Iron, found in two distinct chemical forms, is an indispensable component of human biology. It plays a central part in the transportation of oxygen throughout the body, primarily as a component of hemoglobin in red blood cells. Iron is also necessary for the oxygen-storing protein myoglobin in muscles and is a cofactor for numerous enzymes involved in energy production and DNA synthesis. The body must absorb iron from the diet, and the chemical form dictates how efficiently this process occurs. Understanding the difference between these two forms is important for managing dietary intake and supplementation.
Defining Heme and Non-Heme Iron
Ferrous sulfate, a common iron supplement, is classified as non-heme iron. Heme iron is defined by an iron atom bound within a complex ring-shaped organic molecule called protoporphyrin, which makes up hemoglobin and myoglobin in animal tissues. Non-heme iron is inorganic iron that is not attached to this porphyrin structure. It includes the iron found in plant-based foods, fortified foods, and most oral iron supplements. Ferrous sulfate is an iron salt in its more easily absorbed ferrous (Fe2+) state. Other non-heme supplemental forms include ferrous fumarate and ferrous gluconate.
How the Body Absorbs Different Iron Forms
The difference in chemical structure leads to two entirely separate pathways for iron absorption in the small intestine. Heme iron is absorbed intact into the intestinal cells via a specialized transporter protein. Once inside the cell, an enzyme releases the iron from the porphyrin ring, and the iron then joins the general pool of cellular iron. This direct absorption pathway makes heme iron highly bioavailable, with absorption rates typically ranging from 15% to 35% and being largely unaffected by other foods consumed.
Non-heme iron, such as that from ferrous sulfate, requires several steps before absorption can occur. Inorganic iron often exists in the less soluble ferric (Fe3+) state in the gut lumen, which must first be converted into the ferrous (Fe2+) state by an enzyme on the intestinal cell surface. Only the ferrous form of non-heme iron can be transported across the intestinal cell membrane, primarily through a specific transporter. Because of these conversion requirements, the absorption of non-heme iron is lower and more variable, ranging from approximately 2% to 20%.
This process is significantly influenced by other dietary components. Consuming non-heme iron with Vitamin C can substantially enhance its absorption because Vitamin C acts as a reducing agent, helping to convert Fe3+ to the absorbable Fe2+. Conversely, compounds such as phytates found in grains and legumes, polyphenols in tea and coffee, and calcium can bind to non-heme iron in the gut, reducing its uptake. Heme iron absorption is largely resistant to these inhibitory factors.
Primary Sources of Iron in Diet and Supplements
The two forms of iron are found in distinct categories of food. Heme iron is exclusively found in animal products, specifically in the muscle tissue of meat, poultry, and fish. This includes sources like red meat, organ meats, and shellfish.
Non-heme iron is the type of iron present in all plant-based foods, including grains, nuts, seeds, legumes, and green leafy vegetables. It is also the iron used to fortify foods, such as breakfast cereals and breads, and it constitutes the iron in nearly all over-the-counter supplements. Ferrous sulfate is the most common and cost-effective supplemental form used to treat and prevent iron deficiency.
While meat contains both heme and non-heme iron, the majority of the iron consumed in the average diet is non-heme. Individuals who follow vegetarian or vegan diets rely solely on non-heme iron sources, which is why they may require nearly double the recommended dietary allowance compared to those who consume animal products.