Most people enjoy a good trip to the mountains every once in a while: clean air, beautiful views, and fun outdoor activities. In fact, more than 34 million people travel to altitude every year. Unfortunately, there also are downsides when traveling to altitude, especially to higher elevations. Whereas everyone experiences some degree of impairment in exercise performance, many people also experience symptoms of altitude-related illnesses. The good news is that there are certain things that can be done before and during a stay at altitude that can minimize the negative effects of the high altitude, reduce the decrement in exercise performance, and make the mountain feel more like a molehill.
For the purpose of this article, exercise will be defined as activities or events with a high component of sustained aerobic activity. Moderate altitude will be considered as greater than 5,280 ft (1,609 m), and high altitude will be defined as elevations above 8,500 ft or (2,600 m). Numerous vacation destinations in the United States and around the world fall into categories of moderate to high altitude (Table 1), which can place individuals at risk for discomfort and can cause reductions in exercise performance. Although the effects are greater at higher elevations, some individuals can experience mild symptoms of mountain sickness as low as Denver, CO (5,280 ft).
THE SOURCE OF THE PROBLEMS
Several factors can affect exercise performance at altitude. First, lower oxygen pressures at altitude can reduce exercise capacity directly. As elevation increases, barometric pressure falls, causing a decrease in pressure of oxygen in the surrounding air and thus in the air that enters the lungs during breathing. The lower oxygen pressure means that less oxygen will diffuse from the lungs into the blood, and therefore, less oxygen will be transported to the tissues, ultimately decreasing the work capacity of the muscles and the heart. The general rule is that maximal exercise capacity will decrease approximately 3% for every 300 m above 1,500 m. However, sprinting, jumping, and certain throwing events may actually improve at altitude because of the decreased air resistance.
Second, athletes that travel up to high altitude may experience sleep disorders that can cause them to awaken throughout the night. This fragmented sleep pattern is caused by sessions of irregular breathing during which ventilation can be accelerated, slowed, or even interrupted for short periods. Poor sleep quality can result in an inability to sustain physical activity and impair cognitive performance. In addition, independent of sleep quality, altitude per se can reduce concentration, slow rates of thinking, and impair information recall and memory. Such decrements would be especially detrimental to individuals engaged in activities that require high levels of technical skill or focus.
Finally, the reduced oxygen availability also is the primary cause of the cascade of events that leads to high-altitude illnesses such as acute mountain sickness (AMS), high altitude pulmonary edema (HAPE), and high altitude cerebral edema (HACE). HAPE and HACE, characterized by excess fluid accumulation in the lungs and the brain, respectively, are serious conditions that require evacuation to lower elevations and medical care and are beyond the scope of this article. Acute mountain sickness symptoms such as headache, nausea, insomnia, dizziness/light-headedness, and loss of appetite generally dissipate after a few days of high altitude exposure but can be very debilitating for the first 48 to 72 hours. Therefore, reducing symptoms of AMS can dramatically improve exercise capacity during the first few days of exposure.
Why some people experience major decrements in performance and/or get AMS whereas others don't is an area of active investigation. It is likely that factors you can change (e.g., rate of ascent, amount of physical activity, and preacclimatization) and factors you cannot change (breathing response to low oxygen, genetics, age, etc.) determine an individual's risk of AMS and magnitude of decrement in exercise performance (Table 2). According to one study, important factors for susceptibility to AMS are rate of ascent, days spent at high altitude (>3,000 m) during previous 2 months, and prior history of AMS (1). There are sex differences in altitude acclimatization, but they are relatively minor and have minimal impact on exercise performance and AMS responses.
Surprisingly, younger and fitter individuals are not protected from the deterioration in maximal performance at altitude and may be more susceptible to AMS than older less fit individuals. However, their higher incidence of AMS may be more related to behavior than age or fitness level. Younger fitter individuals tend to ascend to altitude more rapidly and engage in intense exercise sooner upon arriving at altitude, putting them at greater risk for AMS. Therefore, although it is important to be as fit as possible when heading to the mountains to be able to accommodate the high stress of altitude activities, it will not provide full protection against the risks. The purpose of this article is to provide strategies that can be used by men and women traveling to altitude to optimize the preservation of performance.
