S129
Nutrition and High Altitude Exposure B. Kayser
Abstract Abstract
B. B.
Kayser, Nutrition and High Altitude Ex-
posure. mt mtJj Sports posure. Med, Vol Vol 13, 13, Suppl Suppl 1, pp S129—S132, Sports Med, 1992.
Altitude exposure leads to considerable weight loss. The different hypotheses that have been put forward to explain this phenomenon are discussed reviewing the literature: 1) a primary decrease of food intake due to loss of appetite caused, directly or indirectly, by hypoxia, changes of menus, comfort and habits, 2) a discrepancy be-
tween energy intake and energy expenditure due to an increased basal metabolic rate and/or high levels of activity which are not matched by an increased food intake, 3) a loss of body water due to increased insensible loss through increaed ventilation in the mountain environment, decreased liquid intake, and/or changes in water metabolism, 4) an impaired absorption of nutrients from the gastrointestinal
intake due to loss of appetite caused, directly or indirectly, by hypoxia, changes of menus, menus, comfort comfort and and habits habits (1, (1, 2, 2, 3, 3,24), 24), 2) a discrepancy between energy intake and energy expenditure due to an increased basal metabolic rate and/or high levels of activity which are not matched by an increased food intake (2, 3, 27), 3) a decrease in body water due to increased insensible loss through increased ventilation, decreased liquid intake, or changes in water metabolism (1, 3, 10, 16), 4) an impaired absorption of nutrients from the gastrointestinal tract (1, 2, 17, 21) and 5) loss of muscle mass due to lack of physical exercise and/or direct effects of hypoxia on protein synthesis (15, 24, 25).
The purpose of this article article is is to to discuss discuss briefly briefly the above hypotheses. Anorexia
ercise and/or direct effects of hypoxia on protein synthesis. It is concluded that altitude weight loss is due to an initial
During acclimatization, food intake is generally reduced (2, 8). Moreover, Moreover, ifif altitude altitude exposure exposure isis sudden sudden and leads to symptoms of acute mountain sickness, food intake may be remarkably low during several days (8). During climbing activities food intake tends to remain low, with oc-
loss of water and subsequently to loss of fat mass and
casional increases during resting days in base-camp (1, 7, 27).
muscle wasting. Up to altitudes altitudes around around 5000 5000 m m the the weight weight loss from fat and muscle seems to be largely avoidable by maintaining adequate intake in a comfortable setting. Primary anorexia, lack of comfort and palatable food, detraining, and possible direct effects of hypoxia on protein me-
It has not been established how far discomfort and lack of palatable food play a role in the decreased food intake at high altitude. To gain insight into this problem, body
tract, and 5) a loss of muscle mass due to lack of physical ex-
tabolism seem to inevitably lead to weight loss during longer exposures at higher altitudes. In order to minimize losses losses it is advisable to acclimatize properly, to reduce reduce the the length length of stay at extreme altitude as much as possible possible and and to to maintain a high and varied nutrient intake. words Key words
Altitude, Altitude, nutrition, weight loss, metabolism, climbing
Introduction Altitude exposure leads to considerable weight Altitude loss in lowlanders, a fact usually considered an inevitable consequence of chronic hypobaric hypobaric hypoxia hypoxia (1, (1,2, 2, 3, 7, 8). This phenomenon has been attributed to: 1) a primary decrease of food ______ Int.J.SportsMed. l3(1992)S129—5132 13(1992)S129—S132 GeorgThieme Verlag New York GeorgThieme VerlagStuttgart StuttgartNewYork
weight, skinfold thickness, limb circumferences and alactic
anaerobic performance of several limb muscles were measured in 8 healthy male Caucasians, before and during a I1
month stay at the Italian Research Laboratory in Nepal at 5050 m (19). This facility is fitted with all equipment necessary for a comfortable stay at altitude. In addition a wide choice of palatable food items was available throughout the sojourn. As
shown in Table 1, no significant changes were observed in either anthropometric measurements or maximum alactic anaerobic performance (19). This suggests that a rich choice of palatable food items in a comfortable setting plays a determinant role in the maintenance of body weight and functional performance of healthy Caucasians during a 1 month stay at 5050 m. During climbing at higher altitudes, it seems that a combination of discomfort, lack of palatable food and primary anorexia will lead to an important negative energy balance (1,27). (1,27). As far as micro nutrients are concerned, some evidence exists for a relative lack of Fe++ in women (8) and for a reduced intake of some vitamins (8). Probably the effects of such temporary deficiencies are small and can largely be overcome through body stores and/or supplementation.
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Dept. de Physiologie, CMU, 1211 Genève 4, Switzerland
S130 mt. J. Sports Med. 13 13(1992) (1992)
B. Kayser
Table 1 Anthropometric Anthropometric and and muscle muscle functional functionaldata data(means±SD) (means SD) from 8 healthy healthy male male Caucasians Caucasians (33.7± (33.7 4.64.6 yr yr SD, height 1.80± SD, height 1.800.08 0.08m) m) before and during a one month stay at 5050 m. The subjects had ad libitum access to a wide choice of palatable food and lived ived in ri relatively comfortable quarters. AM+B = arm muscle plus bone area (from skinfolds and circumferences), VM+B = muscle plus bone volume of the lower limb (from skinfolds skinfolds and and circumferences), circumferences), MVC MVC==isometric isometricmaximum maximumvoluntary voluntarycontraction contractionstrength strengthofofthe thebiceps bicepsbrachii, brachi,AH AH==maximum maximum vertical jumping height. There were no significant differences (from reference 19).
Body fat
AM+B AM+B
VM+B
MVC MVC
AH
(0/c)
(cm2)
(I)
(N) (N)
(cm)
76.4± 19.0 76.4±19.0
16.9±4.1 16.4±4.8
70.3± 8.2
19.2
70.9± 70.9± 15.5
7.84 7.84 1.41 7.49± 1.42
382.9±67.7 396.3±64.6
32.77±5.33 35.21 3521
73.7 18.5
16.4 4.5
69.7 10.9
7.37 1.25
389.4 62.2
36.28 5.64 5.64
73.8±18.1
16.6±4.4
69.1
5.55± 1.34 1.34
394.4±83.3 394.4± 83.3
35.61
76.1
9.3
15—
Table 2 Average daily daily metabolic metabolic rate rate (ADMR), (ADMR),energy energyintake ntake (El), (El),
A 12
9Mj/day Mi/day
'V
63-
5.71
ra
ADMR Energy intake
weight weight loss loss (ABM) (ABM) and andfat fatmass massloss loss(z±FM) (AFM) of 5 subjects during a climb of Mt. Everest. ADMR was measured over 7—10 days with the doubly labelled water technique and El was monitored with food diaries. Before and after the observation interval body mass (BM) was measured with a balance and fat mass (FM) was calculated from skinfolds and BM. Three of the subjects summited within two days after the observation interval. Subject 5 summited for the second time without supplementary oxygen, and the summit was included in the observation interval (adapted from reference 27).
