Certain dietary factors can enhance its uptake, while others can inhibit it.
A primary enhancer of non-heme iron absorption is Vitamin C, also known as ascorbic acid. Vitamin C aids absorption by preventing the formation of insoluble iron compounds and by reducing ferric iron (Fe3+) to the more absorbable ferrous iron (Fe2+) state. Vitamin C can significantly increase iron absorption. Other organic acids, such as citric acid, and the presence of animal proteins like meat, fish, and poultry, can also improve non-heme iron absorption. The “meat factor” may work by stimulating gastric acid production, which helps solubilize iron, and by chelating iron to maintain its solubility during digestion.
Conversely, several dietary components can hinder non-heme iron absorption. Phytates, found in whole grains, legumes, nuts, and seeds, can bind to iron, forming insoluble complexes that reduce its uptake. Even small amounts of phytates can strongly inhibit iron bioavailability. Oxalates, present in leafy greens like spinach and kale, beets, nuts, and chocolate, also impair non-heme iron absorption.
Tannins and other polyphenolic compounds, commonly found in tea, coffee, red wine, and some spices, are potent inhibitors of iron absorption. For instance, coffee can significantly reduce iron absorption. Calcium is another inhibitor, affecting the absorption of both non-heme and heme iron. Higher amounts of calcium can significantly reduce iron absorption.
Optimizing Your Iron Intake
To maximize non-heme iron absorption, strategic food pairing is beneficial. Combining iron-rich plant foods with sources of Vitamin C can significantly enhance uptake. For example, a meal of lentils or beans with bell peppers, tomatoes, or citrus fruits would be effective. Adding strawberries to a kale salad or using a lemon-based vinaigrette can also boost Vitamin C intake and, consequently, iron absorption.
Strategies to mitigate the impact of inhibitors are also valuable. Soaking, sprouting, or fermenting legumes and grains can help reduce their phytate content, thereby improving iron availability. It is also recommended to avoid consuming tea, coffee, or calcium-rich foods and supplements with iron-rich meals. Creating a buffer of one to two hours between non-heme iron foods and calcium-rich options is suggested to minimize interference. For individuals on plant-based diets, focusing on a variety of iron-rich beans, nuts, seeds, and fortified products, combined with iron-enhancing food combinations, supports meeting daily iron requirements.