Iron deficiency in runners

Iron deficiency is the most common micronutrient deficiency in endurance athletes and one of the most significant performance-limiting conditions that often goes undiagnosed and untreated at the recreational level. It does not require full-blown anemia to impair performance. Subclinical iron deficiency – normal hemoglobin but depleted iron stores – is sufficient to decrease maximal aerobic capacity, increase perceived exertion, and impair recovery.

This article discusses why iron deficiency is so common in runners, how to identify it, and what can be done about it.

Why Runners Are at Increased Risk

Several mechanisms combine.

Foot strike hemolysis is one. Repeated impact during running physically destroys red blood cells in the capillaries of the feet. Each step produces pressure that can break some red blood cells, release their hemoglobin, and increase iron loss. Runners covering high weekly mileage and running on hard surfaces are at the greatest risk.

Exercise-induced inflammation is another. Prolonged endurance exercise increases circulating hepcidin – a hormone that dampens iron absorption from the gut. Intense training reduces the body's ability to absorb dietary iron precisely when iron demand is at its highest.

Microscopic gastrointestinal bleeding is a third. Long-distance running causes small amounts of gastrointestinal bleeding in many athletes, especially at higher intensities. It is small but continuous and accumulates over a period of high weekly mileage.

Iron is also lost in sweat in low concentrations. For a runner covering high weekly mileage and sweating profusely during the Finnish summer, the cumulative loss is significant.

Inadequate nutrition completes the picture. Athletes who restrict calorie intake, follow a vegetarian or vegan diet, or avoid red meat are at an increased risk. Plant-based iron is absorbed with an efficiency of about five to ten percent, while animal-based heme iron is absorbed with twenty to thirty percent efficiency.

Performance Impact of Subclinical Deficiency

Iron is needed for hemoglobin synthesis – the oxygen-carrying protein in red blood cells. It is also a component of myoglobin (muscle oxygen store) and a structural part of many enzymes involved in energy metabolism.

When iron stores are depleted but hemoglobin is still within the normal range – a condition known as iron deficiency without anemia – several performance impairments follow. Maximal aerobic capacity decreases because oxygen delivery is impaired. Perceived exertion increases at submaximal intensities. Running economy deteriorates – the same pace requires more energy. Mitochondrial function and the efficiency of fat oxidation decrease. Recovery between sessions slows down. Fatigue feels disproportionate to the training load.

These effects are measurable and significant. An athlete with depleted ferritin stores running at their usual training paces will perceive them as harder, recover more slowly, and halt their progress.

Diagnosis

The most important blood test for assessing iron status in athletes is serum ferritin – a protein that stores iron. Hemoglobin alone is not sufficient to diagnose subclinical deficiency.

Performance-relevant ferritin thresholds for athletes are approximately: below twelve micrograms per liter indicates clear iron deficiency; twelve to thirty indicates subclinical deficiency with probable performance impact; thirty to fifty is a borderline case that should be monitored in runners with high weekly mileage; above fifty is generally sufficient for most.

Finnish athletes can request ferritin measurement through occupational health services or private sports medicine clinics. Annual testing is appropriate for those running high weekly mileage. Testing twice a year is advisable for female athletes who also experience menstrual bleeding.

Treatment

Iron supplementation, guided by a doctor, is the fastest way to correct depleted ferritin. Oral iron – typically ferrous sulfate or ferrous gluconate – taken on an empty stomach with vitamin C to enhance absorption is the standard treatment.

Iron supplementation should not be started at a high dose without confirmed deficiency. Excess iron is harmful, and iron overload is a serious condition.

Optimizing diet is also important. Increase sources of heme iron (red meat, liver, dark poultry) or highly absorbable plant-based sources (lentils, tofu, fortified breakfast cereals) combined with vitamin C to improve absorption. Avoid combining iron-rich foods with coffee, tea, or calcium-rich dairy products, all of which inhibit the absorption of non-heme iron.

During supplementation periods, consider temporarily reducing training load to slow continuous iron loss as stores are built up.

Summary

Iron deficiency is common in long-distance runners, easily detectable with a simple blood test, and significantly performance-limiting even before the development of anemia. Annual ferritin measurement should be a standard part of health monitoring for every serious runner. Treat deficiency with medical guidance and dietary optimization, not guesswork.

References

Peeling P, Dawson B, Goodman C, Landers G, Trinder D. (2008). Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. European Journal of Applied Physiology. 103(4), 381–391.

Sim M, Garvican-Lewis LA, Cox GR, Govus A, McKay AKA, Stellingwerff T, Peeling P. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology. 119(7), 1463–1478.

Burden RJ, Morton K, Richards T, Whyte GP, Pedlar CR. (2015). Is iron treatment beneficial in iron-deficient but non-anaemic (IDNA) endurance athletes? A meta-analysis. British Journal of Sports Medicine. 49(21), 1389–1397.