High-Altitude Training

There is less oxygen at higher altitudes, requiring that athletes undergo a degree of adaptation before training regimens and performance can approach sea-level expectations for aerobic (predominantly endurance) activities. Even for anaerobic events, a degree of adaptation is necessary to adjust to the lower oxygen concentration of high altitude so that altitude illness does not impede training. Because the concentration of oxygen is lower at progressively higher altitudes,a stepwise progression to higher and higher altitudes makes sense to allow for an efficient and illness-free adaptive response.

Athletes training at higher altitudes can expect a faster respiration and faster heart rate, which are adaptations to the lower oxygen being pulled into the lungs with each breath. Only a greater red blood cell concentration will mediate this physiological response, an adaptation that may take days, weeks, or months before a more normal breathing rate and heart rate occur. It should be remembered that several nutrition factors are associated with successful production of red blood cells, including adequate caloric intake, sufficient iron to increase the red blood cell concentration, folic acid, and vitamin B12. A healthy diet may satisfy most of these requirements, but care should be taken that iron in particular be consumed at the level of ~18 milligrams per day. This may be more difficult than it seems because athletes often complain of a loss of appetite at high altitude. Table 10.1 defines what should be considered high altitude.

Cold environments cause heat loss through both convection and conduction, although humans do have systems that help maintain core body temperature and increase heat production.¹ This process of thermoregulation helps ensure survivability when exposed to cold temperatures. With cold exposure, the body attempts to lower the amount of heat loss through peripheral vasoconstriction. However, the reduced blood flow to the skin and extremities predisposes individuals to frostbite, particularly of the fingers and toes. To counteract this risk, the body initiates a process referred to as cold-induced vasodilation (CIVD) after approximately 10 minutes of cold exposure. The pulsing of peripheral vasoconstriction and vasodilation results in the preservation of core temperature, but at the cost of fluctuating temperatures of the skin and peripheral tissues.

Humans mainly produce heat when muscles work. Of the calories used by working muscles, approximately 30 to 40 percent actually results in muscular movement, while 60 to 70 percent is lost as heat. Put simply, as warm-blooded animals we are more efficient at creating heat than creating movement. We can also produce heat by shivering, which is an involuntary central nervous system-induced mechanism that is invoked by a 3 to 4 degree drop in body temperature.^{2, 3} The increase in muscle contraction from shivering results in a 2.5-fold increase in total energy expenditure, most of which is the result of increased carbohydrate oxidation.⁴ Cold stress also increases muscle glycogen utilization as a result of increased plasma catecholamines.⁵

Therefore, consumption of sufficient amounts of carbohydrate is important when exercising in an environment where cold stress and shivering occur.⁶ Older individuals who have experienced some degree of muscle loss fare less well in cold environments, mainly because their lower level of muscle mass reduces their capacity to produce heat, whether from work or shivering.⁷ However, older age would not automatically increase hypothermic risk if a program of regular exercise were instituted to maintain the lean mass.

Cold-weather exposure creates a significant dehydration risk. Soldiers in cold environments commonly lose up to 8 percent of body weight from dehydration.⁸ There are several reasons for this dehydration, including difficulty obtaining adequate amounts of potable water, high levels of water loss (particularly if excess clothing is worn or heavy equipment is being carried), respiratory water loss, and cold-induced diuresis (CID).

Working at high altitude, by itself, increases nutrition risks because the work takes place in a reduced-oxygen environment. Many experienced skiers and mountain climbers are aware of the potential for nausea, confusion, and easy fatigue with high-altitude work. It takes time to adapt to this relatively hypoxic environment, mainly by improving the capacity to deliver oxygen to working tissues. High-altitude exposure increases oxidative stress, a fact that may alter nutrient requirements in favor of more antioxidant intake.⁹ It is estimated that most humans are 80 percent acclimatized after 10 days at altitude and approximately 95 percent acclimatized by 45 days at altitude.¹⁰ People can expect certain normal changes when going to a higher altitude. These include faster breathing, more shortness of breath than the person is accustomed to, higher urination frequency, and altered sleep patterns. The lower barometric pressure of high altitude lowers the oxygen concentration of every breath, forcing a more frequent breathing pattern in an attempt to pull in the same level of oxygen. However, it is impossible to take in the same level of oxygen at high altitude when compared with sea level, no matter how fast the breathing pattern. For this reason, physical work will always be more difficult, and fatigue will occur more quickly at high altitude. A failure to properly acclimatize to the altitude is commonly referred to as altitude sickness and may result in additional symptoms:¹¹

