Performance

1. What is the effect of cold or altitude exposure on muscle strength and endurance?

Cold and high-altitude exposure affects muscle strength and endurance through changes in cardiac output and oxygen uptake. Very cold environments that lower body temperature by more than 0.9°F (0.5°C) may limit maximal cardiac output and result in reduced maximum oxygen uptake. Because moderately cold environments lower muscle temperature, endurance of moderate physical activity actually can be theoretically increased if body core temperature can be maintained. There are conflicting data on the effects of cold exposure on muscle strength, and more research is needed to determine this relationship.

High altitudes can also affect physical performance because they decrease maximum oxygen uptake by approximately 10 percent for every 3,280 ft (1,000 m) increase in altitude. Endurance is also significantly reduced at high altitudes (see Chapter I in this volume). While acclimatization to high altitudes does not improve maximum oxygen capacity, endurance does improve (often as much as 50 percent or more) (see Chapter I in this volume).

2. Can diet influence these changes?

Maintenance of muscle structure and function over the long term depends on muscle strength and exercise. Muscle strength and endurance are influenced by diet through maintenance of muscle mass and the availability of appropriate substrates for muscle activity. Provision of adequate dietary energy under circumstances of either cold or high-altitude exposure will maximize the possibility of maintenance of muscle mass, and thus muscle strength. Conversely, inadequate energy intake will result in loss of muscle tissue with a concomitant decrease in strength and endurance.

Macronutrient composition of dietary intake may influence this process. In the cold or at high altitudes, protein requirements are not elevated above the needs of the individual at ambient temperature or at sea level performing the same level of activity. Nevertheless, dietary protein should be adequate to maintain the muscle mass that supports the strenuous physical activity performed. Fat as a source of energy is well tolerated in the cold, but provision of adequate carbohydrate is important because it is the major fuel needed for shivering, an important method for maintaining body temperature, and thus indirectly affects endurance. At high altitudes, carbohydrate becomes the predominant fuel at rest and during exercise. Failure to supply sufficient energy as carbohydrate (at least 400 g/d) at high altitudes can result in loss of muscle mass and decreased endurance.

3. How does cold or altitude exposure influence appetite?

The term appetite in this context is defined as a desire for food or drink. The traditional wisdom has been that cold climatic conditions lead to an increase in appetite. The evidence for this conclusion is derived from changes in body weight, self-scored questionnaires, and food intake records in cold environments at sea level. However, the reported increase in appetite is also associated with changes in other aspects of the subjects' environment such as altered activity levels, isolation, reduced social interaction, and modifications in diet. Nonetheless, it does appear that food intake is generally increased with cold exposure.

With ascent to altitudes above 10,000 to 12,000 ft (3,048 to 3,658 in), food consumption is reduced regardless of temperature. Body weight loss is common among subjects during the first few weeks at high altitudes, and such weight loss can be avoided only with successful efforts to consume food.

Although some studies have reported weight gain in cold environments, other investigations have found that soldiers operating in cold climates may not consume military rations in amounts adequate to meet energy expenditure. Reports from field training exercises have shown decreased intake of energy relative to need in both cold and high-altitude environments. The factors that influence ration consumption discussed in the CMNR's report on Not Eating Enough (IOM 1995) may be even more significant for operations in the cold and in high altitudes. Encouragement of food discipline through a field feeding doctrine (IOM, 1995) would help soldiers maintain an appropriate level of food intake. With adaptation to altitude, appetite increases but generally food intake is insufficient to regain lost weight or even to maintain the lower weight.

Health and Medical Aspects

4. Is them concern for increased cardiovascular risk when a high fat diet is consumed for intermittent (7-to 14-d) time periods in the cold?

Although this question was not addressed specifically by any participant in the workshop, all available evidence indicates that there should be no concern with higher fat diets for these short periods of time. A major nutritional problem during military operations in the cold is meeting the added requirements for water and food to prevent both dehydration and weight loss. King et al. (1993) reported that in artic field tests, the Army's 18-Man Arctic Tray Pack Ration Module¹ (29 percent of calories from fat), in combination with either a wet-pack (Meal, Ready-to-Eat¹ [MRE, 36 percent of calories from fat]) or a dehydrated (Long-Range Patrol, Improved¹ [LRP 1, 35 percent of calories from fat]) individual ration, met the full daily nutritional recommendations for protein and micronutrients. However, energy needs were not met, and soldiers consistently lost body weight.

