Iron deficiency anemia occurs when your body doesn't have enough iron to produce hemoglobin.Slow, chronic blood loss within the body -- such as from a peptic ulcer, a hiatal hernia, a colon polyp or colorectal cancer -- can cause iron deficiency anemia.Hemoglobin is the part of red blood cells that gives blood its red color and enables the red blood cells to carry oxygenated blood throughout your body.