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19
Maintenance of Body Weight at Altitude: In Search of 500 kcal/day

Gail E. Butterfield1

INTRODUCTION

Weight loss while visiting at elevations not normally inhabited by the sojourner (at altitude) has been considered by some as inevitable (Boyer and Blume, 1984; West et al., 1983). However, acceptance of the inevitability of this weight loss may lead to a decrease in muscle mass (Rose et al., 1988), a decrease in performance capacity (Sridharan et al., 1987), and in the military arena, failure of a military campaign. Thus, the determination of the truth of the dogma of inevitability of weight loss at altitude and the understanding of its causes and consequences becomes important to the success of any endeavor at high elevations.

1  

Gail E. Butterfield, Palo Alto Veterans Affairs Health Care System, Palo Alto, CA 94304



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--> 19 Maintenance of Body Weight at Altitude: In Search of 500 kcal/day Gail E. Butterfield1 INTRODUCTION Weight loss while visiting at elevations not normally inhabited by the sojourner (at altitude) has been considered by some as inevitable (Boyer and Blume, 1984; West et al., 1983). However, acceptance of the inevitability of this weight loss may lead to a decrease in muscle mass (Rose et al., 1988), a decrease in performance capacity (Sridharan et al., 1987), and in the military arena, failure of a military campaign. Thus, the determination of the truth of the dogma of inevitability of weight loss at altitude and the understanding of its causes and consequences becomes important to the success of any endeavor at high elevations. 1   Gail E. Butterfield, Palo Alto Veterans Affairs Health Care System, Palo Alto, CA 94304

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--> ENERGY BALANCE AT ALTITUDE The Scenario of Weight Loss at Altitude Weight loss at altitude is accompanied by anorexia (Boyer and Blume, 1984; Consolazio et al., 1968, 1972; Guilland and Klepping, 1985; Hannon et al., 1976; Kryzywicki et al., 1971; Pugh, 1962; Rose et al., 1988; Westerterp et al., 1992; Whitten and Janoski, 1969) and often diuresis (Boyer and Blume, 1984; Guilland and Klepping, 1985; Koller et al., 1991; Krzywicki et al., 1971), both thought to begin with acute exposure. The weight loss occurs continuously throughout exposure in most studies (Rose et al., 1988); the diuresis may be a transient response (Boyer and Blume, 1984). Where monitored, nitrogen balance (a measure of the maintenance of lean body mass) has usually been negative (Guilland and Klepping, 1985; Hannon et al., 1976), which suggests a breakdown of lean tissue. Basal metabolic rate (BMR) is elevated over sea-level values initially but falls toward sea level over time (Hannon and Sudman, 1973; Stock et al., 1978). Studies on fuel utilization at altitude show glycogen sparing (preferential use of a fuel other than glycogen) and increased circulating glycerol and triglycerides with exercise after acclimatization (Young et al., 1982). These data have been interpreted to mean that the metabolic fuel chosen for maintenance of energy needs at rest and during exercise changes from carbohydrate to fat with acclimatization (Young et al., 1982). The Scenario with Starvation: Consequences of Negative Energy Balance This picture of weight loss during acute exposure to altitude sounds suspiciously similar to the series of events accompanying weight loss due to starvation. Starvation may be defined as a state where energy intake does not match energy need or as a state of negative energy balance. Ancel Keys in his seminal work on starvation in male conscientious objectors (Keys et al., 1950) described the sequelae of events in response to diminished energy intake, which included negative nitrogen balance accompanied by a decline in lean body and fat mass and by a concurrent decrease in BMR. Keys and coworkers (1950) also noted a decrease in overall activity level, which was thought to represent a mechanism to conserve existing energy stores. Further work on starvation by other investigators has shown a shift in energy metabolism toward mobilization and utilization of fat and ketone bodies under circumstances of negative energy balance (Saudek and Felig, 1976). The ultimate adaptation to inadequate energy intake was the failure of appetite (Keys et al., 1950).

