Depletion of iron stores is frequently seen in male and female distance runners. Possible causes of this iron depletion include inadequate iron intake especially among females and increased iron excretion through sweating and gastrointestinal blood loss. Animal studies suggest that iron deficiency without anemia can reduce endurance by lowering tissue cytochromes and the activity of certain muscle tissue enzymes. Iron supplementation appears to be beneficial in reducing blood lactate concentrations following heavy exercise. The amount of iron in the supplement appears to influence the amount of increase in hemoglobin and serum ferritin.