Nutrition at High Altitude

E. Wayne Askew, PhD

Inappropriate thirst and appetite responses, together with increased insensible water loss, transient diuresis and increased energy expenditures can lead to rapid dehydration and glycogen depletion and weight loss at altitude if adequate food and fluid are neglected. Dehydration may intensify the symptoms of altitude sickness and result in even lower food intakes. One of the most effective and practical performance-sustaining measures that can be adopted upon arrival at high altitude is to consume a minimum of 3 to 4 liters of fluid per day containing 200 to 300 g of carbohydrate in addition to that contained in the diet. This should prevent dehydration, improve energy balance, improve the oxygen delivery capability of the circulatory system, replenish muscle glycogen, and conserve body protein levels.

Altitude Sickness

Abrupt exposure to elevations greater than 10,000 ft (3,050 m) is frequently associated with symptoms of altitude illness. Altitude illness is a combination of symptoms, including headaches, anorexia, nausea, vomiting, and malaise. The combined effect of these symptoms is usually a profound depression of appetite and reduction of food intake, just at the time when the climber needs energy the most. Climbers that anticipate the consequences of altitude-impaired appetite may at least minimize the secondary consequences of the cachexia of altitude: reduced energy intake, depleted muscle glycogen stores, negative nitrogen balances, and loss of critical lean body mass.

Gradual acclimatization to progressively higher altitude exposure is the best preventive medicine for high-altitude sickness. Unfortunately, it is not always practical or possible to delay ascent to altitude. Rescue workers frequently must travel abruptly to high altitudes to perform critical tasks. Prior acclimatization is not always possible. Abrupt transportation from sea level to high altitude may be accompanied by debilitating altitude sickness symptoms, including altered mood, appetite, and performance. These uncomfortable symptoms usually increase in intensity for periods of up to 48 hours after altitude exposure and then gradually lessen. Unfortunately, it is usually during the first 48 hours at altitude that critical work must be accomplished. The strenuous activities associated with work or recreation at altitude, plus an initial increase in resting metabolic rate and the lack of adequate food intakes almost invariably result in an initially negative energy balance. Altitude illness can limit volitional activity, but energy expenditures of experienced and motivated climbers who are acclimatized can be quite high, depending upon the activity level achievable under hypoxic conditions.

Effect of altitude on energy balance

Food intakes are typically reduced 10 to 50% during acute altitude exposure depending upon the individual and rapidity of ascent. Rose et al. (1988) observed depressed food intakes and weight loss at altitude even under the controlled hypobaric chamber conditions of Operation Everest II. In this study, work requirements were relatively low, and a thermoneutral hypobaric environment with an adequate quantity and variety of palatable food were provided. Decreased food intake under these conditions indicated that hypoxia by itself was a major factor reducing appetite and food intake. Adequate food intake can be achieved at altitude but it requires a concerted, conscious effort of dietary management and forced eating (Butterfield 1996). The combination of anorexia and reduced food intake can potentially exert a negative effect on work performance at even moderate altitude (Askew 1996).

Numerous pharmacological attempts to reduce acute mountain sickness have been investigated, with limited success. Caffeine has been reported to enhance relatively short-term, high-intensity work at simulated high altitude, perhaps via an influence upon blood glucose availability. High carbohydrate diets have been recommended by some as a "non-pharmacological" method to reduce the symptoms associated with acute mountain sickness. As an adjunct for lessening or preventing altitude illness, high carbohydrate diets should be fed prior to and during the initial 3 to 4-day critical period of acute altitude exposure. It should be noted that only a limited number of investigators have studied high-carbohydrate diets or carbohydrate supplements for the relief of acute mountain sickness and performance enhancement. Some (Consolazio et al. 1969; Askew 1997), but not all (Swenson et al. 1997), have reported some beneficial effects upon symptoms, mood, and performance. Most investigators agree that, at the very least, energy balances can be improved by aggressive carbohydrate supplementation at altitude, particularly via the fluid component of the diet. In addition to improving energy balance, carbohydrate supplementation also improves nitrogen balance in the initial phase of acute altitude exposure. Butterfield et al. (1992) have confirmed that the negative nitrogen balance encountered at altitude is not due to any hypoxia-related decrease in protein digestibility or absorption, but primarily due to a negative energy balance.

The mechanism by which carbohydrate exerts a beneficial effect on relieving symptoms of altitude sickness and prolongs endurance at altitude may be related to improving blood oxygenation. Hansen et al. (1972) showed that blood oxygen tension is increased by a high-carbohydrate diet and Dramise et al. (1975) reported that carbohydrate can increase lung pulmonary diffusion capacity at altitude. Recently, Lawless et al. (1999) have demonstrated that carbohydrate consumption significantly increased oxygen tension and oxyhemoglobin saturation in arterial blood of subjects during simulated altitude (reduced oxygen in inspired air). In addition to improving blood oxygenation, carbohydrate is a more "efficient" energy source at altitude than fat or protein. The energy production per liter of oxygen uptake is greater when carbohydrate is the energy source compared to fat (carbohydrate, 5.05 kcal/l O2; fat, 4.69 kcal/l O2) regardless of the oxygen tension in the inspired air. Taken together, these different lines of evidence suggest that carbohydrate is a more efficient energy source for work at reduced oxygen tension.

