Runner's macrocytosis: a clue to footstrike hemolysis. Runner's anemia as a benefit versus runner's hemolysis as a detriment.

This report of a runner in whom progressive macrocytosis developed with increasing mileage deals with the hematologic adaptations to exercise, introduces the concept of "runner's macrocytosis" as a compensated hemolysis of older red cells, and makes a case for "runner's anemia" as a benefit versus "runner's hemolysis" as a detriment. It presents the characteristic hematologic profile of footstrike hemolysis and explores the influence of racing, different levels of training, and different shoes. It shows that runner's hemolysis can be reduced by reducing mileage but not necessarily by changing shoes, and it suggests that runner's hemolysis can impair race performance by preventing the attainment of an optimal red cell mass and, in time, by evolving into iron-deficiency anemia.