The easiest way for military feeding systems to provide for increased caloric needs during cold-weather operations is to include additional dietary fats. Such an increase in dietary fat is also most expedient, logistically. However, some tested supplements, containing only a modestly higher fat content, did not result in sufficient energy intake to prevent weight loss (Edwards and Roberts, 1991). Cold-weather operations probably require a total energy intake ranging from 45 to 62 kcal/kg body weight/d, but earlier military studies in the Arctic suggested that 4,000 kcal/d or less were actually being consumed (LeBlanc, 1957). Current projections for energy needs in arctic conditions focus on 58 kcal/kg body weight/d (see Chapter I in this volume).

Controversial questions about the relationships between dietary intakes of fat and cholesterol and the pathogenesis of atherosclerosis, strokes, and coronary heart disease have fueled important clinical research studies for several decades. Although recent estimations indicate that the average total fat intake in the United States has declined to 34 percent of total calories (CDC, 1994), the most recent review of national dietary guidelines recommends that an individual consume no more than 30 percent, with an increased intake of complex carbohydrates to provide total energy needs (USDA, 1995). Increased consumption of fruits and vegetables, which would increase the intake of complex carbohydrates, is also recommended.

However, these recommendations for the diet of the general population may not be appropriate for the military and logistical requirements for conducting either short-or long-term field operations in arctic climates. Fresh fruits and vegetables would be impossible to supply. Although diets supplying 58 kcal/kg body weight/d can be formulated with only 30 percent fat, they may prove operationally difficult to provide and the CMNR believes that this guideline is too restrictive for military operational rations. Diets containing more of the high density fat fuels may become an operational necessity. In addition, as pointed out by Edwards et al. (1992), the choice of ration must consider water availability, size and volume of load, resupply schedule, logistics, and the task at hand. Although a higher fat diet is clearly not a nutritional necessity in the Arctic, it may prove to be a logistical need.

From a metabolic point of view, it is probable that the additional fat calories will be metabolized promptly, to satisfy immediate energy needs, rather than being stored in body fat depots. If extra dietary fat is consumed primarily to meet high daily energy requirements and to prevent weight loss during military operations in cold climates, it will not necessarily have important long-term consequences.

Current national dietary recommendations have been in effect for only a few years, and there is no available research evidence to suggest that a temporary deviation from a low fat diet, eaten in order to meet unusually high energy demands, would have a long-term effect on slowly developing cardiovascular pathology.

If this question is viewed from a risk/benefit perspective, the short-term risks to a soldier who must participate in a dangerous military operation in arctic cold are high, and nutritional assistance must be given to help the soldier function at an optimal level. Clearly, inadequate energy intakes and progressive weight losses are not desirable. The immediate benefits of an adequate energy intake far outweigh the possibility that a short-term intake of extra fat calories (eaten to meet the energy demands of cold, arctic climates) might contribute to deleterious health effects several decades later.

5. What nutrients prevent or lessen the symptoms of acute altitude exposure?

Two nutrients have the reputation of being protective against acute mountain sickness (AMS): water and carbohydrate. Because acute altitude exposure is accompanied by diuresis in most individuals, replacement of water lost through diuresis has been reputed to be important in minimizing the symptoms of mountain sickness. Scientific data on this question are minimal. More careful studies have been done on the effect of carbohydrate feeding during acute exposure to altitude, and the general consensus from those studies is that carbohydrate is of benefit in minimizing the symptoms of acute exposure (Consolazio et al., 1969). Because carbohydrate is the primary metabolic fuel at rest and during exercise (Brooks et al., 1991; Roberts et al., in press a, b), and because it provides slightly more energy for the oxygen consumed than does fat (Kleiber, 1961), provision of ample amounts of this macronutrient could be expected to overcome the 500 kcal/d deficit created by exposure to hypobaric hypoxia, maintain body glycogen stores, and assist in the maintenance of muscle mass.

6. Is free radical formation a concern for prolonged (10- to 30-d) military operations at 10,000–15,000 ft (3,048–4,572 m) elevation?

