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11
Macronutrient Requirements for Work in Cold Environments

Peter J. H. Jones1 and Ian K. K. Lee

INTRODUCTION

There is an important need to characterize accurately the energy and macronutrient requirements of individuals living and working in cold climates. In addition, this information is necessary to ensure optimal performance in military field settings where sizable effort is required to transport required food supplies and associated materiel. During field exercises, either excessive or inadequate food supplies will hamper unit performance capacities.

Several previous studies have suggested that caloric needs in military personnel should be increased in the cold; however, estimates range considerably (Johnson and Kark, 1947; King et al., 1993; LeBlanc, 1957; Rodahl, 1954; Swain et al., 1949). The current U.S. Military Recommended Dietary Allowance (MRDA) for males in environments that are colder than 57°F (14°C) is 4,500 kcal/d (AR 40-25, 1985). Knowledge of the impact of cold on energy and macronutrient utilization by military personnel both at rest and

1  

Peter J. H. Jones, School of Dietetics and Human Nutrition, McGill University at MacDonald Campus, Quebec, Canada H9X 3V9



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--> 11 Macronutrient Requirements for Work in Cold Environments Peter J. H. Jones1 and Ian K. K. Lee INTRODUCTION There is an important need to characterize accurately the energy and macronutrient requirements of individuals living and working in cold climates. In addition, this information is necessary to ensure optimal performance in military field settings where sizable effort is required to transport required food supplies and associated materiel. During field exercises, either excessive or inadequate food supplies will hamper unit performance capacities. Several previous studies have suggested that caloric needs in military personnel should be increased in the cold; however, estimates range considerably (Johnson and Kark, 1947; King et al., 1993; LeBlanc, 1957; Rodahl, 1954; Swain et al., 1949). The current U.S. Military Recommended Dietary Allowance (MRDA) for males in environments that are colder than 57°F (14°C) is 4,500 kcal/d (AR 40-25, 1985). Knowledge of the impact of cold on energy and macronutrient utilization by military personnel both at rest and 1   Peter J. H. Jones, School of Dietetics and Human Nutrition, McGill University at MacDonald Campus, Quebec, Canada H9X 3V9

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--> during physical exertion is of great value in establishing energy and macronutrient contents for rations in cold environments. The objective of this chapter was to reassess the older literature and review the most recent literature dealing with energy and macronutrient requirements in the cold, focusing particularly on data obtained in trails that measured energy expenditure levels. Older estimates of requirements based on food intake data may be unreliable due to underestimation of energy consumption. The overall goal was to answer two questions. First, are caloric requirements elevated in the cold, and if so, by how much? As part of this question, mechanisms for any such increase were examined. Second, what is the dietary consumption ratio of protein, carbohydrates, and fat that optimizes performance in cold environments? ENERGY BALANCE AND REQUIREMENT IN THE COLD The energy balance of an individual can be determined using the following equation: where EE is energy expenditure, EI is intake, and ΔS is change in body energy stores over the period of measurement. For individuals maintaining body composition, EE = EI; thus EE, and, therefore, the energy requirement, can be determined using an estimate of metabolizable EI. In situations where individuals change body energy stores, as is the case in many studies to be discussed, EE can still be calculated from EI; however, ΔS needs to be established accurately, which is often difficult using current techniques. Past studies that examined energy needs in military personnel in cold environments have used both EI and EE to estimate these needs. Studies Showing Increased Requirements Using Energy Intake Data Historically, reports concerning effects of environmental temperature on energy needs began shortly after the Second World War when Johnson and Kark (1947) demonstrated a relationship between climate and reported food intake in several groups of military personnel given unlimited access to food rations. These authors suggested an inverse correlation between local mean temperature and energy reported to be consumed per man per day. Intakes ranged from 3,100 kcal/d in the desert (91°F [33°C]) to 4,900 kcal/d in arctic conditions (-29°F [-34°C]). Other earlier studies similarly indicated higher caloric needs of troops in arctic settings. Swain et al. (1949) showed that troops in garrison at Fort Churchill, Northwest Territory, Canada over two

