Many people encounter the term “iron” in vastly different contexts, from the food they eat to the metals used in construction. This can lead to confusion, as the iron found in dietary sources and the iron used as a physical metal are distinct in their forms and functions. While they share the same elemental name, their chemical structures and roles in the world around us are fundamentally different.
The Fundamental Difference
The core distinction between iron in food and metallic iron lies in their chemical states. Elemental iron, as found in metal, exists in a pure, unreacted form (zero oxidation state). In contrast, the iron present in food is typically in an ionic form, such as ferrous (Fe2+) or ferric (Fe3+) ions, which are bound within organic molecules like proteins. The body requires iron in its ionic states, which are chemically reactive and can participate in biological processes.
Iron in Our Food
Iron found in food is categorized into two main types: heme iron and non-heme iron. Heme iron, primarily sourced from animal products like red meat, poultry, and seafood, is part of hemoglobin and myoglobin proteins. Non-heme iron is found in plant-based foods such as beans, lentils, spinach, fortified cereals, nuts, and seeds, and also in animal products alongside heme iron. It is a component of hemoglobin, the protein in red blood cells responsible for transporting oxygen from the lungs to tissues throughout the body. Iron also contributes to energy production within cells, supports a healthy immune system, and is involved in DNA synthesis.
Iron as a Metal
Elemental iron is a lustrous, silver-grey metal known for its strength and magnetic properties. It is prone to rusting when exposed to oxygen and moisture, forming iron oxide. This form of iron is extensively used in various industries, primarily for manufacturing steel. It is a key constituent of structures like bridges, vehicles, tools, and cast iron cookware. Due to its chemical stability and inability to be broken down into absorbable ionic forms by the digestive system, consuming metallic iron is not safe or nutritious.
How Your Body Uses Dietary Iron
The body absorbs dietary iron predominantly in the duodenum and upper jejunum of the small intestine. Heme iron is generally absorbed more efficiently, with absorption rates ranging from 15% to 35%. Non-heme iron absorption is more variable, typically ranging from 2% to 20%, and is significantly influenced by other dietary components. Factors that enhance non-heme iron absorption include vitamin C, which helps convert it into a more absorbable form. Eating meat, poultry, or fish alongside plant-based iron sources can also improve non-heme iron uptake.
Conversely, substances like phytates found in grains and legumes, and tannins in tea and coffee, can inhibit non-heme iron absorption. Once absorbed, iron is transported in the bloodstream bound to a protein called transferrin and stored primarily in the liver, spleen, and bone marrow as ferritin. This stored iron is then released as needed for various physiological functions.