Influence of prolonged physical exercise on the erythropoietin concentration in blood.

Erythropoietin (EPO) and red blood cells were studied in 15 well-trained men before and several times after a marathon run. Changes in red blood cells reflected changes of plasma volume. Immediately after the run, red blood cells were increased due to haemoconcentration, whereas 31 h later the values were decreased due to haemodilution. The EPO concentration was increased 3 h, and more impressive 31 h, after the run. This long-lasting increase in EPO concentration after the marathon run would seem to be responsible for the increased red blood cell mass in long distance runners.