Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations (1993)

Elsworth R. Buskirk¹

This historical review presents some considerations for supplying military personnel with appropriate nutrition as they operate in different climates. Attention is paid to some of the pertinent investigations undertaken related to energy turnover in the 1930s through the 1960s, with a few comments regarding interpretation of results based on more current knowledge.

Many of the earlier appraisals of the nutritional needs of U.S. Armed Forces personnel were published as reports of the various agencies involved. For the purposes of this presentation, the following agencies are identified:

• Quartermaster Food and Container Institute for the Armed Forces, Chicago, Illinois

• U.S. Army Medical Research and Nutrition Laboratory, Fitzsimmons General Hospital, Denver, Colorado

• Aero Medical Laboratory, Wright Air Development Center, United States Air Force, Wright Patterson Air Force Base, Ohio

• U.S. Army Medical Research Laboratory, Office of the Surgeon General, Fort Knox, Kentucky

• Quartermaster Climatic Research Laboratory, United States Army, Lawrence, Massachusetts

• U.S. Army Quartermaster Research and Development Center, United States Army, Natick, Massachusetts

• U.S. Army Research Institute of Environmental Medicine, United States Army, Natick, Massachusetts

Those interested in perusing the various investigations conducted by personnel from these agencies should consult their respective report series because not all summaries of the sponsored work have appeared in the open scientific literature. It is hoped that many of the reports remain on file.

Between 1941 and 1946, reliable data were collected of food intakes for physically fit, active ground troops who chose their foodstuffs from the rations provided in temperate, mountain, desert, jungle, arctic, and subarctic areas in North America, Europe, and Asia. Dietary surveys and Army ration trials had been conducted intermittently throughout World War II. Johnson and Kark (1947) summarized these data and presented a brief critical review of the nutrition of U.S. and Canadian soldiers in 1946 based on their more comprehensive report prepared for the U.S. Army's Quartermaster Food and Container Institute for the Armed Forces (Johnson and Kark, 1946). Their summarized nutrient intake data from several studies appear in Table 6-1. A somewhat abbreviated table was subsequently published in 1947 (Johnson and Kark, 1947) (Table 6-2). They clearly demonstrated the inverse relationship of caloric intake with mean local temperature as ascertained from the dietary surveys and ration trials. Groups of from 50 to 200 men were represented in each study. A consistent reduction in voluntary kcal intake per °F over the range-20° to 100°F was found. Their regression equation was: kcal per day = 4660-15.9 T (°F) where T is the mean external temperature. The higher the mean environmental temperature the lower the voluntary kcal intake, and the lower the mean environmental temperature the higher the voluntary kcal intake.

Johnson and Kark (1947) concluded that the large difference in caloric intake could not be explained by differences in basal metabolic rate (a difference of 10 to 20 percent at most), body weight, or type of activity because they contended the ground troops carried out similar tasks in each environment. Unfortunately, they had little data to confirm no differences in physical activity by the troops at the several garrisons. Nevertheless, Johnson and Kark stated that the caloric expenditure for a given task was greater in the cold than in warm climates because of the ''hobbling effect'' of arctic clothing and equipment. They also concluded that more body heat

was required in cold than in warm environments to maintain thermal balance.

A review of Johnson and Kark's additional table clearly shows that the percentage of protein voluntarily chosen was essentially the same in each environment (Table 6-3). Percent fat intake in the warm environments was somewhat less than that in the cold and percent carbohydrate intake somewhat higher. Johnson and Kark's overall conclusion in regard to rations was that essentially the same rations can be supplied regardless of environment, but the colder the environment the more calories are needed. In addition, they emphasized that caloric requirements are in large measure determined by the physical activity in which troops are engaged and that their summary should be regarded as the setting of standards for dietary allowances.

The conclusions of Johnson and Kark, for the most part, appear to be as valid today as when presented in the 1940s. Nutritional knowledge has advanced, food supplies—including military rations—have changed, but many of the tasks required of armed forces personnel still require physical effort, which is the major factor associated with differences in caloric needs. Of consequence, however, is that clothing has been improved, providing better protection in the cold and better potential for allowing heat loss in hot environments. Clothing items are also generally lighter in weight, and various vehicles have somewhat diminished personal load carrying.