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--> successive winters required caloric intakes of over 5,000 kcal/d. About 100 men participated in these studies, with each man spending about 3 hours each day outside. Food and beverages issued and food waste returned were recorded in the canteen daily for 10 days for each man. However, uniform records of activity were not obtained, which precluded precise assessment of movement or exercise. Weight records were kept for 16 individuals during one of these trials. It was found that over a 20-wk time period, body weight increased by an average of 1.55 kg. If this weight gain represented an increase in fat mass alone, it would account for, at most, approximately 100 kcal excess per day. Therefore, the majority of calories consumed would have been expended as energy. Overall, studies that determined caloric needs using estimates of EI have demonstrated an increase in energy requirement for individuals working in cold climates as just described. However, these studies are not without exception as described below. Studies Showing Minimal Effects of Cold Environments on Energy Requirement Determined by Energy Intake Not all studies of caloric requirements in the cold have found an increase. LeBlanc (1957) suggested that food intakes of groups of persons living in the cold and taking part in military exercises were about 3,900 kcal/d. Similarly, Rodahl (1954) examined the nutritional requirements of a group of airmen and infantrymen in garrison in Alaska. These individuals had resided in Alaska for about a year prior to the study. Men slept in warm quarters, with outdoor temperatures ranging from -14° to 32°F (-26° to 0°C), and had low activity levels. Food was carefully weighed, and intake data were collected at mealtime, with subjects recording additional snack items separately. Mean caloric intake ranged between 3,100 and 3,400 kcal/d, with a mean of 3,200 kcal/d and no overall body weight change. Similar intakes were observed in native Eskimos. Rodahl concluded that previous studies probably overestimated caloric needs for troops in arctic conditions. He mentioned that these troops were accustomed to the terrain and operations in the Arctic. However, it cannot be ruled out that energy intakes were underestimated; thus, caloric consumption data erred on the low side. At least some of the food consumption data were self-reported, which cannot be considered reliable, as will be demonstrated. More recently, King et al. (1993) reported that troops living in Alaska in tents during the winter, with temperatures as low as -18°F (-28°C), recorded intakes of 3,000 to 3,300 kcal/d (Table 11-1). In contrast, the energy expenditures in a subgroup of these individuals averaged 4,253 kcal/d as measured by the technique of doubly labeled water (DLW) described below.

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--> TABLE 11-1 Military Recommended Dietary Allowances and Reported Intakes of Subjects Consuming the Meal, Ready-to-Eat and Long-Life Ration Packet Nutrient MRDA* MRE† LLRP‡ Energy, kcal 4,500 3,271 3,035 Protein, g 100 134 111 Carbohydrate, g 619§ 375 376 Fat, g 175§ 138 123 * Military Recommended Dietary Allowance for males > 17 years old for a cold environment (<57°F [<14°C]) (AR 40-25, 1985). † Meal, Ready-to-Eat subgroup (provided ad libitum). ‡ Long-Life Ration Packet subgroup (provided ad libitum). § Military feeding guidelines suggest energy intake to be 50 to 55 percent from carbohydrate and 35 to 40 percent from fats (AR 40-25, 1985). SOURCE: Adapted from King et al. (1993). The difference in calories consumed versus those expended in the subgroup appeared to be made up by calories lost as fat (1.4 kg over 10 days). However, once again, under rugged living conditions, it is difficult to determine accurately the quantity of food consumed. Inaccuracies in self-reported food and beverage intake may be a significant factor in explaining the results of this and other studies that suggest lower food intake, and thus food requirements, in the cold. Discrepancies in these results may be due to this important factor that has emerged only recently. Underreporting of Energy Intakes Results of considerable numbers of recent studies suggest that underreporting of energy intake is an important limitation of nutrient intake assessment as obtained by self-reported food intake instruments. Underreporting has been shown both in civilian and military populations. Schoeller et al. (1990) have demonstrated that overweight individuals, in particular, significantly underreport their food intake. For instance middle-aged women (29 percent body fat) who were maintaining body weight reported energy intakes (as determined by self-reported food consumption instruments) that were 25 percent lower than values derived from DLW (Martin et al., in press) (Figure 11-1). In this study, individuals were carefully instructed and repeatedly reminded to be thorough in the self-reporting of food intakes. Military personnel who were training for jungle warfare similarly reported caloric intakes that were about 85 percent of expenditure levels as determined using DLW (Forbes et al., 1989).

