Do You Need Iron Supplements? Evidence on Iron Deficiency and Exercise Performance

Iron is the building block of hemoglobin—the protein in red blood cells that carries oxygen—and is essential for the mitochondrial electron transport chain. Iron deficiency reduces hemoglobin levels, impairing oxygen delivery to muscles. This leads to decreased endurance performance, elevated heart rate at the same exercise intensity, and increased fatigue. Even 'latent iron deficiency' (low ferritin without anemia) causes fatigue and reduced endurance (Pasricha et al., 2014).

+3.9 ml/kg/min

VO2max improvement from iron supplementation (in deficient individuals)

Women with regular menstrual blood loss face elevated iron deficiency risk; endurance athletes (especially long-distance runners) compound this through intravascular hemolysis from foot-strike impact. Vegetarians and vegans consume only non-heme iron from plants, which has approximately 1/2 to 1/3 the absorption rate of heme iron from animal sources. Blood donors and those with gastrointestinal bleeding also deplete iron rapidly.

1/2–1/3

absorption rate of non-heme iron vs. heme iron

Iron supplements are effective when iron is deficient, but do not improve exercise performance in iron-sufficient individuals. Excess iron increases oxidative stress, causes gastrointestinal symptoms (constipation, nausea), and raises the risk of hemochromatosis. Rather than self-directed supplementation, a blood test including serum ferritin and hemoglobin is strongly recommended to first assess current iron status.

Heme iron found in animal foods (beef, liver, bonito, salmon) has a high absorption rate of 15–35%. Non-heme iron in spinach, legumes, and tofu has 2–3× higher absorption when consumed with vitamin C (ascorbic acid). Conversely, tannins in coffee/tea and calcium inhibit iron absorption, so spacing iron-rich meals apart from these is ideal.

15–35%

absorption rate of heme iron

2–3×

improvement in non-heme iron absorption when taken with vitamin C