GOING EARLY, ASCENDING SLOWLY, AND TAKING IT EASY
Developing AMS can make the first few days at altitude miserable. Nobody wants to start a trip or compete in an event while vomiting with a splitting headache and dizziness. Therefore, one of the best ways to preserve performance is to focus on minimizing the symptoms of AMS. One effective technique (if we could all give up our day jobs) is to go early and ascend slowly. Symptoms of AMS can be reduced if less than 500 m are gained per day above 2,500 m and a rest day is taken every 1,000 m. Climbing to more than 6,000 m in 10 days or less is not recommended (2). If the mode of transportation involves something more than walking (e.g., car or jet), then arriving early before starting intense activity is advisable. However, appropriately timing an arrival to a competitive event is tricky. On the one hand, there are many positive changes that occur over time to allow acclimatization to altitude. On the other hand, at higher elevations, the ability to train hard (at the same absolute workload) decreases because normal sea-level training workloads become a higher percentage of maximal capacity at altitude. Therefore, athletes may detrain over time. As indicated in Table 3, adaptations continue to occur for months or even years at altitude, but several critical ones occur within the first 48 hours. Therefore, arriving at least 48 to 72 hours ahead of an event or expedition to take advantage of early adaptations, but not long enough for training to suffer (1 to 2 weeks), is a prudent strategy. If traveling to altitude with an expedition or travel company, it is likely that the itinerary will already include at least 2 days of acclimatization before starting the trek.
To get the positive adaptations to altitude without the problem of detraining, many athletes use the "live high, train low" (LHTL) technique to improve sea-level performance. As the name implies, LHTL involves providing an altitude stimulus (e.g., living or sleeping at altitude) to cause adaptation while still maintaining heavy exercise loads by training in an environment with plentiful oxygen availability (e.g., running or cycling at a considerably lower altitude). The LHTL technique can be achieved either in a mountain area with easy access to lower elevations or in a simulated altitude environment such as an altitude tent. When trying to improve sea-level performance, LHTL generally uses a moderate altitude stimulus (approximately 2,000 to 2,500 m) with prolonged exposures (e.g., >12 hours/day for more than 3 to 4 weeks) while training at low altitudes (e.g., <1,250 m) once or twice a day (see Levine and Stray-Gundersen (3) for review).
In contrast, many of the studies investigating preacclimatization for subsequent altitude performance have used altitude chambers or hypoxic gas to provide exposure regimens that simulate higher elevations (>4,000 m). Under such conditions, significant benefits such as increased oxygen saturation, improved ventilatory response, and enhanced submaximal performance have been observed by some in as short as 90 minutes/day for 7 days (4). Longer exposure times (4 hours/day, 5 days/week for 3 weeks at 4,300 m) have resulted in increases of greater than 18% in performance measures such as V˙O2max, submaximal time trial performance, and muscle endurance in addition to reductions in AMS at altitude (5). A recent review of preacclimatization techniques concluded that exposures to altitudes of at least 4,000 m for 1.5 hours or greater for 5 to 6 days is required to achieve preacclimatization for subsequent altitude performance (6).
Most people don't have time to do elaborate, prolonged, preacclimatization regimens, but preacclimatization does not necessarily need to be rigidly planned. Even informal trips to the mountains for hiking or camping during the months preceding an expedition or competition are thought to help reduce AMS symptoms (1) and enhance performance during subsequent sojourns, although few data exist on this theory. At minimum, by going up to a similar target altitude in the prior months, individuals can evaluate how much their performance is affected at altitude, judge whether they are susceptible to AMS, and apply the recommendations outlined in this article to determine what strategies work best for them. As with many elements of physiology, there is much variability in how well individuals respond to preacclimatization and the LHTL regimen.