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--> That these two states, exposure to high altitudes and starvation, share such a similar description of symptoms bears investigation (Brouns, 1992). Is the sequence of events that accompanies acute exposure to altitude a result of negative energy balance similar to starvation (Consolazio et al., 1972), and thus preventable (Butterfield, 1990), or is it in some part a response to the hypoxia, and even perhaps an important adaptive mechanism for acute acclimatization, as has been suggested by some (Hackett et al., 1981). The remainder of this review will attempt to answer this question. Negative energy balance may be the consequence of changes on either the intake or the expenditure side of the energy balance equation, or it may be the algebraic sum of both. Testing the hypothesis of the inevitability of weight loss requires matching both sides of this equation as closely as possible and evaluating the physiological changes seen. Few studies have attempted to balance this equation (Consolazio et al., 1972), and those that have (Milledge et al., 1983; Withey et al., 1983) are directed primarily toward the question of fluid homeostasis and have not presented data on food intake, weight change, or components of body composition other than fluid. Anorexia at Altitude: The Intake Side of Energy Balance Anorexia is defined as loss of appetite. As there is no direct measure of appetite, the question of anorexia at altitude has been approached primarily by monitoring food intake. Figure 19-1 represents a compilation of data from studies in which food consumption was essentially ad libitum, and reliable information is available on both sea-level (assumed to represent sea-level need) and high-altitude intakes. As can be seen, energy intakes under the two conditions are closely correlated, and altitude energy intake appears to be about 180 kcal/d lower than sea-level intake in these studies. Note that neither altitude nor activity level was considered in making this figure. The high intakes were reported in studies involving training at sea level and at altitude, whereas the lower intakes represent experiments done using more sedentary individuals. The lowest values are derived from a study in women (Hannon et al., 1976). Energy values for studies conducted at altitudes as low as 3,500 m (11,483 ft) (Sridharan et al., 1987) fall on the same line as the mean intakes for subjects studied at the barometric equivalent of the top of Mount Everest (Rose et al., 1988). Attempts to Correct Anorexia Attempts to prevent or correct this anorexia have been frustrating. Before 1968 Consolazio and colleagues (1968) attempted an enforced feeding study with U.S. Army recruits using standard Army rations, but the diet was so unpalatable

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--> FIGURE 19–1 Relationship between seal-level energy intake (kcal/d) and high-altitude energy intake (kcal/d) compiled from studies involving ad libitum food intake and providing accurate measures of both parameters. Numbers next to boxes indicate reference used: 1, Consolazio et al. (1968); 2, Krzywicki et al. (1969); 3, Consolazio et al. (1972); 4, Hannon et al. (1976); 5, Sridharan et al. (1982); 6, Boyer and Blume (1984); 7, Guilland and Klepping (1985); 8, Rose et al. (1988). that energy intakes dropped to less than 2,000 kcal/d in these individuals training at altitude. Whitten and Janoski (1969) provided a liquid formula (33 percent fat), which was so unpalatable that mean energy intake for 9 days at altitude was only about 750 kcal/d, whereas sea-level intake had been about 2,700 kcal/d, presumably on the same diet. Rate of weight loss in the study of Whitten and Janoski (1969) approached 0.5 kg/d. Data from these experiments were not used to generate the regression line in Figure 19-1 because of the abnormal qualities of the diets. A second attempt at providing adequate food intake by Consolazio in 1968 (Consolazio et al., 1972) was more successful. High-altitude energy intakes accomplished with strong encouragement (but still ad libitum), and a food and

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--> formula diet, a diet composed of foods and special liquid supplement, were only about 200 kcal/d less than the intake at sea level. In this short-term study (6 days at altitude), nitrogen balance was positive, possibly due to the training regimen imposed on the recruits. Positive nitrogen balance under circumstances of negative energy balance has been demonstrated by Todd et al. (1984) at sea level in individuals initiating a moderate exercise program. Weight maintenance was not accomplished in the Consolazio experiment, and the rate of weight loss after the first 2 days was similar to that seen in other studies where energy intake was not enforced (146 to 188 g/d; see Table 19-1). Other studies have attempted to maintain food intake by providing extremely palatable food. Rose and colleagues (1988) allowed subjects in the simulated ascent of Mount Everest (Operation Everest II) to request desired foods, but food intake declined with time, and weight loss continued throughout the ascent (Figure 19-2). Most other attempts at providing very palatable food have been equally futile (Kayser et al., 1992), although Hannon et al. (1976) found that food consumption at altitude returned to sea-level intakes within 7 days in women given standardized frozen dinners. These authors suggested, as a consequence of these and other data, that women may adapt better to altitude than do their male counterparts. More recent attempts by Butterfield and associates (1992) to feed moderately active male subjects sufficient energy as a formula and food diet (individually designed to meet energy need as determined from sea-level energy requirement and changes in BMR at altitude) were successful in curtailing weight loss after the initial week at 4,300 m (14,110 ft) in four of seven subjects. Weight loss in the other three was slowed to a rate of less than 50 g/d, a rate much lower than that found in the ad-libitum feeding studies (see Table 19-1). Subjects in this experiment were highly motivated and involved, and food intake was strictly enforced. Additionally, the investigators joined the subjects in their enforced dietary regimen, and thus increased the motivation and compliance of the subjects. In a second study by the same team, enforcing food intake to meet individual needs from day 1 at altitude was successful in halting weight loss completely in a similar group of sedentary men (Roberts et al., in press b). In active individuals performing at altitude, however, energy needs are higher than those of sedentary sojourners, making attainment of adequate energy intake more difficult. Studies using doubly labeled water suggest that the total energy requirement at altitude is 2.2 to 2.3 times (Westerterp et al., 1992) the sea-level basal requirement (total energy requirement in working individuals at sea level for similar lifestyles would be 1.8 to 2.0 times basal energy requirement [Butterfield et al., 1992; Schutz et al., 1981]). Worme and coworkers (1991) were able to encourage an expeditionary team to increase energy intake above sea-level need using a special high-carbohydrate military ration (mean energy intake at altitude was about 3,600 kcal/d whereas sea-level intake was 2,600 kcal/d). However, dissatisfaction with the rations and gastrointestinal

