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
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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