At altitude, the body has a natural tendency to lose fluid. There is increased water loss through breathing of cold dry air (called insensible water loss) that can reach 1 to 2 liters/day (7). Increased sweat rates resulting from high levels of physical activity and rapid evaporation increase the risk of dehydration. Because dehydration can compromise performance, attention should be paid to keeping properly hydrated. A general recommendation for water intake at altitude is 3 to 5 liters/day, which takes into account insensible water loss, urine output, sweat loss, and water for heat regulation (7). However, individual athletes should tailor their fluid intake to meet their own specific needs and level of activity. Measuring urine specific gravity, urine osmolality, and/or first morning body weight provide an accurate way for athletes to monitor their hydration status (for specific details on these methods, please read the ACSM Position Stand, "Exercise and Fluid Replacement" 8).
Interestingly, as Table 3 indicates, one of the early adaptations to altitude exposure is a reduction in plasma volume (PV) because of increased urine output. The reduction in PV hemoconcentrates blood to promote increased oxygen carrying capacity per unit of blood. Therefore, the advice to drink additional fluids at altitude may seem counterproductive. However, it is important to let the body regulate those changes. Volitional dehydration to speed the reduction in PV before the appropriate hormonal and blood adaptation signals could draw fluid from unintended body compartments with detrimental effects. Conversely, excess dietary salt consumption during early exposure could impair the natural diuretic response to altitude and slow the acclimatization process.
At altitude, it is common to have an energy deficit of 300 to 500 Kcal/day which, over time, often leads to weight loss. Several mechanisms contribute to this caloric deficiency. First, basal metabolic rate, the energy used to support body functions while lying quietly in bed, increases by approximately 200 Kcal/day. Second, most people travel to altitude to compete or engage in recreational activities (such as skiing, climbing, trekking, etc) that can increase energy requirements. Third, despite the increased caloric output, caloric intake is frequently lower because low oxygen availability suppresses appetite and food can be difficult to carry and prepare in the field. Although the energy deficiency may not affect athletes or mountaineers going to altitude for a weekend, during longer stays of weeks or months, consuming adequate calories becomes more important. Overall, it is useful for mountaineers to be aware that their appetite may not accurately reflect their true nutritional needs.
Maintaining adequate carbohydrate (CHO) intake while at altitude is critical. Carbohydrate is the predominant source of energy to sustain exercise regardless of elevation, but the reliance on CHO may be even greater at high altitude. In the short term (up to 3 weeks), overall caloric deficit may not negatively impact performance as long as glycogen stores are adequately maintained (9). Whereas some theorize that eating additional CHO at altitude may ease the symptoms of AMS, available data are mixed regarding this strategy. It is recommended that complex CHOs make up greater than 60% of total caloric intake at altitude. Further, whereas it is recommended that athletes consume adequate amounts of protein (12% to 20% of their diet), the protein requirements do not differ from sea level (10).
Although no specific recommendations exist for fat intake per se at altitude, fat is an important fuel. Because fat is more calorically dense (9 Kcal/g) compared with other macronutrients (4 Kcal/g), inclusion of additional fat in the diet can help provide sufficient calories at altitude especially when pack weight and space for food are limited during field events or excursions. However, for people experiencing symptoms of AMS, small amounts of low-fat bland food every couple of hours are preferable to heavy fat-rich meals until the symptoms diminish (10).
SUPPLEMENTS AND DRUGS
Adequate iron storage is important for long-term acclimatization at altitude. Athletes with iron deficiency were not able to increase their red blood cell volume/mass after 4 weeks at 2,500 meters (11). Athletes with poor iron status can benefit from eating iron-rich food such as meat, poultry, fish, and somewhat less effectively, some plant-based food such as broccoli, spinach, and kale. However, athletes with iron deficiency (serum ferritin, <20 μg/L, or transferrin saturation, <16%), who have less than 3 weeks to prepare for competition at altitude, would benefit more from taking iron supplements before and during altitude exposure (11). This strategy may be especially important for those at higher risk for being iron deficient such as women of childbearing age and some vegetarians.