Free radical formation, the consequence of oxidative stress, might be expected to increase in cold or in high altitude environments, due to (1) the elevation in metabolic rate that results from an increased energy expenditure; (2) the stress of hypoxia at altitude; and (3) the increased exposure to ultraviolet radiation at altitude or on snow-covered ground. As recently reviewed by Askew (1995), some limited evidence does suggest an increase in oxidative stress in high altitude environments. During a 6-wk polar expedition, an increase in production of malonaldehyde, a product of lipid peroxidation believed to be a marker for oxidative stress, was measured in erythrocytes and plasma, followed by decreased blood concentrations of vitamin E (Panin et al., 1992 as reported by Askew, 1995). Simon-Schnass (see Chapter 21 in this volume) also reported increased exhalation of pentane, another marker for oxidative stress, with prolonged stays at high altitudes. Further research is needed to assess the physiological significance of such markers in terms of actual oxidative tissue damage as well as the potential long-term consequence of such damage, and the likelihood for significant oxidative damage during the timeframe of typical military deployments to high-altitude areas. Thus, there appears to be a potential for increased oxidative stress at high altitudes. However, the possible long-term consequences as well as the extent to which any ensuing damage would be decreased or prevented by providing additional antioxidant nutrients are not known at this time, but this is an important area for future research.

Thermoregulation and Acclimatization

7. Is cold/altitude acclimatization facilitated by prior nutritional status or supplemental nutrients?

There are few data on this topic. Prior nutritional status may affect acclimatization to cold/altitude in that an individual in poor nutritional status may have difficulty in adapting. Nutrients of particular concern would be iron, because of its relationship to hemoglobin and hemoglobin synthesis, and vitamin E, because of its relationship to oxidative stress.

In addition to prior nutritional status, the body composition, recent losses of body weight or lean body mass, and recent health and training history of individual soldiers should be considered prior to their participation in missions or training in cold and in high-altitude environments. In particular, the extreme losses of lean body mass described for some individuals who participated in U.S. Army Ranger Training would need to be regained prior to working in environmental extremes.

8. What nutrients influence thermoregulation?

Thermoregulation involves cardiovascular measures to reduce heat loss (nonshivering thermogenesis), an increase in metabolic heat production through shivering and an increase in voluntary muscular activity.

In short term studies, shivering thermogenesis, like voluntary muscular activity, has been found to be fueled by carbohydrate and, to a lesser extent, fat. There is no evidence for a role for protein in shivering thermogenesis at this time; however, more research is needed to establish whether a specific proportion of nutrients has any advantage in maintaining thermoregulation under field conditions.

Thiamin, niacin, riboflavin, and pantothenic acid all play a critical role in thermogenesis due to their involvement in energy metabolism; however, there is no evidence at this time to support increased intake in the cold of any of these nutrients above MRDA (AR 40–25, 1985) levels. Evidence from studies conducted primarily in laboratory animals has suggested a role for the micronutrients iron, copper, and zinc in nonshivering thermogenesis. There is no evidence at this time to indicate that short-term depletion of any of these micronutrients interferes with thermoregulation in humans; however, more research is needed.

The macronutrient sources of energy (carbohydrate, protein, and fat) also have thermogenic effects. Fat is absorbed slowly but has the lowest postprandial thermogenic effect. Carbohydrates are absorbed most rapidly, but their thermogenic effect is higher and may last for 2 to 3 hours. Protein digestion gives rise to amino acids, which are absorbed more slowly than carbohydrates, but have the highest thermogenic effect, lasting up to 5 to 6 hours after absorption. The use of a high protein snack prior to retiring to sleep has been recommended to aid in thermoregulation.

9. Does the timing of food ingestion influence cold tolerance?

Postprandially induced thermogenesis can be a significant source of heat production within the body. Consumption of a substantial meal high in protein may provide necessary heat during periods of low activity or during sleep (see LeBlanc, Chapter 12 in this volume and the answer to Question 8 above). The consumption of small meals or snacks at intervals throughout periods of moderate activity is useful for maintaining body heat and work performance.