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--> TABLE 19-1 Rate and Composition of Weight Loss at Altitude   Altitude (m [ft]) Time at Altitude (days) Total Weight Loss (kg) Rate of Weight Loss (g/d) Composition of Weight Loss Reference (kg lean) (kg fat) Consolazio et al., 1968 4,300 (14,100) 28 2.66* 95     Whitten and Janoski, 1969 4,300 9 4.27 474     Krzywicki et al., 1969 4,300 12 3.54† 295 2.24 1.29       3.96 330 2.51 1.46 Consolazio et al., 1972 4,300 6 0.88 147           1.13 188     Hannon et al., 1976‡ 4,300 7 1.00 143     Boyer and Blume, 1984 <5,400 (< 17,717) > 5,400 (> 17,717) 23 26 1.90 4.00 83 154 0.56 2.80 1.34 1.20 Guilland and Klepping, 1985 4,800–6,000 (15,748–19,685) 20 3.95 198     Bradwell et al., 1986 4,846 (15,900) 16 4.50 281     Rose et al., 1988 up to 8,846 (29,025) 38 7.40 196 5.05 2.51 Worme et al., 1991 2,400–4.300 (7,874–14,110) 31 1.90 61 0.90 2.80 Westerterp et al., 1992 7,000 (22,966) 10 2.20 220 0.80 1.40 Butterfield et al., 1992 4,300 21 2.20 104     * In this study, speed of ascent was different in two groups. † In this study, diets differing in carbohydrate content were fed. ‡ Study on women.

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--> FIGURE 19-2 Changes in mean energy intake and expenditure in relation to body weight during progressive decompression to the barometric equivalent of Mount Everest. SOURCE: Rose et al. (1988), used with permission. distress thought to be brought on by the diet limited food intake, and the troops were unable to meet energy needs as estimated from changes in body composition (mean energy deficit calculated from change in lean and fat tissue was 850 kcal/d, making total mean energy need for the time at altitude 4,450 kcal/d). The energy intake achieved, however, was sufficient to maintain performance parameters and lean body mass in these training troops, which suggests that the training had a protective effect on protein utilization as has been shown previously at sea level (Todd et al., 1984). Indian investigators have been particularly successful at accomplishing weight maintenance or even weight gain at altitude. Sridharan and colleagues (1987) accomplished weight maintenance and improved work capacity (as measured by exercise tolerance in a Harvard step test [test of endurance by stepping up repeatedly on a 10-inch step]) in road construction workers at altitude (2,750 m [9,022 ft]) by enforcing intake of food slathered in oil (energy intake increased from 3,100 kcal/d to 3,750 kcal/d with the inclusion of additional fat in the foods). Rai and coworkers (1975) actually accomplished weight gain in a group of soldiers working at 3,500 to 4,700 m (11,483 to 15,420 ft) for 4 months when they were fed their usual diet of 3,700 to

