21.19: Humans become acclimated to low-oxygen conditions.

Is it just a coincidence that a large percentage of the world’s most accomplished long-distance runners incorporate high-elevation training in their preparation for important races? No. Although llamas are adapted for high elevations from the day they are born, most humans are not—but we acclimate well, and this has a significant impact on our strength, speed, and stamina.

You don’t need to live in the Himalayas to become physiologically acclimated to the low-oxygen conditions found at high elevations. Training for three to five weeks at high elevations, usually 6,000–7,000 feet above sea level, triggers several physiological changes—a process called acclimation. And, as with llamas, some of the human acclimation to low-oxygen conditions comes from modifications (though they are acquired rather than inherited) to hemoglobin. Other changes include stimulating the production of additional red blood cells, increasing blood and capillary volume, and increasing the number of mitochondria.

High-elevation training causes an increase in a chemical called diphosphoglyceric acid (DPG) in red blood cells. This acid combines with hemoglobin in the red blood cells and alters the protein’s shape ever so slightly. In doing so, it reduces hemoglobin’s stickiness, giving it a lower affinity for oxygen. With this reduced oxygen affinity, the DPG-modified hemoglobin releases—at any elevation—more of the O₂ that it carries (FIGURE 21-39).

Athletes who train at high elevations find that with the additional oxygen released to their tissues, they can improve their performance by about 3%. This difference was manifested dramatically during the 1968 Olympics in Mexico City (elevation 7,500 feet) when the top five finishers in the 10,000-meter race were all year-round high-elevation residents. The benefits of high-elevation training remain after an athlete returns to sea level—increasing performance significantly. But just as humans become acclimated to high elevations, we also become acclimated to low elevations, and the DPG (and the performance enhancement it can bring) returns to low-elevation levels about three to five weeks after returning to sea level.