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--> FIGURE 11-1 Degree of underreporting in middle-aged women where energy expenditure was determined by doubly labeled water, and energy intake was determined by self-reported food intake record analysis. SOURCE: Adapted from Martin et al. (in press). Findings of a study by the authors have shown significant underreporting in subjects who maintained relatively constant body weight over a 10-d period in the Arctic (Jones et al., 1993). Subjects were instructed to record each evening all food and drink consumed throughout the day and were prompted to be thorough in their recollection at each occasion. Compared with mean expenditure levels of about 4,300 kcal/d, self-reported caloric intakes derived from food records averaged only 61 percent of expenditure (Figure 11-2). Some of this difference might be attributable to small changes in body composition. However, because body composition remained reasonably stable, most of the discrepancy was attributable to underreporting. Thus, particularly for experiments conducted in harsh environments, individual energy requirements are better determined using approaches based on energy expenditure, rather than those based on intake and balance. Underreporting of actual food intakes may occur for several reasons in hostile environments. First, subjects are exposed to stress, which may cause them to forget to record certain items. Second, scheduling of operations may disrupt the routine of camp or garrison life in such a way that reporting opportunities are less frequent. Third, at some stage during the test, individuals may have consumed foods or beverages that they did not reveal to authorities. Military troops during intensive training are often not highly motivated study participants.

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--> FIGURE 11-2 Comparison of energy expenditure (Exp.) calculated by doubly labeled water (DLW) with self-reported energy intake (Int.) in DLW group (infantrymen during a cold-weather field exercise on Baffin Island, Northwest Territory, Canada). Values are mean ± SD per total body weight and per kg FFM. FFM, fat-free mass. SOURCE: Jones et al. (1993), used with permission. Studies Measuring Energy Requirements Using Energy Expenditure Data To determine with accuracy the overall caloric needs of military troops in the cold, methods are required that provide integrative, accurate measures of energy expenditure over a representative time period. One such method is DLW, which is well suited to such field studies in that it is noninvasive, does not involve use of radio tracers, and requires minimal subject compliance. The procedure utilizes the difference in rate of elimination from the body between oxygen and hydrogen, measured following administration of water, isotopically labeled with a heavy isotope of oxygen (18O) and hydrogen (deuterium) (D), to calculate the rate of carbon dioxide production of an individual. Caloric expenditure can be determined from carbon dioxide production, given knowledge of the food quotient or respiratory quotient of the subject. This technique has been validated against the methods of respiratory gas exchange (Ravussin et al., 1991; Schoeller and Webb, 1984; Westerterp et al., 1988) and caloric intake balance (Jones and Leitch, 1993; Schoeller et al., 1986) in a number of laboratories. The DLW technique was found to be accurate to within 2 to 6 percent. At least four experiments have determined total integrated energy expenditure in troops exposed to cold conditions. First, DeLany et al. (1989)

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--> used DLW to determine energy expenditure in relatively sedentary troops exposed to outdoor temperatures ranging from 30° to 61°F (-1° to 16°C). Expenditure levels averaged 3,400 kcal/d, or about 45 kcal per kg body weight per day. In another experiment in a more severe cold setting, Hoyt et al. (1991) reported energy expenditure for 23 Marines during strenuous exercise in a higher-altitude operation in Bridgeport, California (7,218 ft [2,200 m]). Ambient temperatures ranged from 5° to 55°F (-15° to 13°C). Energy expenditure measured by DLW over the entire 10-d period averaged 4,919 kcal/d, with a peak level of over 7,000 kcal/d during the initial highly strenuous portion of the study. During less extreme activities, energy expenditure averaged 3,632 kcal/d. Over the entire study these subjects' caloric requirements were 62 kcal per kg body weight per day as measured using DLW. The subjects also lost an average of 2.5 kg of body weight, with 1.7 kg loss as fat. Third, in a more severely cold environment, troops at Fort Greely, Alaska were observed to have expended 4,253 kcal/d as measured by DLW during a 10-d trial period where the average temperature was -18°F (-28°C) (King et al., 1993). Troops were fairly active over the test period. This result represents an expenditure level of 54 kcal per kg body weight per day. Subjects lost about 1 kg of fat mass on this training exercise. Studies in this laboratory have determined caloric expenditure and requirements at temperatures colder or similar to those of the Fort Greely experiment (Jones et al., 1993). Energy expenditure and requirements were determined in a group of Canadian infantrymen during a cold-weather field exercise on Baffin Island, Northwest Territory, Canada. During most of the experiment the outdoor temperature rarely exceeded -13°F (-25°C), with the average temperature remaining below -22°F (-30°C). Subjects slept in tents where the temperature ranged from below freezing to 59°F (15°C) and on one or two nights slept in unheated igloos. The subjects performed outdoor exercises during this period which ranged in severity (Table 11-2); however, most of these activities were not highly strenuous. Mean caloric expenditure level was 4,317 kcal/d or 57 kcal per kg body weight per day. This expenditure was about equivalent to that provided in a single day's standard-issue individual meal pack (IMP) rations (4,350 kcal/d) if all foods were consumed. In this trial a fairly high level of sharing of ration pack items was casually observed. Individuals with caloric requirements below the average may have passed uneaten food items to those with higher needs. For this reason, it may not be necessary to provide energy at a level that meets the needs of all individuals within the population. Encouraging food-sharing practices among personnel at mealtime permits the standard to be set close to the population mean requirement, while saving food production and transport costs and reducing