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--> 3,900 kcal/d with an additional fat intake of 45 to 324 g (400 to 2,900 kcal/d) as butter or hydrogenated oil. Thus, anorexia appears to be an immediate consequence of altitude exposure. The continuation of anorexia over time may be a consequence of continued negative energy balance, as is seen with starvation. A Shift in Appetite Anecdotal reports of climbers suggest that appetite switches from a preference for fat to carbohydrate after several days at altitude (Gill and Pugh, 1964). Carbohydrate represents the most oxygen-efficient fuel (Brooks and Fahey, 1984), and some investigations have shown that carbohydrate has a protective effect against clinical symptomology associated with hypoxia, ameliorating acute symptoms of mountain sickness (Consolazio et al., 1969), and positively affecting arterial oxygen concentration (Hansen et al., 1972) and pulmonary function (Dramise et al., 1975). In studies where composition of diet has been considered, if a voluntary switch in composition has occurred between sea level and altitude, it has most frequently been to maintain or increase absolute (Boyer and Blume, 1984; Worme et al., 1991) or relative (Guilland and Klepping, 1985) carbohydrate intake at the expense of fat and protein. A major exception to this observation are the results of the simulated ascent of Mount Everest (Operation Everest II), where subjects decreased the proportion of their diminishing food intake provided by carbohydrate (Rose et al., 1988). However, shifts in composition of diet may be dependent, at least in part, on the foods served (Hannon et al., 1976; Worme et al., 1991). In studies where food composition has been manipulated to enforce decreased carbohydrate intake at altitude, exercise performance has been adversely affected (Bigard et al., 1993). Increased Energy Need: The Output Side of Energy Balance Basal Energy Needs Several reports in the literature suggest that BMR (the energy required to maintain body functions in the most minimal state) increases at altitude (Butterfield et al., 1992; Gill and Pugh, 1964; Hannon and Sudman, 1973; Stock et al., 1978), especially during the first week. The early work in this area is confounded by a lack of standardized methodology for determining BMR: some studies collected data on resting metabolic rate (measured after arising and moving around), others on true BMR (measured before arising), and still others measuring one parameter at sea level and another at altitude. In studies where valid measurements were made across several weeks, the

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--> elevation in energy expenditure in the resting state declined to near sea level by 2 to 3 weeks of exposure (Hannon and Sudman, 1973; Stock et al., 1978). The literature on measurement of BMR at altitude has been reviewed elsewhere (Butterfield, 1990). The cause for the return toward sea-level values in BMR after the first few days at altitude is thought to be associated, at least in part, with a loss of metabolically active tissue consequent to energy imbalance and weight loss (Butterfield, 1990), as in the circumstances of starvation. This change in metabolic energy need has not been monitored or addressed in most studies attempting to match food intake to energy requirement at altitude. In fact, energy needs and energy intake may interact. In contrast to previous reports where energy intake was reduced compared to sea level, Butterfield and coworkers (1992) found that when energy intakes were maintained at or above sea-level values throughout 3 weeks of exposure to altitude, BMR rose initially, then fell slightly, but remained elevated above sea-level values by 17 percent (see Figure 19-3). The level at which basal energy needs stabilized in this study (221 ml of oxygen per kg per hour) is similar to that found by other investigators in individuals living chronically at high altitudes (247 ml of oxygen per kg per hour in miners living at 4,900 m [16,076 ft] for greater than 4 months [Picon-Reategui, 1961]; 232 ml of oxygen per kg per hour in climbers at 5,800 m [19,029 ft] for several weeks FIGURE 19-3 Basal oxygen consumption at sea level and at 4,300 m 14,110 ft). Values are means ± SE for seven subjects. *, Significant difference from sea level (P < 0.025). SOURCE: Butterfield et al. (1992), used with permission.

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--> [Gill and Pugh, 1964]). Interestingly, Butterfield and coworkers (Roberts et al., in press b), in a study subsequent to that mentioned above, matched energy intake with energy need (including increased basal needs) from day 1 of a 3-wk exposure to 4,300 m (14,110 ft) and found that BMR rose upon acute exposure to altitude, but did not spike on the first days at altitude. BMR rose to and remained elevated at about 17 percent above sea level for the duration of the study. Thus, adequate feeding may in fact, increase overall energy needs at altitude by maintaining increased basal needs. These needs must be addressed in designing food programs for high-altitude residents. Decreased Absorption If absorption of nutrients were hampered at altitude, increased intake would be required to cover this increased need. However, malabsorption at altitude is primarily reported anecdotally. Pugh (1962) reported ''greasy stools" at altitude in the subjects on the Himalayan Medical and Mountaineering Expedition. Boyer and Blume (1984), in an attempt to evaluate the possibility of fat malabsorption at 6,300 m (20,669 ft), studied fecal fat in three individuals, all of whom appeared to have symptoms of malabsorption even at sea level (mean sea-level net fat absorption was 79 percent, when normal net fat absorption would be expected to be about 95 to 96 percent). In several reports carefully evaluating the issue, no evidence of malabsorption of fat at altitudes from 3,500 m (11,483 ft) (Sridharan et al., 1982) to 4,700 m (15,420 ft) (Rai et al., 1975) could be found using fecal fat as a measure, nor was fat absorption affected at 5,500 m (18,045 ft) (Imray et al., 1992) as monitored by 13C-palmitate absorption. Attempts to show malabsorption of protein at 3,500 m (11,483 ft) (Sridharan et al., 1982), 4,300 m (14,110 ft) (Butterfield et al., 1992), or 5,000 m (16,404 ft) (Kayser et al., 1992) have proven futile, as has the quest to identify alterations in the absorption of carbohydrate (Butterfield et al., 1992) or total energy (Butterfield et al., 1992; Kayser et al., 1992; Sridharan et al., 1982) below 5,000 m (16,404 ft). Few studies of absorption have been conducted above that altitude. Thus, while it has been contended that energy requirements at altitude are elevated due to decreased availability of nutrients through the gastrointestinal tract, the available data confirm that this is not of concern at the altitudes at which military maneuvers will occur. Energy Expended in Activities Energy expended in activities could be affected by altitude in two ways. High altitudes could increase the energy required to do any activity, or they could change overall energy requirement by affecting the amount of activity performed. Regarding the latter possibility, most studies at altitude have

