Runners, particularly those involved in endurance events, frequently experience low iron levels, a condition sometimes called “sports anemia” or “runner’s anemia.” Iron is a component of hemoglobin, the protein in red blood cells responsible for transporting oxygen from the lungs to the muscles. Having sufficient iron is therefore directly linked to a runner’s energy production and overall performance capacity. The combination of increased iron loss during running and the body’s altered response to intense training creates a complex challenge to maintaining adequate iron stores.
Physical and Mechanical Iron Losses
The constant impact of the foot striking the ground causes mechanical stress on the red blood cells circulating in the capillaries of the feet, a process known as foot strike hemolysis. This physical trauma leads to the premature breakdown of red blood cells, releasing hemoglobin into the bloodstream. While the body can recycle some of this free hemoglobin, the chronic nature of distance running creates a sustained demand that can outpace the recycling capacity, resulting in a net loss of iron over time.
Iron is also lost through sweat, and for athletes engaged in long-duration exercise, this cumulative loss can become significant. Although the concentration of iron in sweat is low, a runner can lose multiple liters of sweat during a long run or race, especially in hot and humid conditions. Studies suggest that some male endurance runners may lose about 1 milligram of iron daily through sweat alone, which adds to the body’s total iron deficit.
Intense running can also cause minor internal bleeding in the gastrointestinal (GI) tract. During prolonged exercise, blood flow is redirected away from the digestive system and toward the working muscles. This reduced blood flow, or splanchnic hypoperfusion, can cause minor damage to the intestinal lining. Evidence of this microbleeding is seen by an increase in fecal hemoglobin levels after a race, indicating blood loss.
Physiological Factors Affecting Iron Status
Beyond direct physical loss, the body’s systemic response to intense training affects iron availability. Strenuous exercise induces a temporary state of inflammation, which triggers the release of Hepcidin from the liver. Hepcidin is the master regulator of iron metabolism, blocking the body’s ability to absorb dietary iron and preventing stored iron from being released into the bloodstream.
Hepcidin levels typically rise significantly within three to six hours after a prolonged run and can remain elevated for several hours. By restricting iron transport, Hepcidin effectively creates a functional iron deficiency where iron is present in storage but unavailable for red blood cell production. This post-exercise block on absorption can reduce the amount of dietary iron a runner absorbs by over a third.
Dilutional pseudoanemia is not a true iron deficiency but an adaptation to training. Endurance training causes an increase in plasma volume, the liquid component of blood, which expands the overall blood volume. Since the increase in red blood cells may not match the proportional increase in plasma, the concentration of hemoglobin appears lower. This phenomenon, which can enhance oxygen delivery, causes iron markers like hemoglobin to appear low in blood tests.
The high demands of training increase the body’s need for iron. Iron is needed for oxygen transport, to produce new red blood cells, and to support the energy-generating machinery (mitochondria) within muscle cells. This constant, elevated demand, combined with the losses and the Hepcidin-mediated block, places runners at risk for developing a true iron deficiency.
Dietary and Supplemental Strategies
Runners can proactively manage their iron status by focusing on the type and timing of their dietary iron intake. Dietary iron comes in two forms: heme iron, found in animal sources like meat and fish, and non-heme iron, found in plant sources, fortified foods, and eggs. Heme iron is absorbed more readily by the body, with an absorption rate that can be two to three times higher than non-heme iron.
To maximize the absorption of non-heme iron, runners should consume it alongside a source of Vitamin C, such as citrus fruits or bell peppers. Conversely, substances like calcium, tannins found in tea and coffee, and phytates in whole grains can inhibit iron absorption. It is beneficial to avoid consuming these inhibitors close to an iron-rich meal or supplement.
Iron supplementation should be approached with caution and ideally only after a blood test confirms a deficiency, as excessive iron can be harmful. If a supplement is necessary, timing is important to bypass the Hepcidin-induced block on absorption. Taking a supplement later in the day, or well outside the window of intense exercise, can help ensure that the iron is absorbed more efficiently.