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--> TABLE 11-2 Type and Duration of Activities of Infantrymen during an Arctic Field Exercise Study Day Activity and Average Duration 0 Arrival at hanger 1 Deployment to camp (4), camp setup (6) 2 Skiing (3), ski-during (3), compass training (3) 3 Skiing (3), ski-during (3), compass training (3) 4 Snowmobiling-hunting (5) 5 Snowmobiling-hunting (5) 6 Ski-during (3), snowmobiling (2), ice-fishing (5), hunting (3) 7 Igloo building (8), hunting (3) 8 Snowmobiling (3), return to hanger (4) 9 Public relations exercises (6) 10 Public relations exercises (6) NOTE: Average duration of each activity in hours is given in parentheses. Ski-during is a troop transport system involving skiers towed behind skidoos. Public relations exercises were held at Iqaluit, Baffin Island, Northwest Territory, Canada. SOURCE: Jones et al. (1993), used with permission. waste. Food sharing also helps ensure that individual needs for nutrients other than energy are met, as levels of consumption are, to a certain extent, tied to energy intake. Mechanism of Action of Effects of Cold on Metabolism Johnson and Kark (1947) were the first to speculate on the cause of increased energy requirements in the cold. They suggested that greater energy needs resulted from the hobbling effect of winter terrain, clothing and equipment, as well as the added heat required to maintain the body's thermal equilibrium. Subsequent studies have provided data to support these original suggestions. Locomotion in snow is energy-intensive due to terrain and friction-related factors. A number of older experiments have characterized the energy cost of specific activities in snow and cold conditions. For instance, the energy costs of snowshoeing and skiing have been reported to be quite significant, though

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--> variable, depending on snow conditions and geography (McCarroll et al., 1979). In addition to the energy cost of activity itself, heavy clothing worn in the cold restricts movement. Indeed the hobbling influence of winter clothing was determined to be a factor contributing to increased energy costs in studies by Gray et al. (1951). They demonstrated that caloric output for a given amount of external work performed at a constant temperature increased about 10 percent between light and heavy clothing, regardless of the environmental temperature. Also, the caloric output for a given amount of external work performed in a given outfit of clothing increased about 4 percent from a working temperature of 90°F (32°C) to one of -15°F (-26°C). These findings suggest that the influence of temperature on energy metabolism involves both hobbling and other factors. Similarly, and more recently, Teitlebaum and Goldman (1972) demonstrated that the energy cost expended walking while wearing a multilayered arctic clothing system was greater than that expended when subjects carried an equivalent weight as a single-layer outfit plus a belt with added weight. An increase of 16 percent in energy cost was measured in individuals carrying out the same activity while wearing the clothing over that of the belt. This increase was attributed to friction drag between layers of material and interference with movement of body joints because of the bulky clothes. However, the room temperature was 63°F (17°C), which may have produced some overheating in the more heavily dressed subjects. The second factor that potentially contributes to increased energy requirement in the cold is the need to maintain thermoregulation. It has been established that exposure to cold increases oxygen consumption and metabolic rate (Gray et al., 1951; Timmons et al., 1985). The importance of this second component will depend on such factors as adequacy of thermal insulation of clothing worn to protect against heat loss, as well as environmental temperature, windchill, and duration of cold exposure. It has been suggested that the lack of increased energy requirement observed in some studies is the result of subjects spending long periods indoors (Rodahl, 1954). Thus, when specific logs of activity are not maintained (that is, not only type and duration of activity, but environmental conditions as well), the results of energy requirement studies are of questionable usefulness in determining energy needs under field conditions. This is particularly true for studies based on energy intake data. OPTIMAL MACRONUTRIENT RATIO IN THE COLD The second question that must be addressed concerning macronutrient needs in the cold is the pattern (or proportion) of macronutrients required to optimize survival and performance. This question is important because energy