0-
Fig. 1 Average daily metabolic rate (ADMR) and energy intake (El) of 5 subjects during a climb of Mt. Everest. ADMR was measured over 7—10 days with the doubly labelled water technique and El was monitored with food diaries. The energy deficit acquired during the climb of Mt. Everest, illustrated by the difference between ADMR and and El, El, was was paid paid dor dor with with energy energy frmm frmm body body fat fat and and lean lean mass. mass. E.Eq.ABW = energy equivalent of fat and lean mass lost during the climb. climb.
Subject 1
2 3 4 5 mean mean
ADMR ADMA (MJ/day)
El (MJ/day)
L\BM (kg)
12.3 14.9
8.2 9.7
11.7 15.6 13.5
5.8 6.9 6.9
—2.5 —4.0 —1.0 —3.0 —2.5
— 1.0 —1.0
13.6
7.5
—2.6
—1.4
7.0
AFM (kg) —
-1.3 -1.7 -1.7 —2.4
SD
Energy Intake and Expenditure It seems difficult to maintain energy balance above 4500 m (1, 2, 3, 7, 8, 27). It is not clear whether this discrepancy is mainly the result of a lowered energy intake or of
an increased energy expenditure as well. Different reports have indicated a possible increase in resting metbolic rate (RMR) at high altitude (3, 8), which may lead to an increase in average daily metabolic rate (ADMR). If such an increase is
not matched by an adequate rise in energy intake the energy balance would turn negative. In fact, artificially matching the intake to the expenditure has proven to be successful in minimizing weight loss at high altitude (2). Recently the energy expenditure and intake of 5 subjects was studied while climbing
Mt. Everest, using the doubly labelled water technique (27). The activity of the subjects, as reflected by the ADMR/RMR ratio was was high high(2.4 (2.4 0.1, 0.1,see seeTable Table2) 2) and and comparable comparable to that of oxygen consumpconsumpof endurance athletes at sea level. Estimated oxygen
tion during climbing climbing activities activitiesreached reached30 30mlmlmin1 mm1 kg kg which amounts to 50—60% of sea level VO2max as measured in
elite climbers. This is probably near the maximum aerobic power of the subjects at altitude and is compatible with the subjects perception of intense exertion while climbing. By con1 .5 MJ/day, MJ/day, trast, energy intake was surprisingly low (7.5 1.5
see Table 2). The loss of fat mass was related to this discrepancy and the energy balance equation fitted (expenditure = intake + bodystores consumed, see Figure 1). intake As far as the partition of energy in the diet is concerned, a spontaneous shift towards a diet rich in carbohy-
drates has been observed several times (7, 8). This could be favourable as it increases the metabolic respiratory quotient and and hence the energy yield per mole of oxygen consumed consumed and and raises PAO2 (26).
Water Changes in water metabolism during altitude exposure can interfere with measurements of energy balance
and body weight changes (6). During acclimatization, a decrease in intra- and extra-cellular water occurs, as well as a decrease in circulating plasma volume. These changes result in a weight loss of 1—2 1—2kg kg(l, (1,2,3, 2,3, 8, 12, 16). By contrast, a temporary rary increase increase of of body body water water may may be be observed observed during during the the initial initial stage at altitude if there are symptoms of acute mountain sickness (8). It has been estimated that during climbing water loss would be very high due to the inhalation of dry air and to hyperventilation, imposing water intakes up to 7 liters/day (21). This figure seems too high when using the formula proposed by Ferrus et al. (5). In fact, an increase in ventilatory rate from 13 to 26 breaths/mm, a 50% decrease in air density, a drop in
air temperature from 20 to 0 °C and a drop in partial water pressure increase respirarespirapressure in the air from 8.5 to 0 Torr, would increase tory water loss only from 288 to 410 ml/day. mI/day. A group of mountaineers recently observed during a climb of Mt. Everest lost 2.6 to 4.0 1/day I/day and ingested 1.8 to 2.9 I/day 1/day (27). (27). However, However, at the the high metabolic rates observed (see Table 1), the the production production of of metabolic water was —. 1.01/day and as the subjects also lost body mass which contains water, the water balance fitted.
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Before 1st week 2nd week 4th week
Weight (kg)
Intl ISportsMed. mt. Sports Med.13 13(1992) (1992) S131 S131
Nutrition andHigh Altitude Exposure
Some observations during a Mt. Everest expedition dition (1), (1), and and the the reported reported relationship relationship between between aa decrease decrease in in xylose absorption from the gut in sea level hypoxic patients (1, 17) have led to the hypothesis of malabsorption of nutrients from from the gastrointestinal tract at high altitude. FatFat- and and carbocarbo-
hydrate-digestibility at altitude are apparently normal up to altitudes of 5000 to 5500 m (11, 14, 22), whereas some evidence dence exists exists of of fatfat- and and carbohydrate-malabsorption carbohydrate-malabsorption at at 6300 6300 m m (1). Protein- and energy-digestibility in 6 Caucasians at 5000 m
were unchanged when compared to sea level (15). On the whole, it seems unlikely that malabsorption plays an important role at high altitude although at extreme altitudes (above 7000 m) the gut may lose some of its absorptive capacity due to very low Pa02.