10. What is the relationship between fluid intake and thermoregulation in the cold and at altitude?

With acute exposure to both cold and high altitudes, fluid losses may result in a hypohydrated state. Diuresis is a common consequence of acute exposure to both conditions. Additional water is lost to the dry air through respiration, especially with the hyperventilation of exercise. Body water loss is also increased through sweating, especially if the individual is wearing excess clothing and engages in physical activity. Finally, fluid intake is often limited under these circumstances because of the response to stress, lack of fluid availability, or desire to control urine formation. The resulting reduction in body water, including blood and plasma volume, will decrease the ability to sweat. Thermoregulation is also affected by a decrease in body water due to the decrease in body heat transfer to the periphery with the decrease in blood volume because it is the blood that carries the body heat to the periphery, where it is given up to the environment through evaporative heat loss. Body fluid losses of greater than 10 percent of total body water are life threatening.

Some of this lost water will be replaced with metabolic water which is produced in greatest amounts with the burning of carbohydrate, the fuel of choice at altitude, and a fuel of significance in the cold. In spite of this, water balance is difficult to attain at altitude or in the cold due to the excessive losses, and the difficulties in supply.

11. What is the effect of cold and altitude exposure (at rest) on basal energy requirements?

Exposure to either cold or high altitudes significantly increases the energy needs of the body. In both cases, basal energy needs² (BMR) are elevated by as much as 15 percent after acclimatization. In the cold, this elevation in energy requirements is consequent to the need to maintain body temperature. The cause of the increase at altitude is not as clearly defined but may be associated with the increased respiratory rate and difficulty in sleeping.

During acute exposure to high altitudes, both the magnitude and time frame of changes in the BMR will vary with total energy intake and environmental conditions. Generally, the BMR increases by 20 to 40 percent over BMR during the first days at high altitudes, and then falls somewhat over the ensuing 3 to 10 days. There may be some loss of lean body mass during this time period, occurring simultaneously with inadequate energy intake, as BMR begins to decline toward the level that existed prior to altitude exposure. In experiments where energy intake has been matched to increased needs, basal needs remain elevated throughout the time spent at high altitudes. Individuals who are native to high-altitude environments show an elevated basal energy requirement in comparison to low-altitude natives of similar body size. The basal energy needs of soldiers can, therefore, be expected to be elevated in cold, high-altitude environments.

Nutritional Requirements

12. What are typical energy requirements for work in cold and in high-altitude environments?

Work in the cold or at high altitudes may result in very high energy requirements. When doubly labeled water techniques were used to determine energy expenditures, mean total energy requirements of 3,400 to 4,300 kcal/d (or 2.5 to 3 times BMR) were recorded in sedentary male military personnel in the cold or at high altitudes. Under training conditions, the energy requirements increased to as much as 5,000 kcal/d. The individual requirement will depend on body size, clothing, and activity level, but energy requirements of 54 to 62/kcal/kg/d are recommended for these environments. Individual requirements may reach as much as 4 to 7 times BMR for short periods of time, especially when activities are being performed in clothing that restricts movement. No available studies define the total energy requirements during military operations under conditions of both intense cold and high altitudes. It should be noted, however, that there is no evidence that the actual energy expenditure of the work done is increased under the conditions of altitude exposure, although the hobbling effect of working in protective gear in the cold may increase appreciably the energy expended in given activities.

13. Does cold or altitude exposure alter the requirement for nutrients other than energy?

With the possible exception of vitamin E, there appears to be little scientific basis at this time to indicate that cold or altitude exposure changes the nutritional requirements for any vitamins or minerals. Questions have been raised about increased needs for vitamin C, iron, zinc, and copper in cold and in high-altitude environments. The MRDAs for operational rations (AR 40–25, 1985) supply generous amounts of nutrients over the requirements in normal conditions and should be adequate to meet any small increases in requirements due to cold or altitude.

14. What is the sodium requirement for hard physical work in a cold environment?

Sodium requirements in the cold have not been the subject of specific investigation. Although there has been monitoring of sodium status in individuals participating in metabolic research in cold environments, the focus of these studies was not to determine sodium requirements, and thus dietary intake of sodium was not controlled.

There is good reason to conduct research on sodium requirements in cold environments especially where hard physical work is required. Excessively high sodium intake can lead to increased diuresis, which is a major concern in cold environments. On the other hand, it is well known that significant sodium loss can occur during heavy physical activity. This loss of sodium through sweating occurs in cold-weather conditions when body heat is allowed to build up in heavy clothing. The loss will likely be reduced after acclimatization occurs. In the absence of more data, it is recommended that sodium intake be maintained as recommended in the MRDAs with no additional amounts given for hard physical activity. It is unlikely that electrolyte complications will occur, such as those associated with hard physical work in hot environments.