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--> involved recruits in training or mountaineers performing strenuous exercise and thus involve situations where the energy requirement for activity is increased over sea-level requirement by the protocol. There are few reports in the literature of activity patterns in moderately active individuals subjected to altitude. Butterfield and colleagues (1992) found a significant decrease in the energy expended at strenuous activities during discretionary time by their sedentary subjects at altitude as compared to sea level, which was possibly an adaptive mechanism to offset the increase in basal energy needs. Such a mechanism acting in other studies may have resulted in a diminished negative energy balance and a moderation of weight loss, giving a false impression of the overall effect of altitude on changes in body weight or total energy need. The question of energy efficiency at altitude has been more thoroughly investigated. Early work suggesting an increased energy requirement for work at fixed power outputs at altitude (Billings et al., 1971) was done using subjects walking on a treadmill, with and without added weights, a situation where metabolic response is known to be dependent on mass and mechanical efficiency (Brooks and Fahey, 1984). This theoretical increase in need was successfully ruled out by West and associates (1983), who showed that the linear relationship known to exist at sea level between oxygen uptake and power output had the same slope and y intercept at altitudes as high as 6,300 m (20,669 ft). This question has been reviewed recently elsewhere (Butterfield, 1990), with the conclusion that energy expended in activities of comparable intensity is constant throughout the range of altitudes where humans may be expected to work, and that this aspect of the energy balance equation does not increase energy needs at altitude. WEIGHT LOSS AT ALTITUDE Thus, as a consequence of decreased energy intake and increased basal energy needs, most individuals acutely exposed to high altitudes experience a significant negative energy balance and subsequent weight loss. In the discussion above, the daily energy deficit between sea-level and high-altitude intakes was estimated to be approximately 180 kcal/d. Basal energy need, however, has been shown by Butterfield and colleagues (1992) to be elevated by about 17 percent (about 300 kcal/d) in individuals who consume adequate energy when acutely exposed to altitude, giving a total potential energy deficit of about 480 kcal/d in individuals allowed to eat ad libitum at altitude. Such an energy deficit would be sufficient at sea level to cause a weight loss of about 0.5 kg of fat tissue per week (or about 70 g/d). As will be seen below, the actual rate of weight loss in most studies is somewhat higher than this amount, which suggests that there may be additional elements to be considered in the issue of weight loss at high altitudes.

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--> Magnitude of Weight Loss The magnitude of weight loss at altitude appears to depend to some extent on the duration of the stay, the conditions of the ascent, and the final altitude attained. Table 19-1 depicts data from studies that have provided adequate data to assess rate of weight loss. Although rates of weight loss range from 85 to 474 g/d, this table shows that the average rate of weight loss in ad libitum studies is about 200 g/d, or about 1.4 kg/wk, about three times greater than would be predicted from the potential negative energy balance estimated above. This weight could have an energy equivalent of as much as 1,800 kcal/d if it were assumed to be all fat, or as low as 200 kcal/d if it were assumed to be lean tissue, 73 percent of which is assumed to be water. Composition of Weight Loss The composition of the tissue lost at altitude is controversial, and results obtained depend on the methodologies used to determine body composition. Because most body composition measures are indirect, assumptions are made as to the density and electrolyte composition of the tissues being measured. These assumptions are essentially negated by shifts in fluid from one compartment to another, which may change the concentration of solutes in the tissue in an unknown way. Such fluid shifts are well documented at altitude (Krzywicki et al., 1971; Withey et al., 1983). In addition, acute exposure to altitude results in a diuresis of varying magnitude (Boyer and Blume, 1984; Guilland and Klepping, 1985; Koller et al., 1991; Krzywicki et al., 1971), which may increase weight loss and complicate the determination of composition of that weight loss. Diuresis is not found in all studies, however (Consolazio et al., 1968; Hannon et al., 1976), and may be related to exercise level (Withey et al., 1983), diet (Krzywicki et al., 1971), degree of acclimatization (Koller et al., 1991), or gender of the subjects (Hannon et al., 1976). Unfortunately, measurement of body composition at altitude by routine field methods (skinfold measurement [Fulco et al., 1985]; electrical impedance [Fulco et al., 1992]) has been found to be unsatisfactory. The composition of weight loss at altitude has been reviewed recently elsewhere (Butterfield, 1990; Kayser, 1992). The issue of concern in the discussion of weight maintenance at altitude, however, is which of these compositional changes are a consequence of exposure to hypoxia, and thus unavoidable, and which are associated with inadequate energy intake. Discussion below will concern itself with direct measures of changes in composition, that is, with measures of lean body mass (nitrogen balance) and water loss.

