Iron deficiency is probably the most common nutrient deficiency in the western world. Low levels of iron in the body are caused by several mechanisms, and become symptomatic with the onset of iron deficiency anemia. Athletes are a special group with additional reasons for iron or blood loss, such as plasma expansion, increase perspiration, 'foot strike hemolysis, and occasionally--malnutrition. Female athletes have yet another source of blood loss--menstruation. However, the most common cause for low hemoglobin levels in an athlete is dilutional pseudoanemia, which is caused by exercise-induced fluid retention. Athletes are more sensitive to the effects of anemia and iron deficiency, as exercise performance depends on maximal oxygen carrying capacity to the active muscle, and efficient oxygen utilization. Iron deficiency without anemia can also reduce athletic performance. Diagnosis is ultimately made by a blood count and red blood cell parameters, with ferritin serving as an index of body iron stores. Treatment requires iron supplements, as it is nearly impossible to refill the iron stores through diet alone.