15. What is the relationship between fluid intake and food intake in the cold or at altitude?

Requirements for food are clearly distinct and separate from requirements for water, even though some foods may partially satisfy water requirements and generate metabolic water after they are consumed. The distinction between needs for water and food is most evident in hot, arid desert-like conditions, where water needs are greatly increased because much body water is lost by physiological mechanisms used to maintain normal body temperatures. This distinction is equally necessary for working in cold and in high-altitude environments.

Extremes of both cold and high-altitude environments have independent effects upon the nutritional and physiological requirements for food and water. As detailed in several chapters (for example, see Chapters 7, 10, and 11 in this volume), physiological responses to extreme cold induce metabolic heat production, which in turn increases the need for an adequate intake of dietary energy. Cold stress also leads to dehydration because of cold-induced diuresis, a phenomenon stimulated by several possible physiologic mechanisms (see review in Chapter 1 in this volume). Body water losses are also increased in the cold because of increased losses of respiratory water as well as losses induced by sweating. Cold conditions tend to reduce fluid intake because of logistical difficulties in supplying water and in preventing it from freezing. Water discipline is as important during cold stress as it is during heat stress.

The pair of problems created in meeting food and fluid needs is exaggerated when high-altitude stress is superimposed upon cold stress. Fluid needs become complicated by physiologic processes and hormonal effects that induce an antidiuretic effect in some individuals (see Anand and Chandrashekhar, Chapter 18 in this volume). This effect can contribute to AMS as well as to high-altitude pulmonary edema. More commonly, however, dehydration may become a potential military problem. Dehydration can result from several causes (see Cymerman, Chapter 16 in this volume), including reduced thirst, inadequate fluid intakes (from both water and foods), and increased sensible and insensible water losses associated with exercise. Again, water discipline is a military necessity.

Increased energy needs are also a separate but important issue at high altitudes. Weight loss is a common reality that must be met by increasing fuel intakes to meet additional energy needs (see Butterfield, Chapter 19 in this volume). Dehydration may also result in anorectic symptoms and lowered food intake. The provision of high calorie, energy dense snack-type foods was recommended by several participants in this workshop and by the CMNR in a previous report (IOM, 1995) as a potential means of providing extra food energy. Thus, the needs for supplying fluids and for supplying food must each be approached as equally important, and logistical support for cold and high-altitude work in the military must take into consideration the distinct differences in effort that are required for the adequate provision of each.

These issues can be summarized in two general questions:

1. Aside from increased energy demands, do cold or high-altitude environments elicit an increased demand or requirement for specific nutrients?

Cold and/or high-altitude environments can increase the needs for two important nutrients, water and carbohydrate (see answers to Questions 2, 5, 8, 10, and 15 in this chapter). Additional fat may sometimes be required to supply energy needs under certain circumstances (see reply to Question 4). While preliminary studies of increasing vitamin E intake to 400 mg/d show research promise in providing protective effects at high altitudes, considerable additional research is needed before questions regarding efficacy and effective doses are fully addressed and before implementing a supplement policy (see reply to Question 6). The needs for certain other single nutrients, i.e., vitamin C, iron, zinc, copper, and sodium, may also be increased (see comments on Questions 7, 13, and 14), but because currently available data are inadequate, additional research will be needed to identify and define any increased needs for these nutrients. There is no evidence at this point to indicate a need for any of the nutrients to be provided at levels beyond that included in the MRDAs. For further elaboration see Chapter 1.

2. Can performance be enhanced in cold or high-altitude environments by the provision of increased amounts of specific nutrients?