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--> Lean Tissue The few studies in which nitrogen balance has been measured show that the decline in body weight with exposure to altitude is accompanied by an increase in urinary nitrogen (and a decrease in the balance between nitrogen intake and total nitrogen output as urine and feces, or nitrogen balance). These results suggest a degradation of lean tissue (Consolazio et al., 1968; Guilland and Klepping, 1985), reflective of the energy deficit incurred. The work of Butterfield and colleagues (1992) suggests that this decline in nitrogen balance can be avoided if sufficient energy is provided in the diet. Figure 19-4 shows nitrogen balance at various times during their experiment. Subjects were in nitrogen balance during the sea-level control phase, which suggests that the energy intake provided during this time met energy needs (Calloway, 1975). During the first week at altitude, when energy intake was equivalent to that at sea level (and thus slightly lower than need, not having been adjusted for increased BMR), nitrogen balance was negative and weight loss occurred. When energy intake was increased to cover the increased needs imposed by elevated BMR, nitrogen balance became positive and remained FIGURE 19-4 Mean nitrogen balance for 3 days at sea level (SL), 6 days at altitude before increase in energy intake (PRE EΔ), 6 days immediately after increase in energy intake (days 1–6 POST EΔ), and last 5 days of collections (days 7–12 POST EΔ). Values are means ± SE for six subjects. Increase in nitrogen balance from before to after energy intake increase statistically significant (P < 0.05) by repeated-measures analysis of variance. SOURCE: Butterfield et al. (1992), used with permission.

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--> positive throughout the remainder of the 3-wk stay, while weight loss slowed in all and ceased completely in four of the seven subjects. A subsequent study by the same group suggests that both nitrogen balance and body weight can be maintained throughout a sojourn of 3 weeks at 4,300 m (14,110 ft) if aggressive attempts are made to cover increased energy needs by increased energy intake (Roberts et al., in press b). As previously noted, studies at altitude where the training regimen is strenuous have been able to accomplish the maintenance of lean tissue (as measured by nitrogen balance [Consolazio et al., 1972] or densitometry [Worme et al., 1991]) in the presence of a moderate negative energy balance (about 200 kcal/d). These results suggest a protective effect of exercise on protein utilization at high altitudes, as has been previously shown at sea level (Todd et al., 1984). Water Net loss of body water at high altitudes may occur as a result of increased insensible losses to the dry environment (Ferrus et al., 1984) or consequent to diuresis (Krzywicki et al., 1971). The latter is considered to be a mechanism of adaptation to altitude and appears to be controlled, at least in part, by hormones affecting kidney function and thirst (Milledge et al., 1983). However, diuresis is also a common consequence of rapid weight loss (Fisler and Drenick, 1987), and the magnitude of diuresis found in most studies during the initial days of exposure to hypoxic conditions may be the sum of the consequences of altitude and of weight loss due to negative energy balance. The magnitude of respiratory fluid losses to the environment is determined by the partial pressure of water in the surrounding air and the frequency of respiration (Ferrus et al., 1984). Respiratory losses at altitude have been estimated to be about 600 ml/d (Westerterp et al., 1992) and may be compensated for by increases in metabolic water production accompanying increased exercise (Kayser, 1992). Insensible sweat losses, representing losses from the body surface, are somewhat more difficult to measure, but may be substantial (Consolazio et al., 1968). Water balance studies have suggested that total insensible losses (sweat and respiratory losses combined) are at least 1,900 ml/d (Butterfield et al., 1992; Consolazio et al., 1968) at 4,300 m (14,110 ft). Thus, water requirement at altitude may be very high, and diuresis with acute exposure may determine the overall state of hydration even after diuresis ceases. The magnitude of diuresis with acute exposure (and the overall negative water balance) was estimated by Consolazio and coworkers (1968) to be about 200 ml/d in individuals inadequately fed or in those purported to be "adequately" fed (Krzywicki et al., 1971).

