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.
References
Askew, E.W., Environmental
and physical stress and nutrient requirements, Am. J.
Clin. Nutr., 61:631S-637S, 1995
Askew, E.W. Cold weather and
high altitude nutrition: overview of the issues. In:
Nutritional Needs in Cold and in High-Altitude Environments,
B.M. Marriott, and S. J. Carlson (Eds.) National Academy
Press, Washington, DC, 1996, pp 83-93
Askew, E.W. Nutrition and performance
in hot, cold and high altitude environments, In: Nutrition
in Exercise and Sport. 3rd edition. I. Wolinsky
(Ed.), CRC Press, Boca Raton, FL, 1997, pp 597-619
Askew, E.W. Water. In: Present
Knowledge in Nutrition, 7th edition. E.E. Ziegler and
L.J. Filer, Jr.,(Eds) International Life Sciences Institute
Press, Washington, D.C. , 1996 pp 98-108
Berglund, B., High-altitude
Training. Aspects of haematological adaptation, Sports
Med., 14, 289, 1992.
Butterfield, G. E., Gates,
J., Fleming, S., Brooks, G. A., Sutton, I. R., and Reeves,
J. T., Increased energy intake minimizes weight loss in
men at high altitude, J. Appl. Physiol., 72,
1741, 1992.
Butterfield, G.E., Maintenance
of body weight at altitude: in search of 500 kcal/day, Nutritional
Needs in Cold and in High-Altitude Environments, B.M.
Marriott and S. J. Carlson, (Eds)., National Academy Press,
Washington, D.C., 1996, 357.
Chao, W., E.W. Askew, D.E.
Roberts, S.M. Wood, J.B. Perkins. Oxidative stress in humans
during work at moderate altitude. J. Nutr. 129, 2009-2012,1999
Consolazio, C. F., Matoush,
L. O., Johnson, H. L., Krzywicki, H. J., Daws, T. A., and
Isaac, G. J., Effects of high-carbohydrate diets on performance
and clinical symptomatology after rapid ascent to high altitude,
Fed. Proc., 28, 937, 1969.
Dramise, J. G., Inouye, C.
M., Christensen, B. M., Fults, R. D., Canham, J. E., and
Consolazio, C. F., Effects of a glucose meal on human pulmonary
function at 1600 m and 4300 m altitudes, Aviat. Space
Environ. Med., 46, 365, 1975
Giesbrecht, G.G., Fewell, J.E.,
Megirian, D., Brant, R. And Remmers, J.E., Hypoxia similarly
impairs metabolic responses to cutaneous and core cold stimuli
in conscious rats, J. Appl. Physiol., 77, 726, 1994.
Hansen, J. E., Hartley, L.
H., and Hogan, R. P., Arterial oxygen increase by high-carbohydrate
diet at altitude, J. Appl. Physiol., 33, 441-445,
1972.
Hoyt, R.W. and Honig, A., Body
fluid and energy metabolism at high altitude, Handbook
of Physiology, Section 4: Environmental Physiology,
C.M. Blatteis, and M.J Frealy, (Eds.), Oxford
University Press, New York, 1996, 1277.
Lawless, N.P. , T.A. Dillard,
K.G. Torrington, H.Q. Davis, G. Kamimori. Improvement in
hypoxemia at 4600 meters simulated altitude with carbohydrate
ingestion. Aviat. Space Environ. Med.,70, 874-878,
1999.
Marriott, B.M. and S. J. Carlson
(Eds) Nutritional Needs in Cold and High Altitude Environments.
National Academy Press, Washington, DC, 1996
McClelland,G.B., P.W. Hochachka,
and J.M.Weber. Carbohydrate utilization during exercise
after high altitude acclimation: A new perspective. Proc
Nat Acad Sci 95: 10288, 1998
Milledge, J., Respiratory water
loss at altitude, Newsletter Int. Soc. Mountain Med.,
2 (No. 3), 5, 1992.
Pfeiffer, J.M., E.W. Askew,
D.E. Roberts, S.M. Wood, J.E. Benson, S.C. Johnson, M.S.
Freedman. Effect of antioxidant supplementation on urine
and blood markers of oxidative stress during extended moderate
altitude training, Wilderness and Environ. Med., 10,
66-74, 1999.
Roberts, A. C., G. E. Butterfield,
A. Cymerman, J. T. Reeves, E. E. Wolfel, and G. A. Brooks
Acclimatization to 4,300-m
altitude decreases reliance on fat as a substrate J Appl
Physiol 81: 1762-1771, 1996.
Rose, M. S., Houston, C. S.,
Fulco, C. S., Coates, G., Sutton, J. R., and Cymerman,
A., Operation Everest. II Nutrition and body composition,
J. Appl. Physiol., 65, 2545, 1988.
Simon-Schnass, I., Oxidative
stress at high altitudes and effects of vitamin E, Nutritional
Needs in Cold and in High-Altitude Environments, B.M.
Marriott and S.J Carlson, (Eds.), National Academy Press,
Washington, D.C., 1996, 393.
Stray-Gundersen, J., Alexander,
C., Hochstein, A., deLomos, D. And Levine, B.D., Failure
of red cell volume to increase to altitude exposure in iron
deficient runners, Med. Sci. Sports Exer., 24, S90,
1992.
Swenson, E.R., A. MacDonald,
M. Vatheuer, C. Maks, A. Treadwell, R. Allen, and R. Schoene.
Acute mountain sickness is not altered by a high carbohydrate
diet nor associated with elevated circulating cytokines.
Aviat. Space Environ. Med., 68, 499-503, 1997.