Very little research is available to support any need to administer single nutrient dietary supplements in cold or high-altitude environments. As noted in an earlier CMNR report on Food Components to Enhance Performance (IOM, 1994), a number of nutrients have been investigated for these purposes but rarely under environmental conditions of cold or high altitudes. Harris R. Lieberman and his colleagues at the U.S. Army Research Institute of Environmental Medicine (USARIEM) have investigated single dose tyrosine pretreatment (100 mg/kg) in humans subjected to a 4.5-h exposure to cold and hypoxia. In a single controlled study, this large supplement significantly decreased symptoms, adverse moods, and performance impairments (Banderet and Lieberman, 1989). This work has been further expanded in several studies by the Naval Research group (Ahlers et al., 1994; Shurtleff et al., 1994). In rats, tyrosine pretreatment (400 mg/kg) reversed the behavioral depression caused by a forced swim in cold water, although it had no influence on deep body cooling (Rauch and Lieberman, 1990). These preliminary findings are worthy of additional future studies.

Water and Dehydration

• Because cold-induced dehydration can cause serious performance decrements, it must be anticipated. The training of military personnel assigned to cold-weather operations must include water discipline, safe fluid sources because snow or ice are generally unsafe, and the protection of drinking fluids from freezing.
• Water discipline is as important during military operations in intense cold and at high altitudes as it is during desert heat. Training should include water discipline measures following guidelines in military doctrines and means for their enforcement.

Energy and Specific Nutrients

• Because of increased energy demands of cold operations, dietary energy sources must be adequate to meet actual or anticipated needs , including the needs for adequate carbohydrate foods. A field feeding doctrine, as previously recommended (IOM, 1995), should be considered. Pre-exposure diets should insure that muscle glycogen stores become optimized. Meal times should be standardized whenever possible in order to encourage increased food intake.
• Carbohydrate intake should be promoted during military operations conducted in the cold or at high altitudes. The inclusion of a liquid or solid carbohydrate supplement in the rations of troops may be useful in maintaining macronutrient balance and performance over time. Carbohydrate intake should be at least 400g/d under these conditions. When energy expenditures are high and total caloric intake is increased, the CMNR recommends that carbohydrate intake be increased to maintain calories from carbohydrate in the range of at least 40 percent of total caloric intake. This will help provide a palatable diet that is not excessive in fat content.
  The Ration, Cold Weather (RCW), MRE, and LRP I as currently prescribed for cold-weather operations all provide a minimum of 4,300 kcal and 582 g carbohydrate per day (see Table $1$2$3). The percentages of calories in these rations that are contributed by carbohydrate are 49 percent for the MRE, 60 percent for the RCW, and 50 percent for the LRP I. Thus these all meet the recommended criteria.
• Restriction of fat calories to only 30 percent, as recommended for the American civilian population, is not appropriate for some military operational rations where caloric density, ration bulk, and palatability are of concern. This is particularly the case for rations designed for use in cold and in high-altitude environments. The percentages of calories in currently available rations that are contributed by fat are 36 percent for the MRE, 32 percent for the RCW, and 35 percent for the LRP I. These levels appear appropriate, given the situational requirements.
• Sharing of food rations is encouraged as a means of meeting the higher than average caloric needs of some individuals in the field.
• In the absence of more data concerning sodium requirements during heavy exercise in conditions of arctic cold, normal sodium intakes should be maintained.

Education and Logistics

• Because physiologic responses and adaptations differ importantly between moderately high altitudes (8,00$1$2$32,000 ft [2,43$1$2$3,658 m]) and extremely high altitudes (greater than 18,000 ft [5,486 m]), planning for military training or military missions at high altitudes should take these differences into account.
• Individuals who have not yet regained lean body mass lost in prior field operations should not be deployed to cold or high-altitude environments until lean body mass is regained.
• Military troops, leaders, and medical personnel being assigned to high-altitude training or missions should be fully instructed on the symptoms and signs of AMS, subacute mountain sickness, high-altitude pulmonary edema (HAPE), and high-altitude cerebral edema (HACE). In addition, they should be trained in the use of appropriate countermeasures and therapy.
  Because about 25 percent of people seem to be "immune" to AMS, military personnel who have successfully completed a tour at altitude should be the ones selected for assignment to altitude missions of unique military importance. Conversely, those who have developed AMS during training at high altitudes should be excluded in advance from participating in such unique military missions whenever possible.
• Information about possible changes in physical performance, alertness, and emotional stability associated with hypoxia should be provided to all levels of command so that soldiers and their leaders will not be surprised when they occur. Breakdown in troop cohesion should be anticipated.
• Because weight loss is common during military operations at high altitudes, command and logistical practices should attempt to ensure, whenever possible, that the availability of palatable foods and fluids, as well as the social setting at mealtimes, are optimized to insure adequate dietary intakes (see IOM, 1995).
• Logistical measures for cold-weather operations must put primary emphasis on the delivery and maintenance of sufficient food stores and unfrozen dietary fluids.
• Military rations are the fuel for the soldier and emphasis should be placed on adequate availability and consumption of operational rations to maintain performance in these harsh environments.