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--> However, work by Butterfield and associates (Roberts et al., in press a, b) suggests a greater difference between the urinary losses of individuals inadequately and truly adequately fed. Figure 19-5 depicts urine volumes from two studies conducted at 4,300 m (14,110 ft) in which fluid and food consumption requirements were similarly enforced and environmental conditions were comparable. In the first study, energy intake was adjusted for increased needs after the first 7 days of altitude exposure, and weight loss occurred over the first week. In the second study, energy intake was adjusted from day 1 of exposure, and weight loss was prevented in 9 of 11 subjects studied. As can be seen from Figure 19-5, fluid losses in urine were significantly decreased under the circumstances of adequate energy intake, which suggests that at least some of the diuresis reported in the past may be a consequence of weight loss and may not be necessary under circumstances of hypoxia. The diuresis that did occur during the second study, however, was not accompanied by weight loss in most of the subjects studied. FIGURE 19-5 Total daily urine volume produced by men acutely exposed to 4,300 m (14,110 ft). Inadequately fed group (n = 7) were required to drink 4 liters fluid from food and water each day and consumed sea-level energy intake as food and formula for first 7 days at altitude. Adequately fed group (n = 11) had same fluid requirements, but energy intake was matched to energy need from the first day at altitude. Diet composition was the same in both groups.

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--> FUEL USE AT HIGH ALTITUDES The individual forced to consume energy sufficient to cover energy needs at altitude appears to respond differently to altitude than does the inadequately fed individual. Basal energy needs remain elevated in the adequately fed individuals, diuresis is minimized, and body weight and composition are maintained. These observations suggest that acceptance of the dogma of the inevitability of weight loss at altitude may have led to a misinterpretation of the consequences of altitude exposure on other physiological parameters as well. For example, it is commonly accepted that the primary source of energy for exercise changes from carbohydrate to fat with acclimatization (Young et al., 1982). This dogma is based on inferential measures of (1) glycogen stores in muscle that are maintained after exercise at altitude, and (2) circulating levels of glycerol, free fatty acids, and triglycerides that are increased with exercise after acclimatization to high altitudes. However, the same metabolic picture accrues in the circumstances of starvation (Saudek and Felig, 1976). Thus, in a circumstance where most of the individuals previously studied were in negative energy balance, it is not unexpected that they would be utilizing body stores of fat as a predominant energy source. However, Brooks and colleagues (1991) found that in men fed sufficient energy to cover need, the primary source of energy at rest and during exercise appeared to be carbohydrate, specifically glucose (see Figure 19-6), after 3 weeks of acclimatization to 4,300 m (14,110 ft). Teleologically, this choice of fuel is more economical in an oxygen-poor environment, as more energy is derived per liter of oxygen from carbohydrate than from any other metabolic fuel source. Using substrates labeled with stable isotopes to evaluate fuel utilization, both acutely and after 3 weeks of exposure to altitude, these investigators have further shown that in fact fatty acid consumption at rest and during exercise decreases markedly with acclimatization in men fed sufficient energy to cover need and that glucose is indeed the "fuel of choice" under these circumstances (Roberts et al., in press a, b). Thus, the acceptance of the inevitability of weight loss at altitude may have several significant outcomes. First, the physiological response to hypoxia may be confused with the physiological response to negative energy balance, giving an incorrect picture as to the metabolic consequences of high-altitude exposure. This consequence is primarily a scientific one and may be acceptable. But more important to the individuals who must live and work at high altitudes, the acceptance of weight loss as a consequence of hypoxia creates the possibility of acceptance of decreases in lean tissue mass, with accompanying decreases in strength. Such a scenario may result in diminished performance in a survival situation and could result in injury or death to the high-altitude sojourner. This consequence is not acceptable. A shift in the dogma followed by education of the high-altitude sojourner as to the causes

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--> FIGURE 19-6 Effects of altitude, acclimatization, and exercise on arterial blood glucose MCR (ml/min/kg) (MCR = Rd/[glucose]; means ± SE of six or seven subjects at each point). *, Different from sea level; **, different from sea level and acute altitude; t, different from rest; MCR, metabolic clearance rate; Rd, rate of disappearance. SOURCE: Brooks et al. (1991), used with permission. and consequences of weight loss at altitude, as well as provision of palatable rations and enforced food intake, may allow for a better matching of energy intake and energy requirement and may well be the difference between life and death for these individuals. AUTHOR'S CONCLUSIONS AND RECOMMENDATIONS Causes for Weight Loss at Altitude A variety of causes for weight loss at altitude have been previously delineated (Butterfield, 1990). They generally include anorexia, increased energy need due to increased maintenance requirements or malabsorption of energy components in the diet, fluid losses due to diuresis and insensible losses, and detraining. Altitude appears to depress appetite by about 200 kcal/d in most studies, but this anorexia can be successfully overcome and weight loss prevented by education, strong encouragement, and the provision of palatable and easily consumed (formula) foods.