imize imize losses, it is advisable to acclimatize properly, properly, to to reduce reduce the length of stay at extreme altitude as much as possible and to keep the nutrient intake high and varied. References Boyer S. J., Blume F. F. D.: D.: Weight Weight loss loss and and changes changesin in body body compocompohigh altitude. altitude. JApplPhysio/57: JApp/Physio/57: 1580—1585, sition at athigh 1580—1585,1984. 1984. - Butterfield G. G. E.: E.: Elements Elements of of energy energy balance balance at at altitude, altitude, in in Sutton Sutton J. R., Coates G., Remmers J. E. (eds): Hypoxia, the adaptations. Burlington, B.C. Decker Decker mc, mc, 1990, 1990, pp pp 88—93. 88—93. Consolazio, C. F., Matoush L. 0., Johnson H. L., Daws T. A.: Protein and water balance balance of of young young adults adults during during prolonged prolongedexposure exposure 1: 154—161, to high altitude altitude (4300 (4300 m). m).AmJC/in AmJCIinNutr2 Nutr2l: 154—161, 1968. 1968. 'I Ferretti Ferretti G., G.,Hauser HauserH., H.,di diPrampero Prampero P. P. E.: E.: Maximal Maximal muscular muscular ml J Sports power before and after exposure to chronic hypoxia. mt Med (suppl I): S31—S34, S31 —S34, 1990. Medl11 I (suppi Ferrus L., Commenges D., Gire J., Varène Varéne P.: Respiratory water
'
loss as a function of ventilatory or environmental factors. Resp
Protein Metabolism 6
A sizeable fraction of the altitude altitude related related weight loss is due to a reduction in muscle mass (1, 4, 24). In elite climbers this loss may decrease the thigh cross sectional area by 17% (4, 24). Recently the question whether muscle loss at altitude could be at least partly a result of detraining was addressed. It may be hypothesised that climbers, in good training conditions before leaving for a mountaineering expedition, undergo a loss of muscle mass at altitude due to a relative lack of exercise which may lead to muscle hypotrophy (24). On the other hand, there are some indicators that hypoxia "per se" may influence influetice amino acid metabolism. For example, acute byhypobaric hypoxia decreased the turnover and uptake of leucine from from the the forearm forearm muscle muscle compartment compartment (23). (23). In In this this respect, respect, itit is noteworthy that also normobaric hypoxia, as observed in chronic obstructive pulmonary disease patients, is often accompanied by marked muscle loss due to similar changes in protein metabolism (18). In addition, supplementation with
8
Physlol56: 11—20,1984. PIzyslolS6: 11—20,1984. Fulco C. S., Cymerman A., Pimental N. A., Young A. J., Maher J. Fulco vial Space Space Environ Environ T.: Anthropometric changes at high altitude. A riot Med56: 220—224, 1985. Med56:220—224,
Guilland J. C., Klepping Klepping J.: J.: Nutritional Nutritional alterations alterationsatathigh highaltitude altitude 1985. 517—523,1985. inman.EurfApp/Physiol54: inman.EurfApplPhysiol54:517—523, Horvath S. S. M., M.,Yousef Yousef at high Hannon J. J. P.: Nutrition Nutrition at Hannon high altitude, altitude, in in Horvath Physiology: Aging, Aging, Heat Heat and AltiM. K. M. K. (eds): (eds): Environmental Environmental Physiology: tude. Amsterdam, Amsterdam, Elsevier Elsevier Noord Noord Holland Holland Inc., Inc., 1980, tude. pp309— 309— 1980, pp 327. 327.
HoppelerH., Hoppeler H., Kleinert Kleinert E., E., Schlegel Schlegel C., C., Claasen Claasen H., H., Howald Howald H., H., of human human Kayar Morphological CerretelliP.: P.: Morphological adaptations adaptations of Kayar S. R., Cerretelli chronic hypoxia. hypoxia. fin mt Jj Sports Med skeletal muscle to chronic Med 11 II (suppl (suppl 1): I):
°
S3 —S9, 1990. 1990. 53—59,
° Hoyt W.,Durkot DurkotM. M.J.,J.,Kamimori KamimoriG. G.H., H.,Schoeller SchoellerD. D. A., A., CyCyHoytR.R.W., decreases intramerman A.: A.: Chronic altitude exposure Chronic altitude exposure (4300 (4300 m) m) decreases cellular and total body water in humans, in Sutton J. R., Coates G., and Houston C. (eds): Hypoxia and moutain medicine. Pergamon Oxford, 1992, 1992,p41. Oxford, p41. '' Press, Imray C. H. E., Chesner I., Wright A., A., Neoptolemeus Neoptolemeus J.J. P., P., BraBradwell A. R.: Fat absorption absorption at at altitude, altitude, aa reappraisal. reappraisal.mt mtJj Sports Sports Med 13: 13:87, 87, 1992. 1992. Body S. C.,Bardhan Bardhan J., J., Swamy Swamy Y. Y. V., V., Krishna B., Nayan N. 5.: Body Jam S.C.,
branched chain amino acids in high altitude trekkers could
12
prevent muscle loss (25). With regard to tissue and animal experiments acute hypoxia decreass protein synthesis in vitro as well as in vivo (20). An unbalance between protein anabolism and catabolism, which is influenced by circulating hormones, could also take place. The concentrations of several hormones
acutealtitude altitudeexposure. exposure. fluid compartments in humans during acute humans during AviatSpaceEnvironMed5 I: 234—236, 1980. 1980. AviatSpaceEnvironMed5l:234236, ° Kayser KayserB., B.,Hoppeler HoppelerH., H.,Claassen ClaassenH., H.,Cerretelli Cerretelli P.: P.: Muscle Muscle struc-
are known to change as a result of exposure to acute and chronic hypoxia. Decreased insulin and increased cortisol,
energy assimilation at 5000 m. In: Sutton J. R., Coates G., and Houston C. (eds): Hypoxia and mountain medicine. Pergamon
catecholamine, growth hormone, thyroid hormone and prostaglandin concentrations, all modulating protein metabolism have been described (see 26 for review). In biopsies of the vastus lateralis muscle of high altitude climbers, the observed muscle loss seemed to result from a decrease in muscle fiber size (9). Small fiber size was also reported in Sherpas (13). As Sherpas do not seem to lose lose weight weight while while climbing climbing (1), (I), it was couid be a useful adapspeculated that a decrease in fiber size could tation to altitude as it decreases the diffusion distance for oxygen from the capillary to to the the mitochondrion mitochondrion(9, (9, 13). In conclusion, altitude ahitude exposure in Caucasians leads to considerable weight loss due to an initial loss of water and subsequently to loss of of fat fat and and muscle muscle tissue. tissue. Up Up to toaltiati-
4
IS IS
6
Press, Oxford, Oxford, 1992 1992(in (in press). press). Kayser B., Acheson K., Décombaz J., Fern E., Cerretelli P.: Protein absorption and energy digestibility at high altitude. JAppiPhysiol, 1992 (in (in press). press). H. J., J., Consolazio Consolazio C. C. F., F., Johnson Johnson H. H. L., L., Nielsen Nielsen W. W. C., C., Krzywicki H.