As a follow-up to the across-climate comparisons of Johnson and Kark, Quartermaster Research and Development Command and Medical Nutrition Laboratory personnel collaborated on a series of studies in desert, temperate, and arctic environments (Buskirk et al., 1956; Iampietro et al., 1956; Welch et al., 1957a,b, 1958). Caloric intake decreased as ambient temperature decreased, but the regression slope was considerably less than that of Johnson and Kark, when either moderate or relatively heavy work was performed. In fact, the regression slope was also considerably less than that emphasized in 1950 by the Committee of Caloric Requirements of the Food and Agricultural Organization (FAO) of the United Nations (1950):

The regressions found in the collaborative study were approximately 4 kcal per °C for both moderate and relatively heavy work. The comparative slopes for FAO were approximately 15 kcal per °C and for Johnson and Kark were 30 kcal per °C. Of interest was that kcal intake was essentially

the same in all climates when calculated on the basis of body weight plus clothing and equipment weight manually transported. A kcal intake of 47 to 49 kcal per kg per day was found for moderate work in the three climates. During relatively heavy work, kcal intake increased from 60 to 62 kcal per kg per day (Welch et al., 1958). They concluded that the differences in energy expenditure among environments are largely accounted for by differences in body weight plus weight of clothing and equipment carried during the performance of duties in the respective environments.

A recent field study showed that troops operating in a warm environment and performing moderate work loads consumed an average of between 44.3 and 47.2 kcal per kg per day (Rose and Carlson, 1986), values that agree with those found by Welch et al. (1958).

An issue that has been investigated over the years with mixed results is the impact of heat on metabolic rate, both during rest and during exercise. A variety of hypothesized causes for different responses of the metabolic rate to exercise in the heat have been proposed and are listed in Table 6-4. The case for a relatively elevated metabolic rate was put forth by Consolazio et al. (1961, 1963, 1970) (Tables 6-5 and 6-6). The primary explanation of the relatively higher energy expenditure in hot compared to cooler environments was the energy expenditure associated with the production and secretion of sweat. Consolazio et al. expanded on the observations of Dill et al. (1931) and Welch et al. (1958). Results from the latter study appear in Table 6-7. Although the differences across climates and locations in the

study by Welch et al. were nonsignificant, there appeared to be a trend for a higher energy expenditure during walking in a hot desert environment when expressed either as kcal per hour or kcal per kg per hour, where kg represents total weight transported. However, more recent studies have failed to find significant differences (Klausen et al., 1967; Rowell et al., 1969; Sen Gupta et al., 1977; Shvartz et al., 1977; Young et al., 1985).

Shvartz et al. (1977) clearly indicated little difference in metabolic rate with ergometer exercise prior to heat acclimation (see Table 6-8). Sen Gupta et al. (1977) raised the possibility that although total energy expenditure during submaximal exercise was not different when the exercise was

conducted in a hot or comfortable environment, the partitioning of energy expenditure was different, that is, less aerobic expenditure and greater anaerobic expenditure in the hot environment (Table 6-9). The results were subsequently confirmed by Dimri et al. (1980). The hypothesized explanation for this partitioning was the diversion of a significant amount of blood from the muscles to the skin for thermoregulation in hot environments. Although

this hypothesis might apply to short periods of exercise such as the 5-minute bouts used by Sen Gupta et al. (1977), it would undoubtedly not apply to longer bouts of exercise, for example, 1 to 8 hours or more when a balance in muscle and skin blood flow would be necessary to sustain the exercise. A relatively elevated anaerobic metabolism and higher blood lactate concentrations would not be present. Thus, the conclusion of Sen Gupta et al. "that during submaximal work in heat, the metabolism becomes more anaerobic and there is reduction in in submaximal and maximal workloads as the heat stress increases" must be qualified at least with respect to duration of submaximal exercise.

Overall, whether energy expenditure is modified during exercise in the heat depends on the circumstances and conditions. Brief intense exercise in a hot environment may elevate energy expenditure by evoking anaerobic processes, but the increment in daily energy expenditure is likely to be negligible. Thus, the earlier investigators posed the problem, but in terms of meeting the daily kcal needs of troops working in a hot environment, the submaximal exercise they perform has no greater impact than if they performed the same tasks in a more comfortable environment.

The possible reasons for either an increase or a decrease in metabolic rate in hot environments are listed in Tables 6-10 and 6-11. A careful appraisal of each military situation would reveal which factors are most important to consider while also bearing in mind the possible causes of different responses previously set forth in Table 6-4.

