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What is iron?

Iron is one of the most recognizable minerals in the body, although an adult may have a little less than a teaspoon’s amount in his or her body. However, quantity should not be associated with importance as the effects of iron deficiency are tragic and severe. In humans, as well as other animals, iron is found as the central component of a very important molecule called heme. Heme is part of larger protein complexes that rank among the most important in the human body. One aspect that makes animals different from plants is the presence of heme. Plants do not have it.


What foods provide iron and what influences its absorption?

Iron is part of both animal and plant foods. The iron found in these foods exists in the form of either heme iron or nonheme iron. Animal foods (i.e., meats) contain both heme and nonheme iron. Good animal sources include beef, chicken (dark meat), oysters, tuna and shrimp. Meanwhile, plants and plant-derived foods contain only nonheme iron. Good plant sources include raisins, tofu, molasses, lentils, potatoes and kidney beans.



What factors influence iron absorption?

The importance in the difference of these two forms of iron is largely in their efficiency of absorption. Nonheme iron is absorbed less efficiently (2 to 20 percent) in comparison to heme iron (25 to 35 percent). However, if the nonheme iron is part of a meal containing vitamin C, meat, fish, or poultry or organic acids such as citric acid, malic acid, tartaric acid and lactic acid, its absorption can increase. Conversely, the presence of phytates and oxalates in some plant foods (vegetables) can interact with nonheme iron in the digestive tract and decrease its absorption. Soy protein and polyphenols in some fruits, vegetables, coffee and tea can also decrease non-heme iron absorption. Many nutritionists recommend that those people taking an iron-containing supplement should do so with a meal that has the least raw plant foods. For many people that meal is breakfast, which may also include citrus juice whose vitamin C may increase the absorption of nonheme iron.

Since the absorption efficiency of both forms of iron is low, it seems likely that the iron content of the body is primarily regulated at the point of absorption. This idea is reinforced by the fact that the efficiency of iron absorption increases during times of greater iron need, such as when iron stores are low. The efficiency of iron absorption also increases during periods of growth and pregnancy.


What are the levels of recommended intake for iron?

The RDA for iron varies depending on age gender and condition. For instance, the RDA for adult men is 8 mg while the RDA for women aged 19 to 50 is 18 mg daily. The recommendation for younger women is dramatically higher than for men to compensate for menstrual losses of iron. Meanwhile the RDA drops to 8 mg for women after the age 50.


What is the difference between heme and non-heme iron in our body?

As with other animals, iron is found in the cells as a part of heme and nonheme molecules. As mentioned, heme is an interesting molecule with iron situated at its core. In fact, iron seems to hold the whole molecule together. One of the most recognizable heme based molecules is hemoglobin in RBCs and myoglobin in muscle. Iron that is not prt of heme is found as part of a few enzymes and stored in molecular iron containers, namely ferritin and hemosiderin.

What are some non-heme iron-containing components of our body?

Iron serves many roles in the body including:


  • Hemoglobin - Hemoglobin is a protein found in RBCs that binds O2 so that it can be transported throughout the body in the blood. A RBC may contain about 250 million hemoglobin molecules, each with the ability to bind four O2, a single RBC could carry roughly one billion O2 molecules.

  • Myoglobin - Myoglobin, is found in muscle tissue and like hemoglobin, it binds O2. This allows myoglobin to act as an O2 reservoir in muscle fibers, which becomes readily available during exercise. When meat is eaten, which is just skeletal muscle of other mammals, much of the iron is derived from myoglobin.

  • Aerobic Energy Production – Iron is part of heme containing molecules called cytochromes that help form the electron-transport chain in mitochondria. Therefore not only is iron important in delivering oxygen (hemoglobin) to cells for aerobic energy metabolism, it is also a key component of much of the aerobic ATP manufacturing machinery itself.

  • Antioxidant Protection – Iron has an antioxidant role as part of an antioxidant enzyme called catalase found in many tissue. Catalase can metabolize hydrogen peroxide to water and oxygen.

  • Immunity - Iron is also fundamental in proper immune function.



What happens if too little iron is consumed?

A poor iron intake over time will result in a reduction of blood hemoglobin levels. Anemia is the medical term used to describe a condition whereby hemoglobin levels fall well below normal levels. Normal hemoglobin levels for men and women are less than 14 and 12 mg/100 mL of blood, respectively. In an anemic state (< 7-9 mg/100 mL) there is a decrease in the O2 carrying capability of our blood. Less oxygen is able to reach cells and anemic people will often complain of lethargy as well as early fatigue when they exercise. Beyond oxygen transport in the blood, iron deficiency decreases the ability of cells to make ATP by aerobic means.


How can our body iron status be assessed?

Lower levels of iron in the body are indicated several ways. For the longest time we assessed hemoglobin levels and hematocrit (percent of blood that is RBCs) and used these as indicators of iron status. However, today we know that reductions in hemoglobin and hematocrit levels tend to occur later on as the body’s iron status becomes more severely compromised. There are three additional ways to assess body iron status from a sample of blood.


  • Transferrin - Transferrin is an iron transport protein and has the capacity to pick up iron from tissues through­out the body. Each transferring molecule can carry multiple atoms of iron much like a bus can carry multiple passengers.

  • Total iron binding capacity (TIBC) - TIBC indicates the potential for iron transport above what is currently being transported on transferrin. For instance, if transferrin levels are somewhat normal yet the capacity to bind iron (TIBC) is relatively high, this suggests poor iron status. This is similar to having plenty of buses driving around but carrying fewer people than normal. The total people carrying capacity would be high.

  • Ferritin – Perhaps the most sensitive indicator of iron status is the level of ferritin in our blood. Ferritin is a large complex that stores iron in cells, such as in the liver. Therefore, the more iron in the tissues the more ferritin in the body. Now then, some of the ferritin seems to leak into the blood and can be used to gauge iron status in the body as it reflects tissue iron content. High levels of ferritin in the blood implies that more iron is in the body.


Can too much iron be consumed?

Recently, a fair amount of attention has been focused on what happens when there is too much iron in the body. For instance, researchers reported that men in Finland who have higher levels of ferritin in their blood were more likely to experience heart attacks in comparison to men with lower levels.


In more extreme examples of having excessive body iron, people in certain sub-Saharan countries noted for drinking beer with a high iron content seem to develop cirrhosis of the liver beyond what would be expected from excessive alcohol consumption alone. Further evidence is genetic-based disorders in which iron absorption is dramatically enhanced. This can lead to excessive body iron content in these people. The disorder is referred to as genetic-based hemochromatosis and is apparent in as many as 12 of every 1,000 people of European descent. This disorder is associated with severe liver disease and early death.