Barnhart R. A.: Water metabolism in humans during acute high 17
' 20
tudes around 5000 m the loss of fat and muscle may be avoidable by increasing food intake. Primary anorexia, discomfort and lack of palatable food, detraining and possible direct effects of hypoxia on protein metabolism seem to inevitably lead to weight loss at higher altitudes. In order to mm-
ture ture and and performance performance capacity capacityof ofHimalayan Himalayan Sherpas. Sherpas. JAppiPhysJAppiPhysio/70: 1938—1942,1991. iol7O: Kayser Kayser B., Acheson K., Bernini A., Fern E., Cerretelli P.: Dietary
21
altitude exposure (4300 809, 1971. (4300 m). m). JAppiPhysiol JAppiPhysiol 30: 30:806— 806—809, Milledge J. S.: Arterial oxygen desaturation and intestinal absorption ofxylose. BritMedJ3: 557—558, 1972.
Morrison W. L., Gibson J. N. A., Scrimgeour C., Rennie M. J.: C/in Set Sd 75:41 5—420, 1988. Muscle wasting in emphysema. Clin 75:415—420,
Narici M., Kayser B., Cibella F., Grassi B., Cerretelli P.: No changes in body composition and maximum alactic anaerobic performance during a 4 week sojourn at altitude. mt IntJSports Sports Med Med 13: 13: 87, 1992. Preedy V. S., Smith D. M., Sugden P. H.: The effects of 6 hours hypoxia on protein synthesis synthesis in in rat rat tissues tissues in in vivo vivoand andin invitro, vitro.Bloc/i BiochJJ 22&179—185,1985. 228:179—185,1985. Pugh L G. C.: Physiological and medical aspects of a Himalayan
scientific and mountaineering mountaineering expedition. expedition. Brit Brit Med MedJJ 2: 621 —637, 1962.
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Digestibility
(1992) S132 mt. J. Sports Med. 13 (1992)
28:242—245,1975.
23 Rennie M. J., Babij P., Sutton J. R., Tonkins J. J., Read W. W., Ford
C., Halliday D.: Effects of acute hypoxia on forearm leucine metabolism.ProgClinBiolRes 136:317—323,1983. tabolism.ProgClinBiolRes 136: 317—323,1983. 24 Rose M. S., Houston C. S., Fulco C. S., Coates G., Sutton J. R., Cymerman A.: Operation Everest II: Nutrition and body composition.JApplPhysiol65:2545—2551, 1988. tion.JApplPhysiol6S:2545—2551, 1988. 25 25 Schena F., Guerrini F,, F., Trenaghi P., Kayser B.: Branched-chain amino acid supplementation during trekking at high altitude: the effects on weight loss, body composition, and muscular power. EurJApp/Physiol EurJApplPhysiol 1992 (in press).
26 Ward M. P., Milledge J. S., West J. B.: High altitude medicine and physiology. Chapman and Hall Medical, London, 1989. 27 physiology. Chapman and Hall Medical, London, 1989. 27 K., Kayser Westerterp K., Kayser B., B., Brouns Brouns F., F.,Herry Herry J.-P., Saris W.: Energy Westerterp expenditure climbing climbing Mt. Mt. Everest. Everest. JAppiPhysiol, JAppiPhysiol, 1992 1992 (in (in press). press). expenditure
Bengt Kayser Kayser Bengt Dept. de de Physiologie Physiologic Dept.
CMU CMU
CH-l2ll Genève4 CH-1211 Genève4 Switzerland Switzerland
Fluid Balance and Exercise R. J. Maughan Department of Environmental and Occupational Medicine, University Medical School, Foresterhill, Aberdeen AB9 2ZD, Scotland
Introduction Abstract
R. .J. Maughan, Fluid Balance and Exercise. IntJ Sports Med, Vol 13, SuppIl, pp S132—S135, 1992.
The rate of metabolic heat production during prolonged exercise maybe increased to 15—20 times that at rest. Evaporation of sweat secreted onto the skin can effectively limit the rise in body temperature which would otherwise occur, but results in the loss of water and electrolytes from the body. Dehydration and an increased thermal load
can accelerate the onset of fatigue during exercise. The available evidence supports the idea that ingestion of fluids
during prolonged exercise can improve performance. Heart rate and rectal temperature will generally be lower, and plasma volume will be better maintained when fluids are given. There is, however, no general agreement on the optimum formulation nor on the frequency or volume of drinking that is mosv appropriate. In practice, the ideal solution will depend on a number of factors, including the duration and intensity of the exercise, the environmental conditions and the characteristics of the individual. The varia-
tion between individuals is, however, large and the optimum strategy can only be established by subjective experience. Key words
Sweating, fluid balance, dehydration, fatigue Int.J.SportsMed. 13(1992)5132—S135 Georg Thieme Verlag StuttgartNew York
Many factors influence the ability to perform exercise, and among these are the environmental conditions under which the exercise is performed. When the ambient temperature and humidity are high, the capacity to perform prolonged exercise is reduced: in this situation, dehydration and tilermoregulatory problems rather than depletion of energy stores or any other factor may be the cause of fatigue.
Fluid loss during exercise is linked to the need to maintain body temperature within narrow limits. During exercise, the rate of heat production can be increased to many times the resting level. A 70 kg runner completing a marathon in 2 h 30 mm requires an average oxygen consumption of about 4 1/mm (10): heat production will be about 1000 W. Similar high levels of energy expenditure are sustained in cycling and cross country skiing. In spite of this, body temperature seldom rises by more than 2—3 °C, indicating that heat is lost from the body almost as fast as it is being produced.
At high ambient temperatures, heat loss occurs only by evaporation. For the 2 h 30 mm marathon runner with a body weight of 70 kg to balance his heat production by evap-
orative loss alone requires sweat to be evaporated from the skin at about 1.6 I/h: at such high sweat rates, an appreciable fraction drips from the skin without evaporating, and a sweat secretion rate of about 2 1/h is likely to be necessary to achieve this rate of evaporative heat loss. This is possible, but would result in the loss of 51 of body water. Water (and heat) will also be
lost by evaporation from the respiratory tract. During prolonged hard exercise in a hot dry environment, these losses may be significant even in the absence of sweating.