A finding that has been repeatedly documented is that unacclimatized personnel suffer the consequences when suddenly exposed to stressful environments, whether the environmental stress is heat, cold, or altitude. The psychological and physical stresses associated with combat only complicate the adverse situation. At issue is inadequate acclimatization, which with sudden exposure to heat, not only perpetrates physiological strain but lessens initiative and appetite, which negatively affects nutritional status including water balance. The acclimatization process with exposure to hot environments proceeds rapidly, being virtually complete in the working soldier within 10 days (Adolph, 1947; Buskirk and Bass, 1957; Dill, 1938). During this time, body weight is invariably lost due to undernutrition, but the weight may be subsequently regained in toto or in part. Johnson (1946), in his review, concluded that following acclimatization, dietary require-

ments are qualitatively similar in hot and temperate areas but may remain quantitatively lessened in tropical climates by the sustained high loss of sweat and anorexia.

The question of whether heat acclimatization (outdoors or in the field) or acclimation (indoors or in laboratories) has an effect on metabolic rate during rest and exercise has been studied intensively with mixed results. Some pertinent studies are cited from among the many in the literature.

Robinson et al. (1945) and Eichna et al. (1950) found that heat acclimation lowered the metabolic rate associated with exercise in the heat by 4 to 8 percent. Shvartz et al. (1977) studied, using cycle ergometry, several groups of men who varied widely in physical fitness and were exposed to 8 days of heat acclimation. Despite interindividual differences in physical fitness, the postacclimation oxygen uptakes were invariably slightly less in all of the environments studied including a 39.4°C (103°F) environment (see Table 6-8).

Sawka et al. (1983) reevaluated the problem. They concluded that heat acclimation, if it had an effect at all, slightly lowered metabolism associated with performance of exercise in the heat. The conclusion was based not only on their studies of 42 subjects of both genders, but on a review of the literature as well. Young et al. (1985) arrived at essentially the same conclusion (see Table 6-12).

Presumably, the small reduction in metabolism is caused by the lesser respiratory and cardiac work caused by more efficient evaporative cooling, peripheral circulation, regulation, and the lowering of body temperature, although as Sawka et al. (1983) have pointed out, the role of such factors is

TABLE 6-12 Statistical Analysis for Comparison of Main Effects of Heat Acclimation and Environment on Respiratory Measurements of Young Men (n = 13)

Variable	Acclimation	Environment
, liters per minute	Pre > post*	Cool > hot*
RER†	Pre > post*	Cool < hot*
	NS	Cool < hot*
NOTE: Environments: Cool—21°C, 30 percent relative humidity; hot—49°C, 20 percent relative humidity. Exercise: 30 minutes of cycle ergometry at 70 percent ; NS = not significant. * p ≤ 0.05 † RER = respiratory exchange ratio. SOURCE: Adapted from Young et al. (1985).

not clear-cut. They suggested further research to investigate the possible role of modification of motor unit recruitment patterns and muscular efficiency—the latter related to phosphorylation efficiency and contractile-coupling efficiency.

Heat acclimatization/acclimation is a valuable physiological adaptation, but the process plays only a minor role in modifying energy turnover and caloric requirements.

Appetite tends to be adversely affected among unacclimatized personnel who are abruptly exposed to a hot environment, a finding that has been recognized for some time. Taylor in 1946, quoted by Mitchell and Edman (1951), suggested the following:

Kark et al. (1947) recommended maintaining appetite through variety, familiarity, and high quality.

Although appetite, hunger, and satiety are complex processes, they must be addressed with regard to hot environments. Hard work in the heat, particularly for the unacclimatized, challenges ration providers and food preparers to offer in sufficient quantity safe, appealing food of good nutritional quality.

One of the thoughts perpetuated in the 1930s through the 1950s was that the specific dynamic action (SDA) of foods—now commonly identified as the thermic effect of food or dietary-induced thermogenesis—contributed significantly to daily kcal turnover. Swift and French (1954) reviewed the various studies and concluded that the impact of specific dynamic effect (SDE) was overemphasized, but that it remained a significant minor factor, in the range of 2 to 8 percent of ingested energy. When people consume mixed meals, the relative SDE impact of protein, carbohydrate, or fat becomes indistinguishable.

In an early evaluation of basal metabolism in the tropics, MacGregor

and Loh (1941) showed that basil metabolic rate (BMR) declined in certain normal individuals, but not in others. The depression in BMR in those affected appeared to reach a maximum before the end of the first year of residence and was maintained after 2 years in the tropics. Military training for 3 months in the same environment appeared to have no influence on the interindividual patterns of response. Neither alterations in diet nor weight loss accounted for the interindividual differences. The authors concluded that the continued relatively high temperatures and possibly humidities were responsible for the depression of BMR in those susceptible. The environmental conditions in Singapore where the work was done averaged 28.3°C (83°F) during the day and 24.4°C (76°F) at night with frequent high humidity. Others have also reported an impact of environmental temperature on BMR or resting metabolism (for example, Galvao, 1950; Mason, 1934).

