Why do athletes train at high altitude?

Athletes train at high altitude to stimulate increased red blood cell production, which enhances oxygen-carrying capacity and improves aerobic performance at sea level. At high altitude, the partial pressure of oxygen in the atmosphere is reduced, leading to lower arterial oxygen saturation and mild tissue hypoxia. The kidneys detect this relative hypoxia and respond by secreting erythropoietin, a hormone that stimulates the bone marrow to produce more erythrocytes. Over days to weeks of altitude exposure, the red blood cell count and total hemoglobin mass increase, allowing the blood to carry more oxygen per unit volume. When athletes then compete at lower altitudes where oxygen is plentiful, this elevated red cell mass provides a performance advantage. Decreasing viscosity is actually the opposite of what occurs — more red cells increase blood viscosity. Suppressing bone marrow would reduce red blood cell production, which is counterproductive to athletic performance goals. Reducing polycythemia makes no sense in this context — altitude training is intentionally used to induce a form of secondary polycythemia to boost erythrocyte numbers. Therefore, increasing RBC production is the correct and well-established physiological rationale behind altitude training in competitive sports.