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--> sources differ in caloric density. Calories stored as fat provide a higher energy level than the same mass of carbohydrate. If performance is unaltered by the macronutrient ratio, then rations providing a higher percentage of calories as fat would be more compact and lighter. In addition, association of enhanced thermogenesis with a particular macronutrient may play a role in selecting the optimal dietary blend of nutrients. In this context, it is interesting to examine the diet of indigenous peoples of the North. Hoygaard (1941) reported that primitive Eskimos apparently prefer a diet comprised of almost one-half fat and one-half protein. Eskimos maintain that fat is required to keep them warm on long journeys. Whether this blend of macronutrients is one that possesses thermogenic properties for Eskimos, or whether it is actually the only dietary macronutrient source available during the winter is a provocative question. Among military personnel, Johnson and Kark (1947) showed that troops consumed a consistent ratio of macronutrients regardless of environmental temperature (Table 11-3). Similarly, Swain et al. (1949) compiled available data to demonstrate that the ratio of protein, fat, and carbohydrate that was consumed voluntarily by troops in the Arctic did not vary significantly from that of troops in other environments (Table 11-4). The proportion of calories TABLE 11-3 Caloric Consumption and Ratio of Macronutrients Eaten by Representative Groups of Troops in Different Environments       Percentage of Calories Provided by Location, Troops Environment Caloric Intake/Day (kcal) Protein Fat Carbo-hydrate Canada, mobile force ''Musk Ox" Arctic and subarctic 4,400 11 40 49 U.S.A., ground troops Temperate 3,800 13 43 44 Colorado Rockies, infantry Temperate mountain (9,000 ft [2,743 m]) 3,900 13 34 53 Pacific Islands, ground troops Tropics 3,400 13 33 54 Pacific Islands, infantry Tropics 3,200 12 34 54   SOURCE: Reprinted with permission from R.E. Johnson and R.M. Kark, "Environment and food intake in man," Science 105:378–379. Copyright 1947, American Association for the Advancement of Science.

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--> TABLE 11-4 Percentage of Macronutrients Eaten by Troops in Various Environments   Proportion of Macronutrient (%)   Environment, Location Protein Fat Carbohydrate Arctic and subarctic areas       Fort Churchill 13 37 50 Fort Churchill 12 41 47 Fort Churchill 13 42 45 Exercise "Musk Ox" 13 42 45 Temperate areas       U.S. Army Zone of Interior* 13 43 44 U.S. Army Zone of Interior, mountain troops 14 44 42 Tropical areas       U.S. Army on Guadalcanal 13 34 53 U.S. Army on Hawaii 13 33 54 U.S. Army on Guam 13 32 55 U.S. Army on Iwo Jima 13 33 54 U.S. Army on Luzon, Philippines 12 34 54 * Zone of Interior, 1941–1943: accumulated data on various phases of messing operations in the training camps of the U.S. Army. The data were obtained over short, unannounced periods of time ranging from 6–10 days in each mess. The results of each post were reported as a 30-d average of these individual surveys, and are considered to provide accurate representative information on average food consumption. 50 posts and 455 messes were surveyed in this manner. SOURCE: Swain et al., J. Nutrition, 38:63–72, 1949, used with permission. obtained from each macronutrient was similar in arctic, temperate, and tropical diets. Thus, the tendency of Eskimos to consume high-fat and -protein diets in cold climates was not replicated among military personnel. More recently, King et al. (1993) have shown that troops stationed at Fort Greely, Alaska during winter consumed about 16 percent protein, 37 percent fat, and 48 percent carbohydrate. Also Hoyt et al. (1991) reported that subjects fed field rations consumed an average of 13 percent, 38 percent, and 49 percent of total energy as protein, fat, and carbohydrate, respectively. In summary, there appears to be no tendency on the part of military personnel toward a deliberate shift in the pattern of macronutrients consumed in colder climates. However, consistent with the high-fat diet of the indigenous cultures of the North, there is a suggestion that the macronutrient mixture is modified in the cold. Timmons et al. (1985) showed a decrease in respiratory exchange ratio in individuals exposed to 14°F (-10°C) for 90 minutes, indicating a preferential fat oxidation as much as 41 percent higher than that of controls. In contrast, Vallerand and Jacobs (1989) showed that carbohydrate

