Nutritional Enhancement of Soldier Performance at the U.S. Army Research Institute of Environmental Medicine, 1985–1992
The purpose of the workshop on which this volume is based is to provide focus and direction to the joint Science and Technology Objective (STO), responsibility for which is shared by the U.S. Army Research Institute of Environmental Medicine (USARIEM) and the U.S. Army Natick Research, Development and Engineering Center. The thrust of this STO is to sustain and enhance soldier performance in environmental extremes through performance-enhancing food components. Integral to this objective is the prevention of performance degradation (the preservation of pre-deployment performance capability), especially under the stress of sustained field operations at
environmental extremes (U.S. Department of the Army, 1991). This research is necessary to move the soldier of today toward enhanced capabilities in the future. The U.S. Army refers to this overall initiative as The Soldier as a System (U. S. Department of the Army, 1991) and recognizes the importance of the individual items and equipment that the soldier wears, carries, or consumes (U.S. Department of the Army, 1991).
The U.S. Army has always been interested in the enhancement of soldier performance. Until recently, however, this concept primarily encompassed efforts to improve training, doctrine, and equipment, with relatively little emphasis on food as a tactical weapon. The advent of the concept of sports nutrition and the documentation by the scientific community of the influence of nutrients on physical and mental performance have pointed the way toward the application of sports nutrition and nutritional neuroscience strategies to military scenarios. The U.S. Army Research Institute of Environmental Medicine has investigated the application of some of these sports nutrition and nutritional neuroscience principles on soldier performance at high altitudes, in the cold, in the heat, and in conjunction with load-bearing work.
The purpose of this brief chapter is to review what the Army has accomplished in the area of nutrition and performance research conducted at the U.S. Army Research Institute of Environmental Medicine from 1985 to 1992 and to provide a new starting point for further research on the science and technology objective. The other chapters presented in this volume will assist the Army in determining additional avenues of research through their reviews of the current status of nutrition and performance enhancement research in general. It is hoped that this information will permit the Army researchers to focus their efforts on potentially fruitful avenues of research.
Nutrition and performance research at the U.S. Army Research Institute of Environmental Medicine (USARIEM) has focused on three general areas:
dietary macronutrients (carbohydrate),
utritional pharmacology (caffeine), and
nutritional neuroscience (tyrosine).
The data presented in this chapter consist of means±standard deviations. Significant differences (P<0.05) are noted, but only concise descriptions of the experimental designs are given. The reader is referred to the references for additional experimental design details.
MACRONUTRIENT AND PERFORMANCE RESEARCH AT USARIEM
USARIEM scientists have conducted a number of studies on the performance enhancing aspects of dietary carbohydrate. These research efforts are in the following areas:
carbohydrate and work at high altitude,
carbohydrate and thermoregulation in the cold,
carbohydrate and work in the heat,
carbohydrate and load-bearing work, and
carbohydrate and marksmanship.
Carbohydrate and Work at High Altitude
The influence of liquid carbohydrate (CHO) supplements on work performance at high altitudes has been investigated in two USARIEM studies (Askew et al., 1987). These two studies differed principally in the manner in which they measured work performance. A study conducted at the summit of Mauna Kea (4,100 m) evaluated CHO-supplemented and non-supplemented soldiers runn-ing for 2 h/day for the first 4 days of exposure to high altitude (Askew et al., 1987). The performance measure was the total distance run during a 2-hr period (70 percent maximal oxygen uptake at sea level) each day for 4 days. Consumption of the supplement or a placebo beverage was voluntary, but average consumption was about 200 g of CHO per day in addition to that obtained from the diet. The supplemented group ran at an average rate of 12.0±0.8 km/2 h whereas the non-supplemented group ran at an average rate of 10.7±0.5 km/2 h. This difference was significant at P< 0.05.
A second study measured the time that it took a group of test subjects to hike the length of the Barr Trail from an elevation of approximately 305 m to the summit of Pikes Peak (4,300 m) (Baker et al., 1990, Smith et al., 1993). The total distance was 21.7 km and the total time (in minutes) required to hike this route was recorded. The test was conducted before and after 3 weeks of acclimation to high altitude, and the test subjects received approximately 300 g of CHO per day in the form of a liquid glucose polymer supplement. The control group received a placebo drink containing no CHO. Under the conditions of the study, there was no significant difference in hiking times at the beginning or end of the 3-week altitude exposure period, although the carbohydrate supplement did significantly increase total dietary carbohydrate intake (Baker et al., 1990).
Carbohydrate and Thermoregulation During Cold Exposure
Neufer et al. (1988) and Young et al. (1989) investigated the influence of cold exposure on the rewarming response after hypothermia and thermoregulation during immersion in cold water. Neufer et al. (1988) fed test subjects 120 or 600 g of CHO per day and studied the effect of mild hypothermia on the rewarming response of these two groups. They found that the low muscle glycogen levels associated with the low dietary carbohydrate intake did not impair the rewarming time during passive rewarming and suggested that individuals suffering from mild hypothermia rewarm spontaneously despite significant muscle CHO depletion. This study thus provided little evidence for a critical role of CHO in the rewarming process.
