How Does Altitude Training Work? Thin Air, More Blood and a Trade-Off

The proportion of oxygen in air is the same at sea level and on a mountain, at about 21 percent, and what changes is the pressure. Lower atmospheric pressure means a lower partial pressure of oxygen, so each breath delivers fewer oxygen molecules to the lungs and the gradient driving oxygen into the blood is weaker, leaving haemoglobin less fully saturated. At 2,500 metres, arterial saturation in a resting person drops from around 98 percent to the low 90s, and during hard exercise it falls further. The body responds within hours by breathing faster and deeper and by raising heart rate, and over days to weeks it makes a more consequential change: the kidneys detect the shortfall and release erythropoietin, the hormone that instructs bone marrow to produce red blood cells, so that after two to four weeks the blood carries measurably more oxygen per unit volume.

Living high builds red cells and training high does not work well, which is the central difficulty. Because less oxygen is available, an athlete at altitude cannot sustain the same speeds or power outputs as at sea level, and the reduction is substantial, so a session that should be run at race pace is run slower. Training intensity is one of the main drivers of adaptation, and losing it for weeks erodes exactly the qualities the camp is meant to build. Several other costs accumulate: appetite falls and weight is lost, including muscle; sleep quality deteriorates, often with periodic breathing; iron stores are depleted quickly by the demand for new red cells, so supplementation is usually necessary; immune function dips; and dehydration is faster because the air is dry and breathing is harder. A poorly managed camp can leave an athlete worse off than when they arrived.

The resolution was proposed and tested by Benjamin Levine and James Stray-Gundersen in a study published in 1997, and it separates the two effects. Athletes live and sleep at around 2,000 to 2,500 metres, which is high enough to stimulate red cell production and low enough to avoid serious illness, and descend to under 1,250 metres for their hard training sessions, where they can hit full intensity. Their trial found this group improved sea-level performance significantly while groups living and training high, or living and training low, did not. The approach has become standard among endurance athletes, and practical implementations include:

• •Geography, where a suitable mountain sits above a valley with a track, as in parts of Colorado, Utah, the Sierra Nevada and the Alps
• •Nitrogen-diluted altitude houses and bedroom tents, which lower the oxygen fraction rather than the pressure and produce a similar stimulus
• •Supplemental oxygen during hard sessions performed at altitude, which simulates descending
• •A general requirement of at least three to four weeks of exposure, for around twelve to sixteen hours a day, before the blood changes are meaningful
• •Careful timing of the return, since the benefit peaks and decays over a few weeks and athletes differ in when they perform best afterwards

Responses vary enormously between individuals, and the variation is large enough that some athletes gain nothing. Studies distinguish responders from non-responders by how much their red cell mass actually increases, and the difference appears to be partly genetic, partly a matter of iron status and energy availability, and partly whether the dose of altitude was adequate in the first place, since athletes frequently spend fewer hours high than the protocol requires. There is also a live scientific argument about whether the benefit comes from the extra red cells at all, since some studies find performance improvements without measurable changes in blood volume, which has led to proposals that altitude also improves muscle efficiency, buffering capacity or the economy of movement. For sprinters and throwers the calculation is entirely different and simpler: thin air offers less resistance, so competing at altitude helps, which is why the 1968 long jump record stood for twenty-three years.

Altitude training raises red cell count by the same route that the banned hormone erythropoietin does, which invites an obvious question about where the boundary lies. The answer given by the regulators is that the method is permitted because it works through a natural physiological response to an environmental stimulus rather than through an administered substance, and because banning it would require banning living in mountainous countries. Artificial altitude tents were reviewed by the World Anti-Doping Agency's ethics panel in 2006, which found that they violated the spirit of sport, and the agency nonetheless declined to prohibit them, partly because enforcement would be impossible. The practical consequence for athletes is that a camp produces blood values that shift within the athlete's biological passport, and those changes must be declared and explained, since an unexplained rise in haemoglobin is precisely what the passport is designed to flag.

Altitude reduces the pressure of oxygen rather than its proportion, so blood carries less, and after two to four weeks the kidneys drive production of extra red cells. Training high is counterproductive because intensity falls, so the standard method since a 1997 study is to live at 2,000 to 2,500 metres and descend for hard sessions, using mountains, nitrogen-diluted rooms or supplemental oxygen. Responses vary widely between individuals, sprinters benefit from thin air directly, and the method is legal while producing the same blood changes that anti-doping passports monitor.