Many athletes believe that training even at moderate altitudes can improve performance. Acclimating to lower oxygen levels at high altitudes may improve the body's ability to deliver oxygen to muscles during aerobic exercise, which may improve performance. Over three or four weeks, altitude training increases production of the hormone erythropoietin (EPO), which stimulates the production of red blood cells (RBCs) for carrying oxygen to muscles. Furthermore, the lower partial pressure of oxygen at high altitude may cause hemogloblin, the oxygen-carrying protein in RBCs, to bind oxygen more efficiently. The increased concentration of RBC and possible increase in the amount of hemoglobin raise the oxygen-carrying capacity of the blood, which may increase maximal aerobic capacity or VO2 max. Altitude training may have various other physiological effects that could improve athletic performance, including increasing the number of small blood vessels, altering the structure and metabolism of muscle, and improving the ability of muscles to cope with acid buildup (buffering capacity). Breathing at higher altitudes with less oxygen causes the heart to beat faster and the body to burn more energy, which may result in weight loss.
Altitude training is generally considered training at more than 6,600 ft. (2,000 m) above sea level (0 ft. or m), either on a daily basis or as a short-term strategy, although above 8,000 ft. (2,400 m) is preferable. Training is sometimes as high as 10,000 ft. (3,000 m), although there are far fewer suitable locations at such high altitudes. Air contains 20.9% oxygen at all altitudes, but there are fewer of all molecules in the thin air of higher altitudes, so at 17,257 ft. (5,260 m), the atmosphere has only 53% of the oxygen molecules at sea level. Likewise, barometric air pressure and oxygen partial pressure both decrease with increased altitude. At sea level at standard barometric pressure, 100% of the oxygen in air is available. At 6,600 ft. (2,000 m), only 80% of the oxygen is available due to the lower pressure.
Mountaineers have long espoused the lasting effects of exertion at high altitudes. The benefits of altitude training first became widely apparent, however, during the Mexico City Olympics in 1968, when endurance athletes who had lived or trained at high altitude for significant periods performed better than those who were not acclimated to the over 7,000-ft. (2,100-m) altitude of the games. Likewise, long-distance runners from high altitudes in Africa have consistently outperformed other athletes. Tibetans living at high altitudes have several gene variants that enable them to use oxygen more efficiently, especially a version of the gene EPAS1 that regulates production of hemoglobin. Tibetans are believed to have inherited this gene from Denisovans, a human species that went extinct about 40,000 years ago after interbreeding with modern Europeans and Asians.
Oxygen diffusion into the bloodstream is dependent on the higher-to-lower pressure gradient between the lungs and the blood. With increasing altitude, the pressure of inspired air decreases, reducing the pressure gradient between the lungs and bloodstream and lowering the oxygen diffusion rate. This decreases oxygen saturation of the blood, resulting in an inadequate supply of oxygen to the muscles, which should impair performance when returning to lower elevations or sea level. Athletes, however, observed that training or competing at high altitude actually improved performance upon return to sea level, and low-oxygen exposure (hypoxia) came to be recognized as an environmental stressor that stimulated physiological adaptations. In recent decades, altitude training has become increasingly popular among endurance athletes, and many elite athletes regard it as indispensable.
Depending on the protocol, adaptation or acclimation to altitude can involve various physiological changes. Immediate changes occur over several days. Longer-term changes occur over weeks to a few months. Respiratory and heart rates initially increase, both during submaximal exercise and at rest, to help counter the lower oxygen partial pressure. This faster breathing affects the acid-base balance in the body, which takes some time to equilibrate. Altitude also initially decreases the volume of blood plasma, which increases the hemoglobin concentration. EPO levels rise over the first 10 days and then level off after 25–30 days.
Longer-term adaptations to altitude include:
There are three basic altitude training protocols:
Some elite athletes live or train in the mountains; others, including amateur runners, train at high altitudes occasionally, for example before running a high-altitude marathon. Most coaches recommend at least 2 weeks of altitude training, although even 7–10 days can result in physical and mental benefits that last for several weeks after a return to sea level. Many altitude training camps run for three weeks, but adding an extra week may be helpful, because adaptations can occur during the fourth week. Many competitive runners altitude train for weeks or months. Most altitude training centers are above 8,000 ft. (2,400 m). Popular training destinations include:
In lieu of training at high altitudes, altitude simulators can be used. Altitude simulators include:
Benefits from altitude training depend on the individual, the protocol, and the training goal(s). In addition to potential physiological benefits, the lack of oxygen can make training feel harder, which may help athletes adapt to working harder and tolerating more discomfort without additional stress to the joints and muscles. Some sea-level performance benefits may be due simply to training more when at altitude. Although even a very small improvement in performance can be hugely significant for elite athletes, an analysis of multiple studies found that subelite athletes were actually more likely to benefit from altitude training than elite athletes, possibly because they started their training with lower hemoglobin levels.
