In high altitudes, the amount of oxygen in the blood is reduced because there's less oxygen in the air. To compensate for reduction of oxygen in the air, the kidneys secrete more of a hormone called erythropoietin, which causes the body to create more red blood cells.
The average life span of a red blood cell is 90 to 120 days. Runners are often able to train harder and perform better for several weeks after they return from about a month-long stay at altitude because their blood still contains the extra blood cells that were produced when they were training at high altitudes. Runners who trained at high altitudes will have more red blood cells than runners who did not train at high altitudes.
Which Altitude is Best?
Research indicates that the best altitude is between 7,000 to 8,000 feet. When you train at high altitudes, high-quality training becomes more difficult to achieve. Easy and long runs are doable at altitudes between 7,000 and 8000 feet, but the thinner air makes it difficult to do intense speed work.
Runners who train at high altitudes often travel down to about 4,000 feet a few times a week to do their speed work.
Tips for Running at High Altitudes