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--> Energy requirements are elevated at altitude. Basal energy requirements are elevated by at least 17 percent and remain so, especially in adequately fed individuals, resulting in an increased energy requirement for the sedentary individual of 200 to 300 kcal/d. However, there is no convincing evidence for malabsorption of fat, carbohydrate, or protein as a consequence of altitude exposures of less than 5,500 m (18,045 ft). Even above this altitude, the evidence for malabsorption is equivocal. Thus, energy requirement is not further increased as a consequence of malabsorption. Inclusion of the increased basal needs in the calculation of energy requirements at altitude is mandatory for overcoming weight loss. Fluid losses at altitude may approach 2 to 4 liter/d, with insensible losses amounting to about 2 liters and urine accounting for the rest. Diuresis in response to acute exposure may be minimized by feeding, but a degree of diuresis appears to be necessary as a positive adaptive mechanism. This diuresis may be accomplished without measurable weight loss. Detraining will only be an issue in those studies where exercise is not part of the protocol. Allowed to use discretionary time as desired, individuals exposed to hypoxia may decrease strenuous activities as a means of conserving energy stores. Energy required to perform standard activities does not vary with the altitude at which the exercise is performed, and consequently, this mechanism does not alter energy requirement. Under circumstances of adequate energy intake, physiological parameters measured at altitude, such as metabolic fuel source, will more adequately reflect the true response to hypoxia uncomplicated by negative energy balance. Weight loss at altitude is preventable and is unacceptable. Author's Recommendations To minimize weight loss with altitude exposure, the following recommendations are made: Energy requirement at altitude for moderately active individuals is 2.2 to 2.3 times sea-level basal requirements (which can be successfully computed from Harris-Benedict equations [Harris and Benedict, 1919]); further adjustments must be made for other strenuous activities performed, such as marching or climbing. Successful consumption of this energy intake may require strong incentives; frequent encouragement; and special high-calorie, nutrient-dense, palatable products. Education about the causes and consequences of weight loss at altitude may be especially important for ensuring compliance. Compliance with this recommendation should essentially eliminate weight loss at altitude.

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--> The composition of diet should be as follows: Protein intake of 0.8 g/kg for sedentary individuals; 1.2 to 1.5 g/kg for those performing strenuous endurance activities (Meredith et al., 1989) (result is about 12 to 15 percent of total energy intake). Carbohydrate intake should supply around 60 percent of total energy intake to cover increased need for carbohydrate in the adequately fed individual. This carbohydrate should be consumed as complex forms to minimize gastrointestinal distress. Fat intake could be as low as 25 to 28 percent, and such levels of intake may decrease some of the intestinal distress that may accompany large intakes of simple carbohydrate (Worme et al., 1991). Develop a drinkable, high-carbohydrate, moderate protein and fat supplement containing about 500 kcal; inclusion of such a product in the rations of sojourners at altitude will help to remedy the consequences of increased need and decreased appetite. Determine amount of diuresis that is necessary for optimal adaptation to altitude. Provide fluid sufficient to cover high insensible losses and diuresis (4 liter/d). Determine the possible positive effects of feeding on the development of symptoms of acute mountain sickness. Evaluate the effect of meal composition on these symptoms. REFERENCES Bigard, A.X., P. Satabin, P. Lavier, F. Canon, D. Taillandier, and C.Y. Guezennec 1993 Effects of protein supplementation during prolonged exercise at moderate altitude on performance and plasma amino acid pattern. Eur. J. Appl. Physiol. 66:5–10. Billings, C.E., R. Bason, D.K. Mathews, and E.L. Fox 1971 Cost of submaximal and maximal work during chronic exposure at 3,800 m. J. Appl. Physiol. 30:406–408. Boyer, S.J., and D. Blume 1984 Weight loss and changes in body composition at high altitude. J. Appl. Physiol. 57:1580–1585. Bradwell, A.R., J.H. Foote, J.J. Milles, P.W. Dykes, P.J.E. Forster, I. Chesner, and N.V. Richardson 1986 Effect of acetazolamide on exercise performance and muscle mass at high altitude. Lancet 1(8488):1001–1005. Brooks, G.A., and T.D. Fahey 1984 Exercise Physiology: Human Bioenergetics and Its Application. New York: John Wiley and Sons. Brooks, G.A., G.E. Butterfield, R.R. Wolfe, B.M. Groves, R.S. Mazzeo, J.R. Sutton, E.E. Wolfel, and J.T. Reeves 1991 Increased dependence on blood glucose after acclimatization to 4,300 m. J. Appl. Physiol. 70:919–927.

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