Iron-deficient anemia reduces the oxygen-carrying capacity of red blood cells, thus decreasing energy and endurance. When there is not enough iron, the red blood cells are not able to produce enough hemoglobin (the oxygen-transporting pigments found in red blood cells). Those affected may have pale skin and be excessively tired and ineffective at work or school. Some may have heart palpitations and frequent headaches. Although rare, spoon-shaped fingernails and difficulty swallowing are also symptoms of anemia. “Iron-deficient anemia develops slowly so those affected do not notice small, day-to-day changes,” says Nancy Andrews, associate professor of pediatrics at Harvard Medical School and an attending physician in hematology-oncology at Children’s Hospital Boston. Iron deficiency affects as many as one in seven premenopausal women in industrialized countries and an even larger proportion of women in developing nations. Worldwide, approximately 600 million people — twice the population of the United States — are iron deficient, says Andrews.