This systematic review synthesizes the evidence pertaining to the relationship between iron deficiency and physical work capacity in animals and humans. 
Evidence from 29 research reports demonstrates a strong causal effect of severe and moderate iron deficiency anemia on aerobic capacity in animals and humans, which is possibly due to 1) the reduced oxygen transport associated with anemia, or 2) tissue iron deficiency through reduced cellular oxidative capacity. Endurance capacity was also compromised in severe and moderate iron deficiency anemia, but the strong mediating effects of poor cellular oxidative capacity observed in animals have not been demonstrated in humans. Energetic efficiency was affected at all levels of iron deficiency in humans, in the laboratory and the field.