However, like all supplements, more is not necessarily better. In fact, too much iron can be detrimental as well. In one study, nondeficient athletes who took iron supplements actually had decreased total body hemoglobin after 18 days at 1,800 m (12). Although the reason for this was unclear, the authors suggested that the level of iron was too high and might have led to disruption of red blood cell production or hemolysis. Therefore, it is important for athletes to talk with their physician before engaging in strenuous events at altitude and get appropriate laboratory tests (e.g., hemoglobin and ferritin concentrations and hematocrit) to determine if they are iron deficient.
Evidence regarding the benefits of antioxidants (specifically vitamins A, C, and E; selenium; and zinc) at altitude is mixed. Antioxidants are noted for their ability to reduce markers of damage caused by oxidative stress. Spending time at high altitude increases the productions of free radicals (or oxidative stress), and it has been hypothesized that free radical damage at altitude might augment symptoms of AMS and impair performance. Earlier studies suggested that antioxidant supplementation could reduce markers of stress during heavy work at altitude (for review, see Askew 13). However, more recent data from a double-blind placebo-controlled study suggest that antioxidant supplementation does not improve performance or impact AMS at altitude in healthy individuals eating a well-balanced diet (14). Although a final determination has not yet been made, antioxidant supplements do not seem beneficial at this time.
Gingko Biloba is an herbal supplement that has been a popular altitude aid because of its purported antioxidant properties. Although some small studies have demonstrated that Gingko Biloba can reduce symptoms of AMS, the majority have found no benefit (for review, see Bartsch et al. 15).
Acetazolamide (e.g., Diamox) is a carbonic an hydrase inhibitor available by prescription that is commonly used and reported to be highly effective in the prevention and treatment of AMS and for improvement in quality of sleep at altitude. Acetazolamide works partly to accelerate diuresis and partly by increasing the body's breathing response to hypoxia. The side effects of acetazolamide are minor, and exercise performance at altitude does not seem to be impaired by the drug when taken in low doses (125 to 150 mg BID). Therefore, acetazolamide can limit the impact of AMS during initial altitude exposure, especially in individuals who have a previous history of AMS or who will be engaging in activities that place them at higher risk (e.g., rapid and difficult ascent to higher elevations). Generally, the drug is started 2 days before arrival at altitude and continued for 2 to 3 days after arrival. Individuals should consult with their physician regarding the suitability, appropriate dosage, and timing of the drug.
Sildenafil citrate (e.g., Viagra) may improve oxygen delivery and exercise performance in certain individuals who experience altitude-induced pulmonary hypertension at altitudes above 3,800 meters. However, because the drug is not without side effects and may exacerbate symptoms of AMS, additional research is required before a solid recommendation can be made regarding sildenafil use at altitude.
High altitude can be a harsh environment that presents several physiological stresses such as reduced oxygen delivery, potential dehydration, energy deficit, and altitude-related illnesses. By taking a few relatively minor precautions before and during a trip and allowing time for the body to acclimatize, symptoms of AMS can be reduced, and a greater amount of sea-level performance can be preserved. Cumulatively, the tips for your trip can make a difference in how an individual feels and does in the mountains.
CONDENSED VERSION AND BOTTOM LINE
Many people travel to moderate or high altitude for recreation every year. Because the pressure of oxygen is less at altitude, most types of exercise performance are impaired, and people are at risk for altitude-related illnesses. By taking minor steps such as staying hydrated, consuming adequate carbohydrates, and arriving at altitude early to allow time for the body to acclimatize and resolve symptoms of acute mountain sickness, a greater portion of sea-level performance can be preserved. Together, the specific recommendations in this article can help people do their best at altitude and make the mountain feel more like a molehill.
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