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22 Rai R. M., Malhotra Maihotra M. S., Dimri G. P., Sampathkumar T.: Utilization of different quantities of fat at high altitude. Am J Clin Nuir
B. Kayser
Nutrition and High Altitude Exposure B. Kayser
Abstract Abstract
B. B.
Kayser, Nutrition and High Altitude Ex-
posure. mt mtJj Sports posure. Med, Vol Vol 13, 13, Suppl Suppl 1, pp S129—S132, Sports Med, 1992.
Altitude exposure leads to considerable weight loss. The different hypotheses that have been put forward to explain this phenomenon are discussed reviewing the literature: 1) a primary decrease of food intake due to loss of appetite caused, directly or indirectly, by hypoxia, changes of menus, comfort and habits, 2) a discrepancy be-
tween energy intake and energy expenditure due to an increased basal metabolic rate and/or high levels of activity which are not matched by an increased food intake, 3) a loss of body water due to increased insensible loss through increaed ventilation in the mountain environment, decreased liquid intake, and/or changes in water metabolism, 4) an impaired absorption of nutrients from the gastrointestinal
intake due to loss of appetite caused, directly or indirectly, by hypoxia, changes of menus, menus, comfort comfort and and habits habits (1, (1, 2, 2, 3, 3,24), 24), 2) a discrepancy between energy intake and energy expenditure due to an increased basal metabolic rate and/or high levels of activity which are not matched by an increased food intake (2, 3, 27), 3) a decrease in body water due to increased insensible loss through increased ventilation, decreased liquid intake, or changes in water metabolism (1, 3, 10, 16), 4) an impaired absorption of nutrients from the gastrointestinal tract (1, 2, 17, 21) and 5) loss of muscle mass due to lack of physical exercise and/or direct effects of hypoxia on protein synthesis (15, 24, 25).
The purpose of this article article is is to to discuss discuss briefly briefly the above hypotheses. Anorexia
ercise and/or direct effects of hypoxia on protein synthesis. It is concluded that altitude weight loss is due to an initial
During acclimatization, food intake is generally reduced (2, 8). Moreover, Moreover, ifif altitude altitude exposure exposure isis sudden sudden and leads to symptoms of acute mountain sickness, food intake may be remarkably low during several days (8). During climbing activities food intake tends to remain low, with oc-
loss of water and subsequently to loss of fat mass and
casional increases during resting days in base-camp (1, 7, 27).
muscle wasting. Up to altitudes altitudes around around 5000 5000 m m the the weight weight loss from fat and muscle seems to be largely avoidable by maintaining adequate intake in a comfortable setting. Primary anorexia, lack of comfort and palatable food, detraining, and possible direct effects of hypoxia on protein me-
It has not been established how far discomfort and lack of palatable food play a role in the decreased food intake at high altitude. To gain insight into this problem, body
tract, and 5) a loss of muscle mass due to lack of physical ex-
tabolism seem to inevitably lead to weight loss during longer exposures at higher altitudes. In order to minimize losses losses it is advisable to acclimatize properly, to reduce reduce the the length length of stay at extreme altitude as much as possible possible and and to to maintain a high and varied nutrient intake. words Key words
Altitude, Altitude, nutrition, weight loss, metabolism, climbing
Introduction Altitude exposure leads to considerable weight Altitude loss in lowlanders, a fact usually considered an inevitable consequence of chronic hypobaric hypobaric hypoxia hypoxia (1, (1,2, 2, 3, 7, 8). This phenomenon has been attributed to: 1) a primary decrease of food ______ Int.J.SportsMed. l3(1992)S129—5132 13(1992)S129—S132 GeorgThieme Verlag New York GeorgThieme VerlagStuttgart StuttgartNewYork
weight, skinfold thickness, limb circumferences and alactic
anaerobic performance of several limb muscles were measured in 8 healthy male Caucasians, before and during a I1
month stay at the Italian Research Laboratory in Nepal at 5050 m (19). This facility is fitted with all equipment necessary for a comfortable stay at altitude. In addition a wide choice of palatable food items was available throughout the sojourn. As
shown in Table 1, no significant changes were observed in either anthropometric measurements or maximum alactic anaerobic performance (19). This suggests that a rich choice of palatable food items in a comfortable setting plays a determinant role in the maintenance of body weight and functional performance of healthy Caucasians during a 1 month stay at 5050 m. During climbing at higher altitudes, it seems that a combination of discomfort, lack of palatable food and primary anorexia will lead to an important negative energy balance (1,27). (1,27). As far as micro nutrients are concerned, some evidence exists for a relative lack of Fe++ in women (8) and for a reduced intake of some vitamins (8). Probably the effects of such temporary deficiencies are small and can largely be overcome through body stores and/or supplementation.
Downloaded by: University of British Columbia. Copyrighted material.
Dept. de Physiologie, CMU, 1211 Genève 4, Switzerland
S130 mt. J. Sports Med. 13 13(1992) (1992)
B. Kayser
Table 1 Anthropometric Anthropometric and and muscle muscle functional functionaldata data(means±SD) (means SD) from 8 healthy healthy male male Caucasians Caucasians (33.7± (33.7 4.64.6 yr yr SD, height 1.80± SD, height 1.800.08 0.08m) m) before and during a one month stay at 5050 m. The subjects had ad libitum access to a wide choice of palatable food and lived ived in ri relatively comfortable quarters. AM+B = arm muscle plus bone area (from skinfolds and circumferences), VM+B = muscle plus bone volume of the lower limb (from skinfolds skinfolds and and circumferences), circumferences), MVC MVC==isometric isometricmaximum maximumvoluntary voluntarycontraction contractionstrength strengthofofthe thebiceps bicepsbrachii, brachi,AH AH==maximum maximum vertical jumping height. There were no significant differences (from reference 19).
Body fat
AM+B AM+B
VM+B
MVC MVC
AH
(0/c)
(cm2)
(I)
(N) (N)
(cm)
76.4± 19.0 76.4±19.0
16.9±4.1 16.4±4.8
70.3± 8.2
19.2
70.9± 70.9± 15.5
7.84 7.84 1.41 7.49± 1.42
382.9±67.7 396.3±64.6
32.77±5.33 35.21 3521
73.7 18.5
16.4 4.5
69.7 10.9
7.37 1.25
389.4 62.2
36.28 5.64 5.64
73.8±18.1
16.6±4.4
69.1
5.55± 1.34 1.34
394.4±83.3 394.4± 83.3
35.61
76.1
9.3
15—
Table 2 Average daily daily metabolic metabolic rate rate (ADMR), (ADMR),energy energyintake ntake (El), (El),
A 12
9Mj/day Mi/day
'V
63-
5.71
ra
ADMR Energy intake
weight weight loss loss (ABM) (ABM) and andfat fatmass massloss loss(z±FM) (AFM) of 5 subjects during a climb of Mt. Everest. ADMR was measured over 7—10 days with the doubly labelled water technique and El was monitored with food diaries. Before and after the observation interval body mass (BM) was measured with a balance and fat mass (FM) was calculated from skinfolds and BM. Three of the subjects summited within two days after the observation interval. Subject 5 summited for the second time without supplementary oxygen, and the summit was included in the observation interval (adapted from reference 27).
