Every living cell—whether plant or animal—contains iron. Most of the iron in your body is found as part of two proteins called hemoglobin, which is found in red blood cells, and myoglobin, which is found in muscle cells. Hemoglobin in blood carries the oxygen you breathe into your lungs to all tissues throughout the body, and myoglobin in muscle holds and stores oxygen for use during exercise. Myoglobin is particularly important for aerobic muscle fibers that are also called slow-twitch red (or type I) fibers. In fact, it is the myoglobin that makes endurance muscle reddish in color and may help explain why chickens, with white muscle in their breasts, are able to fly just a few feet, but ducks, with dark muscle in their breasts, can fly for hours.
The iron in hemoglobin and myoglobin is key because it has special chemical properties that allow it to carry oxygen and then release it to the tissues as needed. Your cells—particularly your working muscle cells—need a regular supply of oxygen to produce energy. Iron-containing hemoglobin is also needed to assist in the elimination of carbon and hydrogen atoms released during the use of carbohydrate and fat fuels for energy, forming carbon dioxide and hydrogen. Thus, having adequate iron stores is particularly important during exercise when the hemoglobin-rich red blood cells shuttle between your lungs and exercising muscle, bringing in fresh oxygen and eliminating carbon dioxide.
In addition to its vital role in oxygen and carbon dioxide shuttling, iron helps many enzymes in the energy-generating pathways. Iron is also needed to make new cells, hormones, neurotransmitters, and amino acids.
Iron is so important to your body that is has been referred to as the body’s gold: a precious mineral to be hoarded. Following absorption in the intestines, a protein called transferrin escorts it to various tissues. Iron is stored primarily in the liver and bone marrow as part of two other proteins called ferritin and hemosiderin. Some storage also occurs in the spleen and in muscle. A small amount of the storage protein ferritin also circulates in the blood. Serum ferritin can be used to assess iron storage because blood levels typically reflect iron stores in the body, although there is some evidence that this might not be true in endurance-trained athletes because serum ferritin is also influenced by factors other than iron stores. Only a very small amount of unescorted iron circulates in the blood.
The liver packs iron, sent from bone marrow or from its own stores, into new red blood cells, also made in bone marrow, and releases them into the blood. Your red blood cells typically live for three to four months. When they die, the spleen and liver salvage the iron and send it back to the bone marrow for storage and reuse. In this way, iron is truly hoarded. Small amounts of iron, however, are lost daily through the shedding of cells in your skin, scalp, and gastrointestinal (GI) tract and through your perspiration. The greatest loss of iron, however, occurs through bleeding. Normal average daily iron loss is approximately 1 milligram for men and nonmenstruating women, and approximately 1.4 to 1.5 milligrams for menstruating women. Monthly menstrual losses account for the higher average iron loss in women.
Athletes in training may also lose iron through GI bleeding, the destruction of red blood cells, urinary loss, and heavy sweating. GI bleeding is a recognized problem that occurs in endurance runners, cyclists, and triathletes and is thought to be related, at least in part, to regular use of aspirin and anti-inflammatory medications. These medications may have a toxic effect on the gut lining, leaving it raw and bleeding. Red cell destruction, called hemolysis, is thought to be caused by the stress of repetitive foot contact with the ground during prolonged running or marching or by the rapid propulsion of blood cells through the blood vessels that can occur with intense muscular training. This excess force can then rupture some of the red blood cells, which, although not dangerous, can increase iron requirements. Loss of hemoglobin in urine, called hematuria, usually follows hemolysis because the kidneys’ capacity to hoard the hemoglobin released from ruptured blood cells, is temporarily overwhelmed. Iron loss in urine can also occur if muscle cells are ruptured during intense training, resulting in the release of stored myoglobin, or if the inner lining of the bladder becomes irritated, resulting in loss of red blood cells in the urine. Blood loss in urine may occur in any athletic situation that places stress on the bladder and may include prolonged running and running during pregnancy. Finally, iron loss in sweat is also thought to be significant for some athletes during prolonged effort, although not all experts agree. Research in this area suggests that iron loss through sweating increases with exercise duration, particularly in males, and may be as high as 0.3 to 0.4 milligrams of iron lost per liter (4 cups) of sweat. Although very few studies have cumulatively assessed total iron losses in athletes, data collected from endurance-trained athletes indicate that iron losses from feces, urine, and sweat were approximately 1.75 milligrams for men and 2.3 milligrams for women.