• Headches
• Vomiting
• Anorexia
• Malaise
• Nausea

Factors that can increase the risk of developing altitude sickness include the following:

• A fast rate of ascent
• High-fat, high-protein, low-carbohydrate diets
• A long stay at altitude
• A high level of exertion

A related disorder, acute mountain sickness (AMS), which commonly occurs at altitudes exceeding 6,600 feet (2,000 meters), produces the following symptoms:^{12, 13}

• Nausea
• Dyspnea on exertion and at rest
• Poor sleep
• Ataxia
• Headache
• Altered mental status
• Lassitude
• Fluid retention (higher antidiuretic hormone production)
• Cough

An assessment of athletes competing in the Primal Quest Expedition Adventure Race in Colorado found that 4.5 percent had altitude illness at the start of the race; 14.1 percent had altitude illness during the race that required medical treatment (of which 13.3 percent was AMS; .8 percent was pulmonary edema); and 14.3 percent withdrew from the race because of altitude-related illness.¹⁴ This race begins at an altitude higher than 9,500 feet (2,900 meters) and rises to an altitude of more than 13,500 feet (4,100 meters). Illness occurring at high altitude should be treated by descent to a lower altitude and by administering oxygen, if available. Individuals with worsening symptoms should never delay descent because worsening symptoms may evolve to high-altitude cerebral edema (HACE) or high-altitude pulmonary edema (HAPE), both of which are life threatening.¹⁵ Capillary leakage in the brain or lungs is the cause of this edema. Symptoms of HACE, which can progress rapidly and result in death within a matter of a few hours, include the following:¹⁶

• Gait ataxia (walks like someone intoxicated)
• Confusion
• Psychiatric changes of varying degree
• Disturbances of consciousness that may progress to deep coma

The cause of HAPE (fluid in the lungs) is not well understood, but it rarely occurs at altitudes below 8,000 feet (2,400 meters). A failure to treat HAPE immediately, typically by immediate descent, may result in death. Symptoms of HAPE result from a lower oxygen-carbon dioxide exchange and include the following:¹⁷

• Extreme fatigue
• Gurgling or rattling breaths
• Breathlessness at rest
• Chest tightness, fullness, or congestion
• Cough, possibly with pink sputum
• Blue or gray lips or fingernails

Obese individuals are more likely to suffer from acute mountain sickness (AMS) than are nonobese individuals.¹⁸ However, people with periodic altitude exposures appear to adapt and reduce the symptoms of AMS.¹⁹ Other strategies, including magnesium supplementation and Ginkgo biloba supplementation, have been tried for reducing AMS, but these have not been found successful.^{20, 21}

The combined impact of AMS symptoms is a severe appetite depression with a concomitant reduction in foods and fluids. The high caloric requirements and fluid consumption difficulties of cold weather, combined with the anorexia of high altitude, create the two most serious problems of work at high altitude: maintenance of weight and fluid balance.

Even those who are part of well-organized mountain-climbing expeditions and regularly exposed to high altitude typically fail to consume sufficient calories, which leads to a loss of body weight. An assessment of people taking part in a Himalayan trek found that body weight was significantly reduced by the end of the trek, and energy intake was significantly lower at high altitude than at low altitude.²² It has been found that food intakes are usually 10 to 50 percent lower at high altitude, depending on the speed of the ascent. This appears to be true even when people are not exposed to severe cold (as in a hypobaric chamber).²³ Only when there is a conscious effort to consume more food, often with forced eating, do individuals at higher altitude have energy intakes that approach physiological needs.²⁴

The level of sweat loss in extremely cold environments can equal that of hot and humid environments. It has been estimated that moderate to heavy exercise in typically insulated winter clothing will result in a sweat loss of nearly 2 liters per hour.²⁵ Therefore, the principle strategy for ensuring adequate hydration is to have enough fluids readily available so they can be consumed frequently and in appropriate quantities. There are real problems, however, in making enough fluids available and ready to drink in cold, high-altitude environments. Fluids can freeze unless there are means of keeping them fluid . . . not an easy task in environments that are often well below freezing temperatures. In addition, fluids are heavy to transport in sufficient quantity to meet needs. One option is to acquire fluids from the local environment by melting and purifying ice and snow, but this option has been found to be extremely costly in heating fuel. It has been estimated that it could take more than 6 hours and a half gallon (2 liters) of gas to melt enough ice and snow to support the fluid needs of a single person.²⁶