0-
Fig. 1 Average daily metabolic rate (ADMR) and energy intake (El) of 5 subjects during a climb of Mt. Everest. ADMR was measured over 7—10 days with the doubly labelled water technique and El was monitored with food diaries. The energy deficit acquired during the climb of Mt. Everest, illustrated by the difference between ADMR and and El, El, was was paid paid dor dor with with energy energy frmm frmm body body fat fat and and lean lean mass. mass. E.Eq.ABW = energy equivalent of fat and lean mass lost during the climb. climb.
Subject 1
2 3 4 5 mean mean
ADMR ADMA (MJ/day)
El (MJ/day)
L\BM (kg)
12.3 14.9
8.2 9.7
11.7 15.6 13.5
5.8 6.9 6.9
—2.5 —4.0 —1.0 —3.0 —2.5
— 1.0 —1.0
13.6
7.5
—2.6
—1.4
7.0
AFM (kg) —
-1.3 -1.7 -1.7 —2.4
SD
Energy Intake and Expenditure It seems difficult to maintain energy balance above 4500 m (1, 2, 3, 7, 8, 27). It is not clear whether this discrepancy is mainly the result of a lowered energy intake or of
an increased energy expenditure as well. Different reports have indicated a possible increase in resting metbolic rate (RMR) at high altitude (3, 8), which may lead to an increase in average daily metabolic rate (ADMR). If such an increase is
not matched by an adequate rise in energy intake the energy balance would turn negative. In fact, artificially matching the intake to the expenditure has proven to be successful in minimizing weight loss at high altitude (2). Recently the energy expenditure and intake of 5 subjects was studied while climbing
Mt. Everest, using the doubly labelled water technique (27). The activity of the subjects, as reflected by the ADMR/RMR ratio was was high high(2.4 (2.4 0.1, 0.1,see seeTable Table2) 2) and and comparable comparable to that of oxygen consumpconsumpof endurance athletes at sea level. Estimated oxygen
tion during climbing climbing activities activitiesreached reached30 30mlmlmin1 mm1 kg kg which amounts to 50—60% of sea level VO2max as measured in
elite climbers. This is probably near the maximum aerobic power of the subjects at altitude and is compatible with the subjects perception of intense exertion while climbing. By con1 .5 MJ/day, MJ/day, trast, energy intake was surprisingly low (7.5 1.5
see Table 2). The loss of fat mass was related to this discrepancy and the energy balance equation fitted (expenditure = intake + bodystores consumed, see Figure 1). intake As far as the partition of energy in the diet is concerned, a spontaneous shift towards a diet rich in carbohy-
drates has been observed several times (7, 8). This could be favourable as it increases the metabolic respiratory quotient and and hence the energy yield per mole of oxygen consumed consumed and and raises PAO2 (26).
Water Changes in water metabolism during altitude exposure can interfere with measurements of energy balance
and body weight changes (6). During acclimatization, a decrease in intra- and extra-cellular water occurs, as well as a decrease in circulating plasma volume. These changes result in a weight loss of 1—2 1—2kg kg(l, (1,2,3, 2,3, 8, 12, 16). By contrast, a temporary rary increase increase of of body body water water may may be be observed observed during during the the initial initial stage at altitude if there are symptoms of acute mountain sickness (8). It has been estimated that during climbing water loss would be very high due to the inhalation of dry air and to hyperventilation, imposing water intakes up to 7 liters/day (21). This figure seems too high when using the formula proposed by Ferrus et al. (5). In fact, an increase in ventilatory rate from 13 to 26 breaths/mm, a 50% decrease in air density, a drop in
air temperature from 20 to 0 °C and a drop in partial water pressure increase respirarespirapressure in the air from 8.5 to 0 Torr, would increase tory water loss only from 288 to 410 ml/day. mI/day. A group of mountaineers recently observed during a climb of Mt. Everest lost 2.6 to 4.0 1/day I/day and ingested 1.8 to 2.9 I/day 1/day (27). (27). However, However, at the the high metabolic rates observed (see Table 1), the the production production of of metabolic water was —. 1.01/day and as the subjects also lost body mass which contains water, the water balance fitted.
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Before 1st week 2nd week 4th week
Weight (kg)
Intl ISportsMed. mt. Sports Med.13 13(1992) (1992) S131 S131
Nutrition andHigh Altitude Exposure
Some observations during a Mt. Everest expedition dition (1), (1), and and the the reported reported relationship relationship between between aa decrease decrease in in xylose absorption from the gut in sea level hypoxic patients (1, 17) have led to the hypothesis of malabsorption of nutrients from from the gastrointestinal tract at high altitude. FatFat- and and carbocarbo-
hydrate-digestibility at altitude are apparently normal up to altitudes of 5000 to 5500 m (11, 14, 22), whereas some evidence dence exists exists of of fatfat- and and carbohydrate-malabsorption carbohydrate-malabsorption at at 6300 6300 m m (1). Protein- and energy-digestibility in 6 Caucasians at 5000 m
were unchanged when compared to sea level (15). On the whole, it seems unlikely that malabsorption plays an important role at high altitude although at extreme altitudes (above 7000 m) the gut may lose some of its absorptive capacity due to very low Pa02.