Influence of altitude upon substrate utilization and nutrient requirements

Roberts et al. (1996) suggested that work at altitude in acclimatized individuals may be less reliant upon fat metabolism and hence more strongly influenced by carbohydrate availability. Although McClelland et al. (1998) contend that the relative contribution of carbohydrate does not increase after altitude acclimatization and, like at sea level, the relative intensity of exercise is the major determinant of metabolic fuel utilization at high altitude.

There is little evidence that chronic or acute altitude exposure increases the requirement for any specific nutrient other than possibly vitamin E and iron (Marriott and Carison, 1996). Studies of the effects of cold, energy expenditure, UV light exposure, and the reductive atmosphere at altitude indicates that supplementation of vitamins having an antioxidant function may be desirable at high altitude (Simon-Schnass 1996; Pfeiffer et al. 1999; Chao et al. 1999; Bailey and Davies 2001). Supplemental antioxidant vitamins taken during a prolonged stay at high altitude may prevent a "deterioration" of blood flow and subsequent decrease in physical performance associated with free radical damage to cellular antioxidant defense systems (Askew 1995, Simon-Schnass 1996). Manipulations that improve oxygen delivery to tissues under the conditions of hypoxia are generally beneficial to work performance.

In general, dietary treatments that preserve or enhance the fluidity or deformability of red blood cell (RBC) membranes at altitude are beneficial to oxygen delivery to tissues. Exposure to hypoxia and resultant lipid peroxidation of the unsaturated fatty acids in the red blood cell membrane reduces red cell deformity (ability of RBC to bend or flex as they pass through a capillary bed). The improvement of RBC membrane fluidity (increased ability to deform) can be achieved by 2 dietary mechanisms: supplementing the diet with polyunsaturated fatty acids or by protecting existing membrane polyunsaturated fatty acids from free radical peroxidation by supplementing the diet with antioxidant(s) such as vitamin E.

The suggestion that supplementary dietary iron may be beneficial at altitude stems from the observation that there is an increased erythropoietic response to altitude exposure as the oxygen delivery system of the body attempts to support increased hemoglobin synthesis at high altitude. Normal dietary iron intakes are adequate to support increased hemoglobin synthesis for males at high altitude, but females exposed to high altitude may benefit from a dietary iron supplement. All iron deficient individuals regardless of gender, may benefit from iron supplementation prior to going to altitude. Stray-Gundersen et al.(1992) have demonstrated that iron deficient runners regardless of sex fail to exhibit a normal hematopoietic response upon exposure to altitude. Although Berglund (1992) recommended oral supplement iron (ferrous sulfate, 200-300 mg/d) for 2-3 weeks before ascent and continuation of iron supplementation for 2-4 weeks while at altitude, he cautioned that a simultaneous free radical production might be enhanced by excess free iron.

Fluid requirements at altitude

Water requirements at altitude may be greater than those at sea level, due to the low humidity of the atmosphere at altitude and hyperventilation associated with altitude exposure (Hoyt and Honig 1996, Askew 1996). The risk of dehydration is high at altitude due to diuresis and water loss in breath and sweat, coupled with the difficulty of obtaining adequate water. An inappropriate thirst response coupled with an increase in insensible water loss and a transient diuresis during the initial hours of altitude exposure, can result in rapid dehydration if adequate fluid is either unavailable or neglected. The rate of respiratory water loss at altitude is about twice the rate of respiratory water loss for an equivalent activity at sea level (Milledge 1992).

Hypoxia vs. cold

High altitude and cold environments are often similar with respect to the thermal challenge, tempting one to categorize work in the cold at sea level with work under similar cold conditions at altitude. There are some distinct differences, however, which should be considered when planning nutritional support at high altitude. Fat, while tolerated relatively well in the cold at sea level, may not be as well tolerated in diets at high altitude. The symptoms of acute altitude exposure may be exacerbated if fat displaces carbohydrate from the diet. Although high-fat foods are energy dense and reduce the weight/calorie aspect of food carried on climbs, fat requires more oxygen for metabolism than carbohydrate and will place a small, but added, burden upon the already overtaxed oxygen economy of the climber. Fat absorption may also be reduced at extremely high elevations. However, elevations commonly reached by recreational skiers, snowshoers, and backpackers are usually not associated with impaired fat or protein or carbohydrate absorption (Butterfield 1992).

Another difference between cold exposure at sea level and high altitude is the calorigenic response to cold (Giesbrecht et al. 1994). Cold exposure during hypoxia results in an increased reliance upon shivering for thermogenesis due to a reduction in non-shivering thermogenesis at altitude. Perhaps this is due to a reduction in aerobic catabolism of free fatty acids during hypoxia or to an alteration in the neural-hormonal axis thermogenic response.

Wayne is professor of Foods and Nutrition at the University of Utah, Salt Lake City, UT, USA.

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