Anemia is caused by a lack of red blood cells or hemoglobin, the oxygen-carrying protein found in red blood cells. Anemia is most often due to a deficiency in iron, a key component of hemoglobin. (That’s why it is often treated with iron supplements). Anemia can be triggered by blood loss, a folic acid or vitamin B12 deficiency, sickle cell disease, and a genetic disorder called thalassemia, among other causes. In the U.S., 2% to 5% of women and 1% of men have iron deficiency anemia.