The hemoglobin within a red blood cell is responsible for carrying iron. Oxygen then attaches to the iron. Anemia can result from insufficient RBCs, not enough hemoglobin, or not enough iron. A hemoglobin molecule has four protein, or globulin, subunits, each with a heme group and iron atom in the center. The iron atom combines with oxygen molecules.
Oxygen binds to hemoglobin in a cooperative manner, meaning after one oxygen molecule binds, the structure changes shape, making it easier for other oxygen molecules to bind.
Fe3+ , or the ferric form of iron, cannot bind oxygen. It must first be oxidized, so it is best to take ferrous (Fe2+) form.
How much iron do we need? The Recommended Dietary Allowance (RDA) is 8 mg for adult males and 18 mg for adult females. Because of the higher levels of red blood cell destruction in runners, we can get away with supplementing with 100% of the RDA even without deficiency. Iron should be taken in the more absorbable ferrous state. There are three forms of supplemental iron - ferrous fumerate, ferrous sulfate, and ferrous gluconate. Ferrous fumerate has the highest percentage (33%) of elemental iron, meaning it is the best absorbed. Ferrous sulfate and ferrous gluconate have 20% and 12%, respectively.
If taking more than one pill to supplement, it is best to split up the pills - only a set amount of iron is absorbed at one time. When hemoglobin levels are below normal, physicians often measure serum ferritin, the storage form of iron as well.