Iron is the trace mineral found in the greatest amount in the body, about 3–5 grams in adults. Most of this iron is found inside hemoglobin molecules, where it plays a critical role in transporting oxygen from the lungs to the cells. Because very little oxygen dissolves in the plasma, most oxygen molecules must bind to the iron atoms in hemoglobin molecules for transport in the blood. Other iron-containing proteins play important roles in aerobic metabolism, including the cytochromes of the electron transport system and myoglobin, the oxygen-binding protein found in muscles. Iron deficiency anemia, the most common nutritional deficiency in the world, reduces the amount of oxygen available for aerobic metabolism and limits a person’s endurance. Thus, iron is an important nutrient in sports that require endurance for success.