Despite a great deal of research, the overall benefits of altitude training and specific protocols remain controversial.
Runners should try to arrive at least 10 days in advance of high-altitude races to recover from increased stress, determine effort levels, and allow for adaptation. If this is not possible, the opposite may be recommended—arriving at altitude within 18–47 hours of a race to avoid the negative effects of the first 2–7 days at high altitude.
Adequate nutrition is essential for effective altitude training, especially adequate intake of energy, carbohydrates, protein, fluids, and foods rich in antioxidants. Altitude training increases basal metabolic rate, especially at first, so more calories are burned at the same time as hypoxia may be suppressing appetite. Increasing the oxygen-carrying capacity of the blood requires sufficient iron to bind the oxygen in hemoglobin. Therefore, iron-rich foods such as red meat, beans, and dark leafy greens or supplemental iron may be necessary before and during altitude training, especially for women and vegetarians. Supplementing with iron before arriving may help maximize EPO and RBC and help prevent altitude sickness, but iron levels should be tested before using supplements. Guidelines for Olympic athletes call for 120-130 mg of daily elemental iron supplementation in two doses taken with vitamin C. Altitude training can increase the production of free radicals in muscles, which can contribute to fatigue and interfere with recovery, so some athletes prepare for altitude training by taking an antioxidant multivitamin or vitamin E. Athletes are sometimes advised to supplement with branched-chain amino acids (leucine, isoleucine, and valine) to help prevent loss of muscle mass during altitude training.
Direct risks from hypoxia and low-pressure (hypobaric) conditions include illness, infection, and less than optimal adaptation. These risks can be aggravated by increased intensity and duration of training, interrupted sleep, and/or increased exposure to ultraviolet radiation. Dehydration is a serious risk, and some experts suggest drinking 4-5 qt. (L) of fluid daily. Headache, malaise, and decreased appetite affect many people above 8,000 ft. (2,400 m), and mild altitude sickness affects up to 50% of people above 14,000 ft. (4,300 m). Medications can help prevent mountain sickness, and mild forms can usually be treated with rest, hydration, alcohol avoidance, and analgesics such as ibuprofen. Severe altitude sickness can be life-threatening due to pulmonary or cerebral edema.
Other risks of altitude training include:
A 2016 scientific expedition to the summit of Mount Everest in Nepal had planned to collect the highest-ever blood sample and muscle biopsy to study the link between hypoxia and cognitive decline. The expedition was abandoned when, after several weeks of acclimatization, the mountaineer's blood was found to be extremely thick due to very high RBC levels. The climber's brain was getting more oxygen than it would at sea level despite an atmosphere containing only half the amount of oxygen, and he appeared in good health; however, he was at serious risk for a stroke or heart attack from blood clotting. This pre-empted study suggested the possibility that at least some Everest climbers—and perhaps some athletes training at very high elevations—might be dangerously overacclimating.
Data from a 2014 study suggested that optimal acclimatization for sea-level performance following a “live high—train low” protocol at a four-week altitude camp was a target altitude of 6,600–8,200 ft. (2,000–2,500 m). Research suggests that the “live high—train low” model is most effective for several reasons:
Although the traditional explanation for adaptation is an increase in RBC, this increase takes weeks, whereas adaptation to altitude occurs within days. An ongoing study called AltitudeOmics reported in 2016 that, among other findings, even short exposures to high altitude cause a cascade of changes within RBCs that help them cope with low oxygen. These adaptations begin as soon as the first night and last for weeks or perhaps months, even after descending to lower elevations. The adaptations include multiple complex changes in how tightly hemoglobin binds oxygen, and they last for the life of the RBC—about 120 days. Thus, this study confirms the experiences of mountaineers, high-altitude skiers, and altitude-training athletes.
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Margaret Alic, PhD