The amount of iron lost through sweating is negligible. For enduring, powerful contractions, the muscles of an athlete must have constant access to abundant oxygen. The condition of sports anemia actually represents a pseudoanemia. The total number of RBCs in training athletes may even increase slightly, but the same sample volume of blood will contain fewer RBCs compared to before training. Therefore, according to Raymond Browne, MD, a former fellow at the American Sports Medicine Institute in Birmingham, sports anemia is more accurately described as dilutional anemia.
Lower viscosity translates to an increased cardiac output, which maximizes oxygen delivery.
In other words, sports anemia is a normal and even desirable result of training. This is not to say that anemia, as indicated by a lowered hematocrit, in an endurance athlete is attributable only to this phenomenon. Therefore an athlete exhibiting any drop in iron or RBC levels must receive a thorough physical evaluation to rule out causes other than sports anemia. As for sweating iron, during strenuous exercise muscles produce lactic acid and other metabolic byproducts that cause the osmotic pressure in the tissues to increase. This in turn increases the filtration rate of plasma from the blood to the tissues.