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--> oxidation was elevated by as much as 588 percent when seven healthy male volunteers, wearing only shorts, were exposed to 50°F (10°C) temperature with a 1 m/s wind. The reason for the disparity between the results of these studies remains unclear. One possibility that cannot be ruled out is that exercise was a factor in only one of these studies. Finally, Issekutz et al. (1962) showed that a full day of cold exposure resulted in a significant increase in the production of urea, suggesting that protein utilization may become more relevant in prolonged exposure to cold. Continuing efforts should be undertaken to determine further (1) the relative macronutrient requirements to optimize performance and (2) the most acceptable and palatable nutrient ratio. Results of such studies will help to prevent weight loss during highly strenuous cold weather activities. AUTHORS' CONCLUSIONS AND RECOMMENDATIONS Although the majority of research in which energy requirement determination is based on measured or reported food intake is consistent with an increased requirement for energy in colder climates, some studies have found no difference compared to more temperate environments. The factors that contribute to this lack of difference include possible underreporting by study subjects and prolonged time spent indoors at warmer temperatures and low activity levels. To avoid errors that may arise from the underreporting of food intake, energy requirement should be determined by measuring expenditure directly. Several studies that examined expenditure report a range of 45 to 62 kcal per kg body weight per day. Variation in this figure is likely due to both degree of cold and extent of physical activity. Under sedentary conditions in the cold, requirements may range from 3,632 to 4,317 kcal/d or about 46 to 57 kcal per kg body weight per day. In more highly strenuous circumstances in the cold, requirements of 4,200 to around 5,000 kcal/d, or 54 to 62 kcal per kg body weight per day, may be required. The estimate of energy requirement for strenuous circumstances is not out of line with current MRDAs of 4,500 kcal/d or 56 kcal per kg body weight per day for a soldier weighing 80 kg (AR 40-25, 1985). During low-activity situations, particularly in less cold environments, the mean requirement may decrease to almost 900 kcal/d below the current MRDA for cold weather. However, in either circumstance, it is likely that the distribution of requirements will result in the need for some individuals to exceed the MRDA for cold environments. Sharing of foods between personnel should be encouraged to ensure that the requirements of all individuals are met. Food sharing will also reduce food waste and the labor involved in transporting and preparing foods.

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--> Studies show a weight balance difference between troops in garrison and troops in actual field studies. In garrison, troops reside in a more insulated, low-activity environment, causing no noticeable weight loss. Conversely, troops in field studies are engaged in strenuous activities within the natural arctic environment. This environment causes accelerated weight loss due to increased energy expenditure. Macronutrient preference does not appear to change with cold climate in settings where subjects freely select their food. In the absence of adequate clothing, carbohydrate oxidation increases substantially during exercise, although increases in fat oxidation have also been reported in response to cold in exercise situations. Further study is required to elucidate the interaction of cold and physical activity on macronutrient utilization. Regardless of which macronutrient is oxidized preferentially in cold conditions, a major objective of field feeding studies should be to minimize body weight loss and, if possible, performance capacity. Thus, given that there is no natural shift in macronutrient intakes in the cold, food acceptability in the field is more important than macronutrient ratio, particularly in high-activity situations. Food should be prepared in as palatable a form as possible to minimize the gap between level of caloric expenditure and that of intake. REFERENCES AR (Army Regulation) 40-25 1985 See U.S. Departments of the Army, the Navy, and the Air Force. DeLany, J.P., D.A. Schoeller, R.W. Hoyt, E.W. Askew, and M.A. Sharp 1989 Field use of D218O to measure energy expenditure of soldiers at different energy intakes. J. Appl. Physiol. 67:1922–1929. Forbes-Ewan, C.H., B.L.L. Morrissey, G.C. Gregg, and D.R. Waters 1989 Use of doubly labeled water technique in soldiers training for jungle warfare. J. Appl. Physiol. 67:14–18. Gray, E. LeB., F.C. Consolazio, and R.M. Kark 1951 Nutritional requirements for men at work in cold, temperate and hot environments . J. Appl. Physiol. 4:270–275. Hoygaard, A. 1941 Studies on the nutrition and physio-pathology of Eskimos. Det norske videnskapsakademis skrifter. Mat.-Naturv. Klasse no. 9, Oslo. Hoyt, R.W., T.E. Jones, T.P. Stein, G.W. McAninch, H.R. Lieberman, E.K. Askew, and A. Cymerman 1991 Doubly labeled water measurement of human energy expenditure during strenuous exercise. J. Appl. Physiol. 71:16–22. Issekutz, B., Jr., K. Rodahl, and N.C. Birkhead 1962 Effect of severe cold stress on the nitrogen balance of men under different dietary conditions. J. Nutr. 78:189–197. Johnson, R.E., and R.M. Kark 1947 Environment and food intake in man. Science 105:378–379.