imize imize losses, it is advisable to acclimatize properly, properly, to to reduce reduce the length of stay at extreme altitude as much as possible and to keep the nutrient intake high and varied. References Boyer S. J., Blume F. F. D.: D.: Weight Weight loss loss and and changes changesin in body body compocompohigh altitude. altitude. JApplPhysio/57: JApp/Physio/57: 1580—1585, sition at athigh 1580—1585,1984. 1984. - Butterfield G. G. E.: E.: Elements Elements of of energy energy balance balance at at altitude, altitude, in in Sutton Sutton J. R., Coates G., Remmers J. E. (eds): Hypoxia, the adaptations. Burlington, B.C. Decker Decker mc, mc, 1990, 1990, pp pp 88—93. 88—93. Consolazio, C. F., Matoush L. 0., Johnson H. L., Daws T. A.: Protein and water balance balance of of young young adults adults during during prolonged prolongedexposure exposure 1: 154—161, to high altitude altitude (4300 (4300 m). m).AmJC/in AmJCIinNutr2 Nutr2l: 154—161, 1968. 1968. 'I Ferretti Ferretti G., G.,Hauser HauserH., H.,di diPrampero Prampero P. P. E.: E.: Maximal Maximal muscular muscular ml J Sports power before and after exposure to chronic hypoxia. mt Med (suppl I): S31—S34, S31 —S34, 1990. Medl11 I (suppi Ferrus L., Commenges D., Gire J., Varène Varéne P.: Respiratory water
'
loss as a function of ventilatory or environmental factors. Resp
Protein Metabolism 6
A sizeable fraction of the altitude altitude related related weight loss is due to a reduction in muscle mass (1, 4, 24). In elite climbers this loss may decrease the thigh cross sectional area by 17% (4, 24). Recently the question whether muscle loss at altitude could be at least partly a result of detraining was addressed. It may be hypothesised that climbers, in good training conditions before leaving for a mountaineering expedition, undergo a loss of muscle mass at altitude due to a relative lack of exercise which may lead to muscle hypotrophy (24). On the other hand, there are some indicators that hypoxia "per se" may influence influetice amino acid metabolism. For example, acute byhypobaric hypoxia decreased the turnover and uptake of leucine from from the the forearm forearm muscle muscle compartment compartment (23). (23). In In this this respect, respect, itit is noteworthy that also normobaric hypoxia, as observed in chronic obstructive pulmonary disease patients, is often accompanied by marked muscle loss due to similar changes in protein metabolism (18). In addition, supplementation with
8
Physlol56: 11—20,1984. PIzyslolS6: 11—20,1984. Fulco C. S., Cymerman A., Pimental N. A., Young A. J., Maher J. Fulco vial Space Space Environ Environ T.: Anthropometric changes at high altitude. A riot Med56: 220—224, 1985. Med56:220—224,
Guilland J. C., Klepping Klepping J.: J.: Nutritional Nutritional alterations alterationsatathigh highaltitude altitude 1985. 517—523,1985. inman.EurfApp/Physiol54: inman.EurfApplPhysiol54:517—523, Horvath S. S. M., M.,Yousef Yousef at high Hannon J. J. P.: Nutrition Nutrition at Hannon high altitude, altitude, in in Horvath Physiology: Aging, Aging, Heat Heat and AltiM. K. M. K. (eds): (eds): Environmental Environmental Physiology: tude. Amsterdam, Amsterdam, Elsevier Elsevier Noord Noord Holland Holland Inc., Inc., 1980, tude. pp309— 309— 1980, pp 327. 327.
HoppelerH., Hoppeler H., Kleinert Kleinert E., E., Schlegel Schlegel C., C., Claasen Claasen H., H., Howald Howald H., H., of human human Kayar Morphological CerretelliP.: P.: Morphological adaptations adaptations of Kayar S. R., Cerretelli chronic hypoxia. hypoxia. fin mt Jj Sports Med skeletal muscle to chronic Med 11 II (suppl (suppl 1): I):
°
S3 —S9, 1990. 1990. 53—59,
° Hoyt W.,Durkot DurkotM. M.J.,J.,Kamimori KamimoriG. G.H., H.,Schoeller SchoellerD. D. A., A., CyCyHoytR.R.W., decreases intramerman A.: A.: Chronic altitude exposure Chronic altitude exposure (4300 (4300 m) m) decreases cellular and total body water in humans, in Sutton J. R., Coates G., and Houston C. (eds): Hypoxia and moutain medicine. Pergamon Oxford, 1992, 1992,p41. Oxford, p41. '' Press, Imray C. H. E., Chesner I., Wright A., A., Neoptolemeus Neoptolemeus J.J. P., P., BraBradwell A. R.: Fat absorption absorption at at altitude, altitude, aa reappraisal. reappraisal.mt mtJj Sports Sports Med 13: 13:87, 87, 1992. 1992. Body S. C.,Bardhan Bardhan J., J., Swamy Swamy Y. Y. V., V., Krishna B., Nayan N. 5.: Body Jam S.C.,
branched chain amino acids in high altitude trekkers could
12
prevent muscle loss (25). With regard to tissue and animal experiments acute hypoxia decreass protein synthesis in vitro as well as in vivo (20). An unbalance between protein anabolism and catabolism, which is influenced by circulating hormones, could also take place. The concentrations of several hormones
acutealtitude altitudeexposure. exposure. fluid compartments in humans during acute humans during AviatSpaceEnvironMed5 I: 234—236, 1980. 1980. AviatSpaceEnvironMed5l:234236, ° Kayser KayserB., B.,Hoppeler HoppelerH., H.,Claassen ClaassenH., H.,Cerretelli Cerretelli P.: P.: Muscle Muscle struc-
are known to change as a result of exposure to acute and chronic hypoxia. Decreased insulin and increased cortisol,
energy assimilation at 5000 m. In: Sutton J. R., Coates G., and Houston C. (eds): Hypoxia and mountain medicine. Pergamon
catecholamine, growth hormone, thyroid hormone and prostaglandin concentrations, all modulating protein metabolism have been described (see 26 for review). In biopsies of the vastus lateralis muscle of high altitude climbers, the observed muscle loss seemed to result from a decrease in muscle fiber size (9). Small fiber size was also reported in Sherpas (13). As Sherpas do not seem to lose lose weight weight while while climbing climbing (1), (I), it was couid be a useful adapspeculated that a decrease in fiber size could tation to altitude as it decreases the diffusion distance for oxygen from the capillary to to the the mitochondrion mitochondrion(9, (9, 13). In conclusion, altitude ahitude exposure in Caucasians leads to considerable weight loss due to an initial loss of water and subsequently to loss of of fat fat and and muscle muscle tissue. tissue. Up Up to toaltiati-
4
IS IS
6
Press, Oxford, Oxford, 1992 1992(in (in press). press). Kayser B., Acheson K., Décombaz J., Fern E., Cerretelli P.: Protein absorption and energy digestibility at high altitude. JAppiPhysiol, 1992 (in (in press). press). H. J., J., Consolazio Consolazio C. C. F., F., Johnson Johnson H. H. L., L., Nielsen Nielsen W. W. C., C., Krzywicki H.