To further such observations, an experiment was designed to evaluate the changes in resting metabolism during the day when food and exercise are taken as usual (Buskirk et al., 1957). Comparisons were made across climates varying from a cold (arctic) to a hot (desert) environment. It was concluded that specific dynamic action or dietary-induced thermogenesis assessed by periodic measurements of oxygen consumption was primarily responsible for the upward trend in energy turnover at rest during the day. A small ''diurnal'' elevation in oxygen consumption occurred during fasting, with or without exercise. Climate per se did not appear to influence the pattern of resting metabolism.

Dietary deficiencies produce various symptoms; however, evidence of gross nutrient deficiency is usually delayed for a considerable period of time unless the deficiency is water, carbohydrate, or total kcal. Johnson summarized the more prominent effects of gross nutrient deficiencies, and his listing was modified by Young (1977) and then adapted here (see Table 6-13). Water deficiency has an almost immediate effect, whereas kcal and carbohydrate deficiency effects are seen in a matter of days. Protein and fat deficiencies produce symptoms within weeks and months, respectively. Based on this early information, attention was paid to water, kcal, and carbohydrate deficiencies in a variety of early studies involving hard work by soldiers in either temperate or warm/hot environments (Grande et al., 1957; Taylor and Keys, 1958). The combination of hypohydration and undernutrition was shown to be particularly compromising with respect to physical performance. Significant nitrogen loss in urine and sweat associated with weight loss and, presumably, skeletal muscle hypotrophy was observed. The nitrogen losses found by Grande et al. (1957) are reported in Table 6-14. Should such nitrogen loss continue, troops would be physically com-

promised. Unpublished investigations from the University of Minnesota in the 1950s revealed that a loss of 125 g nitrogen was associated with measurable physiological deterioration, including a significant reduction in walking endurance and aerobic power. A review of the effects of prolonged semi-starvation has been set forth in a classic study by Keys et al. (1950). A further discussion of negative nitrogen balance based on the experience of those working at the University of Minnesota was prepared by Taylor and Keys (1958).

Undernutrition is always a problem in military operations for various reasons, among them psychological stress, supply problems, food prepara

tion problems, and coping with threats and emergencies. A group led by Sargent and Johnson at the University of Illinois spent considerable time and effort in the 1940s and 1950s working on undernutrition (as well as more normal nutrition). They established fundamental physiological, nutritional bases for an all-environment survival ration (Sargent and Johnson, 1957):

Although the focus was on adequate carbohydrate supply during the 1940s and 1950s, largely to avoid the debilitating effects of ketosis, there was also concern about adequate protein and preservation of body tissue including skeletal muscle mass. Mitchell and Edman (1949, 1951) said about protein:

Consolazio and Shapiro (1964) found in the summary of their studies of men exercising under different climatic conditions that protein intake in the hot climate exceeded the National Research Council's recommended allowance of 100 g per day. In contrast to the conclusion of Mitchell and Edman (1949, 1951), Consolazio and Shapiro felt that increased protein intake in the heat was due not to an innate desire for protein, but to the relatively greater caloric intake. Recently, Paul (1989) suggested that because protein and amino acids contribute from 5 to 15 percent of energy for prolonged exercise—with the higher values perhaps associated with glycogen depletion—adequate protein intake is important when exercising in the heat. He pointed out that urine and sweat urea increase during prolonged, relatively intense exercise. Nevertheless, there appears to be no evidence that protein

intakes in excess of from 1 to 1.5 g per kg body weight offer any advantage to the mature military person. One possible disadvantage of high protein intakes is the obligatory urine volume required to excrete protein breakdown products, including urea.

The comment of D. B. Dill (1985), a former colleague who was well versed in the desert environment, provides a fitting reminder to the readers of this brief historical review.

Unfortunately, military personnel engaged in combat or under the threat of combat may not have the luxury of contemplating beauty, but they nevertheless must deal with the "incredible intensity of the desert."

Finally, as the nutritional situation during the recent operations of Desert Shield and Desert Storm is reviewed, a comment by R. M. Kark (1954) comes to mind.

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