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--> Jones, P.J.H., and C.A. Leitch 1993 Validation of doubly labeled water for measurement of caloric expenditure in collegiate swimmers. J. Appl. Physiol. 74:2909–2914. Jones, P.J.H., I. Jacobs, A. Morris, and M.B. Ducharme 1993 Adequacy of food rations in soldiers during an arctic exercise measured using doubly labeled water. J. Appl. Physiol. 75:1790–1797. King, N., S.H. Mutter, D.E. Roberts, M.R. Sutherland, and E.W. Askew 1993 Cold weather field evaluation of the 18-man Arctic Tray Pack Ration Module, the Meal, Ready-to-Eat, and the Long Life Ration Packet. Mil. Med. 158:458–465. LeBlanc, J.A. 1957 Effect of environmental temperature on energy expenditure and caloric requirements. J. Appl. Physiol. 10:281–283. Martin, L.J., W. Su, P.J. Jones, G.A. Lockwood, D.L. Tritchler, and N.F. Boyd In Comparison of energy intakes determined by food records and doubly labeled water in Press women participating in a dietary intervention trial. In Am. J. Clin. Nutr. McCarroll, J.E., R.F. Goldman, and J.C. Denniston 1979 Food intake and energy expenditure in cold weather military training. Mil. Med. 144:606–610. Ravussin, E., I.T. Harper, R. Rising, and C. Bogardus 1991 Energy expenditure by doubly labeled water. Validation in lean and obese subjects. Am. J. Physiol. 261:E402–E409. Rodahl, K. 1954 Nutritional requirements in cold climates. J. Nutr. 53:575–588. Schoeller, D.A. 1990 How accurate is self-reported dietary energy intake? Nutr. Rev. 48:373–379. Schoeller, D.A., and P. Webb 1984 Five-day comparison of the doubly labeled water method with respiratory gas exchange. Am. J. Clin. Nutr. 40:153–168. Schoeller, D.A., R.F. Kushner, and P.J.H. Jones 1986 Validation of doubly labeled water for measuring energy expenditure during parenteral nutrition. Am. J. Clin. Nutr. 44:291–298. Swain, H.L., F.M. Toth, F.C. Consolazio, W.H. Fitzpatrick, D.I. Allen, and C.J. Koehn 1949 Food consumption of soldiers in a subarctic climate. J. Nutr. 38:63–72. Teitlebaum, A., and R.F. Goldman 1972 Increased energy cost with multiple clothing layers. J. Appl. Physiol. 32:743–744. Timmons, B.A., J. Araujo, and T.R. Thomas 1985 Fat utilization enhanced by exercise in a cold environment. Med. Sci. Sports Exerc. 17:673–678. U.S. Departments of the Army, the Navy, and the Air Force 1985 Army Regulation 40-25/Naval Command Medical Instruction 10110.1/Air Force Regulation 160-95. "Nutrition Allowances, Standards, and Education." May 15. Washington, D.C. Vallerand, A.L., and I. Jacobs 1989 Rates of energy substrates utilization during human cold exposure. Eur. J. Appl. Physiol. 58:873–878. Westerterp, K.R., F. Brouns, W.H.M. Saris, and F. Ten Hoor 1988 Comparison of doubly labeled water with respirometry at lowand high-activity levels. J. Appl. Physiol. 65:53–56.