Barnhart R. A.: Water metabolism in humans during acute high 17
' 20
tudes around 5000 m the loss of fat and muscle may be avoidable by increasing food intake. Primary anorexia, discomfort and lack of palatable food, detraining and possible direct effects of hypoxia on protein metabolism seem to inevitably lead to weight loss at higher altitudes. In order to mm-
ture ture and and performance performance capacity capacityof ofHimalayan Himalayan Sherpas. Sherpas. JAppiPhysJAppiPhysio/70: 1938—1942,1991. iol7O: Kayser Kayser B., Acheson K., Bernini A., Fern E., Cerretelli P.: Dietary
21
altitude exposure (4300 809, 1971. (4300 m). m). JAppiPhysiol JAppiPhysiol 30: 30:806— 806—809, Milledge J. S.: Arterial oxygen desaturation and intestinal absorption ofxylose. BritMedJ3: 557—558, 1972.
Morrison W. L., Gibson J. N. A., Scrimgeour C., Rennie M. J.: C/in Set Sd 75:41 5—420, 1988. Muscle wasting in emphysema. Clin 75:415—420,
Narici M., Kayser B., Cibella F., Grassi B., Cerretelli P.: No changes in body composition and maximum alactic anaerobic performance during a 4 week sojourn at altitude. mt IntJSports Sports Med Med 13: 13: 87, 1992. Preedy V. S., Smith D. M., Sugden P. H.: The effects of 6 hours hypoxia on protein synthesis synthesis in in rat rat tissues tissues in in vivo vivoand andin invitro, vitro.Bloc/i BiochJJ 22&179—185,1985. 228:179—185,1985. Pugh L G. C.: Physiological and medical aspects of a Himalayan
scientific and mountaineering mountaineering expedition. expedition. Brit Brit Med MedJJ 2: 621 —637, 1962.
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Digestibility
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28:242—245,1975.
23 Rennie M. J., Babij P., Sutton J. R., Tonkins J. J., Read W. W., Ford
C., Halliday D.: Effects of acute hypoxia on forearm leucine metabolism.ProgClinBiolRes 136:317—323,1983. tabolism.ProgClinBiolRes 136: 317—323,1983. 24 Rose M. S., Houston C. S., Fulco C. S., Coates G., Sutton J. R., Cymerman A.: Operation Everest II: Nutrition and body composition.JApplPhysiol65:2545—2551, 1988. tion.JApplPhysiol6S:2545—2551, 1988. 25 25 Schena F., Guerrini F,, F., Trenaghi P., Kayser B.: Branched-chain amino acid supplementation during trekking at high altitude: the effects on weight loss, body composition, and muscular power. EurJApp/Physiol EurJApplPhysiol 1992 (in press).
26 Ward M. P., Milledge J. S., West J. B.: High altitude medicine and physiology. Chapman and Hall Medical, London, 1989. 27 physiology. Chapman and Hall Medical, London, 1989. 27 K., Kayser Westerterp K., Kayser B., B., Brouns Brouns F., F.,Herry Herry J.-P., Saris W.: Energy Westerterp expenditure climbing climbing Mt. Mt. Everest. Everest. JAppiPhysiol, JAppiPhysiol, 1992 1992 (in (in press). press). expenditure
Bengt Kayser Kayser Bengt Dept. de de Physiologie Physiologic Dept.
CMU CMU
CH-l2ll Genève4 CH-1211 Genève4 Switzerland Switzerland
Fluid Balance and Exercise R. J. Maughan Department of Environmental and Occupational Medicine, University Medical School, Foresterhill, Aberdeen AB9 2ZD, Scotland
Introduction Abstract
R. .J. Maughan, Fluid Balance and Exercise. IntJ Sports Med, Vol 13, SuppIl, pp S132—S135, 1992.
The rate of metabolic heat production during prolonged exercise maybe increased to 15—20 times that at rest. Evaporation of sweat secreted onto the skin can effectively limit the rise in body temperature which would otherwise occur, but results in the loss of water and electrolytes from the body. Dehydration and an increased thermal load
can accelerate the onset of fatigue during exercise. The available evidence supports the idea that ingestion of fluids
during prolonged exercise can improve performance. Heart rate and rectal temperature will generally be lower, and plasma volume will be better maintained when fluids are given. There is, however, no general agreement on the optimum formulation nor on the frequency or volume of drinking that is mosv appropriate. In practice, the ideal solution will depend on a number of factors, including the duration and intensity of the exercise, the environmental conditions and the characteristics of the individual. The varia-
tion between individuals is, however, large and the optimum strategy can only be established by subjective experience. Key words
Sweating, fluid balance, dehydration, fatigue Int.J.SportsMed. 13(1992)5132—S135 Georg Thieme Verlag StuttgartNew York
Many factors influence the ability to perform exercise, and among these are the environmental conditions under which the exercise is performed. When the ambient temperature and humidity are high, the capacity to perform prolonged exercise is reduced: in this situation, dehydration and tilermoregulatory problems rather than depletion of energy stores or any other factor may be the cause of fatigue.
Fluid loss during exercise is linked to the need to maintain body temperature within narrow limits. During exercise, the rate of heat production can be increased to many times the resting level. A 70 kg runner completing a marathon in 2 h 30 mm requires an average oxygen consumption of about 4 1/mm (10): heat production will be about 1000 W. Similar high levels of energy expenditure are sustained in cycling and cross country skiing. In spite of this, body temperature seldom rises by more than 2—3 °C, indicating that heat is lost from the body almost as fast as it is being produced.
At high ambient temperatures, heat loss occurs only by evaporation. For the 2 h 30 mm marathon runner with a body weight of 70 kg to balance his heat production by evap-
orative loss alone requires sweat to be evaporated from the skin at about 1.6 I/h: at such high sweat rates, an appreciable fraction drips from the skin without evaporating, and a sweat secretion rate of about 2 1/h is likely to be necessary to achieve this rate of evaporative heat loss. This is possible, but would result in the loss of 51 of body water. Water (and heat) will also be
lost by evaporation from the respiratory tract. During prolonged hard exercise in a hot dry environment, these losses may be significant even in the absence of sweating.
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22 Rai R. M., Malhotra Maihotra M. S., Dimri G. P., Sampathkumar T.: Utilization of different quantities of fat at high altitude. Am J Clin Nuir
B. Kayser