Water and Dehydration

Further research is needed:

• to define the best strategies (including pharmacological ones) to avoid cold-induced dehydration.
• to define the water needs of the body during the early phases of exposure to high altitudes, along with its relationship to the diuresis experienced by many subjects, and importantly, to the development of acute mountain illnesses.
• to define the potential "value" of dehydration in association with long-term stays at moderate altitudes and to define the limits whereby such dehydration might be preventable, beneficial, or detrimental.

Energy

Further research is needed:

• to assess the applicability to the military, both men and women, of the finding that it may be possible to maintain body weight, nitrogen balance, and muscle protein mass at optimal values during high-altitude missions.
• to define energy requirements during military operations in which simultaneous exposures to intense cold and high altitudes occur, by validation of the "free-living" estimation of energy requirement based on Hoyt and Honig's proposed use of body weight, foot strike, and terrain (see Chapter 20 in this volume).
• to understand the metabolic aspects of shivering.

Specific Nutrients

Further research is needed:

• to define more precisely the carbohydrate intake required to maintain body glycogen stores and to replenish stores depleted by exercise in the cold and at high altitudes.
• to establish whether a specific proportion of calories from fat, carbohydrate, or protein has a clear-cut advantage in maintaining thermoregulation in cold environments.
• to determine the optimum intake of micronutrients for improving performance in the cold. Such studies must control for nutrient status prior to and at the time of testing, the training level of subjects, and intensity and duration of any exercise to be tested.
• to determine sodium requirements during heavy exercise in intensely cold conditions and the possible advantages of restricting sodium during the first few days at altitude.
• to determine the possible beneficial effects of anorexia at altitude.
• to determine whether supplemental doses of vitamin E have any protective effects on humans exposed to oxidative stress.
• to determine if supplements of vitamin C, iron, zinc, copper, and/or other nutrients could improve performance during the stresses of extreme cold and high altitudes.

Performance and Medical Conditions

Further research is needed:

• to explore the merits of some potentially useful pharmacological compounds such as theophylline, caffeine, and ephedrine, as well as the potential value of prestress tyrosine administration.
• to evaluate possible pharmacological, physiological, and nutritional methods, either in the field or in altitude chambers, to predict, prevent, and/or treat AMS.
• to consider the pathophysiological problems of salt and water balance and the intercompartmental shifts in body fluids at altitude. Physiological mechanisms requiring additional study include cardiovascular, renal, endocrine, metabolic, and biochemical responses.
• to resolve conflicting data on possible effects of cold exposure on muscle strength and endurance.
• to examine the relationship between the aging process and acclimatization. Research in this area would not only be beneficial to the military but the general American population.
• to improve the understanding of mood and performance changes over time in subjects exposed to high altitudes and hypoxia, using animal models for the purpose of identifying the neurochemical cause(s) of those changes.
• to develop follow-up drug and nutrient intervention strategies for ameliorating chemical changes, and thus, ideally mood and performance decrements at high altitudes.
• to develop and evaluate diet-focused pharmacologic countermeasures (e.g., tyrosine and caffeine), with the ultimate goal of applying such countermeasures in field situations to stem the decline in cognitive function that accompanies the exposure to adverse environmental conditions.
• to define the effects of acclimatization to high altitudes in terms of altered performance measures and to optimize nutrition for more rapid acclimatization.
• to address the impact of preexisting malnutrition on the performance of soldiers at environmental extremes of cold and altitude, possibly through the use of key nutrient deficiency screening procedures to be administered to individuals prior to their participation in unique military missions or training in the cold and at high altitudes.

Military Ration Development and Guidance

Further research is needed:

• to insure that consumption of rations specially developed by the Army for use in cold-weather conditions provides intakes of energy, protein, and micronutrients that fully meet the increased requirements of troops operating in the field.
• to optimize packaging, delivery, and serving of these rations to insure that adequate amounts are consumed.