Why mountain air changes everything
The fraction of oxygen in the air is always the same, about 21 percent, from sea level to the top of a mountain. What falls with altitude is the barometric pressure. Less pressure means fewer oxygen molecules pushed into the lungs with each breath. The result is hypoxia: less oxygen reaching the blood and muscles.
The body notices this drop almost immediately. Respiratory and heart rates rise at rest, and aerobic performance falls. In well-trained athletes, maximal oxygen uptake (VO2max) decreases on the order of 6 to 7 percent per 1000 meters of altitude gain, approximately linearly at moderate altitudes.
The adaptation: EPO, hemoglobin and more red blood cells
The central response to hypoxia is hormonal. The kidneys release erythropoietin (EPO), the hormone that orders the bone marrow to produce more red blood cells. EPO spikes in the first hours at altitude, peaks around 24 to 48 hours and then retreats toward baseline over one to three weeks.
What matters for performance is not the EPO peak, but the final product: an increase in hemoglobin mass (the total amount of hemoglobin in the body, which carries oxygen). That gain is slow. The literature indicates something around 3 to 3.5 percent of increase after about three weeks of adequate exposure, and the altitude range considered most efficient for that sits between 2000 and 2500 meters. Below that, the stimulus is weak. Much above, sleep and recovery worsen.
Live high, train low: the model that won
Here is the key turn of the 1997 study. The researchers compared three strategies over the 4 weeks:
- Live high, train low: sleep at 2500 meters and descend to train at 1250 meters.
- Live high, train high: live and train at 2500 meters.
- Live low, train low (control): live and train near sea level.
The two altitude groups gained red cell mass and VO2max. But only the live high, train low group actually improved the time in the 5000 meter test, with a gain of 13.4 seconds. The explanation is elegant: sleeping high delivers the hypoxic stimulus that manufactures blood, while training low allows maintaining the intensity and speed of workouts, which fall when you try to train hard in the thin air. Those who lived and trained high gained blood, but lost training quality, and the race balance was neutral.
That is the practical lesson. Living high and training high (live high, train high) still increases hemoglobin and is used by many athletes, but usually sacrifices the intensity of the fastest workouts. The live high, train low model tries to capture the best of both worlds.
How to measure whether it worked: the carbon monoxide test
Knowing whether an altitude camp truly increased hemoglobin mass is not guesswork. The reference standard is the carbon monoxide rebreathing method (CO-rebreathing), standardized by Schmidt and Prommer in 2005. The athlete inhales a controlled, minuscule dose of carbon monoxide for about two minutes. Since CO binds to hemoglobin predictably, measuring how much hemoglobin was occupied allows calculating the body's total hemoglobin mass, with a typical error around 2 percent. If the topic of CO-rebreathing interests you, it deserves a text of its own, because it is what separates marketing from real adaptation.
The bad side: hydration and altitude sickness
Altitude is not just gain. The dry air and accelerated breathing increase water loss through respiration, and urine production rises in the first days. Dehydration is common and, besides hindering training, is a recognized risk factor for acute mountain sickness.
Acute mountain sickness (AMS) usually appears above 2500 meters, with symptoms arising 6 to 12 hours after the ascent: headache (the most frequent), nausea, dizziness, fatigue and poor sleep. Gradual ascent and adequate hydration are the most consistent defenses. It is worth remembering that the sequence of poorly slept nights in the first nights at altitude also degrades recovery, which reinforces why sleeping high and training low makes sense only up to a certain point of altitude.
The honest summary
What stays
The mountain does not reward whoever merely goes up, it rewards whoever goes up with a plan: in the study by Levine and Stray-Gundersen, two groups gained blood by ascending, but only those who slept high and trained low lowered the 5000 meter time, by 13.4 seconds. The entire difference lies in keeping training speed intact while the body quietly manufactures hemoglobin, a process that takes weeks, not days, and that EPO alone does not guarantee. Before booking an altitude camp thinking about a record, it is worth asking: does the plan protect training intensity, or does it only change the air you breathe?