Young et al. (1989) studied the influence of high or low muscle glycogen levels produced by a combination of exercise and diet (low-carbohydrate diet, 15 percent CHO; high-carbohydrate diet, 65 percent CHO) on thermoregulation during immersion in cold water. The treatments produced muscle glycogen levels of 144±124 and 543±53 mmol of glucose per kg of dry tissue. There was, however, no significant difference between the low- and high-carbohydrate treatment groups in the shivering response, metabolic rate, or maintenance of body core temperature during this experimental exposure to the cold. Young et al. (1989) concluded that the thermoregulatory response to cold stress was not impaired by a substantial reduction in muscle glycogen levels. Apparently, other metabolic substrates such as fat can adequately fuel the heat-generating response when muscle glycogen stores are low. This study did not, however, establish whether there is some minimal muscle glycogen level that is obligatory for sustaining muscle metabolism during this experimental exposure to the cold.
Carbohydrate and Work in the Heat
Rose et al. (1987) examined the thermoregulatory and hydrational status of men during sustained work in a hot (37°C), dry (20 percent relative humidity) environment. They studied 11-heat acclimated young men engaged in 24 h of sustained, 45-min bouts of treadmill walking (1.56 m/sec) interspersed with 15-min rest periods each hour. The subjects consumed either a nutrient solution (24.8 g of CHO, 24 mEq of sodium per liter) or a placebo solution to maintain a constant body weight during the period of sustained activity. Subjects consumed approximately 700 ml (17.4 g CHO/h) of the test solution per hour during the period of sustained activity. Only 2 out of 11 subjects were able to complete a full 24-h period of sustained activity. Although these two subjects happened to receive the nutrient solution, the
mean endurance times for those receiving the placebo control and nutrient solutions (16±3 versus 17±4 h) were not significantly different. Likewise, there was no significant difference in metabolic rate, plasma volume, fluid intake, sweat rate, plasma glucose concentration, or skin or rectal temperature. There was also no significant difference in the gastric emptying rates of the test or placebo solutions (Levine et al., 1991). The relatively low level of carbohydrate used in the study may account for the lack of an anticipated ergogenic effect; however, it is also possible that the nature of the sustained work test chosen for this study contributed to the lack of sensitivity to carbohydrate. Foot and leg soreness, chafing, blistering, and heat rash, rather than exhaustion were the main reasons for terminating the sustained treadmill walking.
Carbohydrate and Load-Bearing Work and Marksmanship
Moore, R.J. et al. (1991) studied the effects of low (250 g), moderate (400 g), and high (550 g) carbohydrate diets on load-bearing work and perceived exertion. Tharion and Moore (1993) also studied laser marksmanship1 as a function of the carbohydrate content of the diet in this same study. A total of 13 test subjects in a double-blind, repeated-measures experimental design each consumed three test diets for three 5-day study phases: Phase 1:250 g CHO diet; Phase 2:400 g CHO and Phase 3:550 g CHO in the same order for all subjects. In terms of work, all phases consisted of days 1 to 3, road marching (19 km/day) while carrying 45-kg packs; day 4 encompassed treadmill running without a pack and metabolic measurements related to the lactate threshold; and on day 5, test subjects carried a 45-kg load while walking until exhaustion at 5.6 km/h on a treadmill set at a 5 percent grade. Subjects rested for 10 min during each hour during the 30–194-min march. Relative perceived exertion (Borg scale) readings were recorded at regular intervals throughout the test period. Laser marksmanship determinations were begun within 5 min following the treadmill walk to exhaustion. Marksmanship accuracy was measured by the tightness of the shot group (area, mm2 of the shot group). Accuracy was better maintained in subjects on a higher level of carbohydrate intake, but the results did not reach statistical significance. Although the study provided some suggestion of a beneficial effect of carbohydrate on perceived exertion and possibly fine motor coordination, it
suffered from a relatively small sample size, precluding definitive conclusions or generalization of the data.
The results are listed in Tables 3–1 and 3–2. The results of the study thus indicated that while there was no significant effect of diet on endurance, the localized perception of the degree of difficulty of the work was greater for those on the low-carbohydrate diet.
NUTRITIONAL PHARMACOLOGY (CAFFEINE RESEARCH)
The ergogenic effect of caffeine during cycle ergometer work at a simulated high altitude in an altitude chamber (Fulco et al., 1989) as well as during marching on the Pikes Peak Barr Trail (King et al., 1993) has been studied by scientists at USARIEM. Caffeine has also been evaluated as an agent to enhance human performance of tasks that require a high level of vigilance (Lieberman et al., 1993).
Fulco et al. (1989) studied the effect of a placebo or caffeine (4 mg/kg) on the endurance times of eight test subjects on a cycle ergometer (80–85 percent ) at sea level, after 1 h of simulated altitude (4,300 m) exposure (acute altitude exposure), and after 2 weeks at the summit of Pikes Peak (4,300 m). At sea level, there was no significant effect of caffeine on the mean endurance time to exhaustion (26.3±11.9 versus 27.5±15.6 min). During
TABLE 3–1 Influence of Carbohydrate on Endurance and Perceived Exertion During Load-bearing Work
Grams of Dietary Carbohydrate
Endurance time min*
Relative perceived exertion†
NOTE: Data from subjects completing at least 110 min of load-bearing work (n=6).
* No significant differences, P>0.05.
† Lower extremity rating. The relative perceived exertions for groups receiving 400 and 550 g were significantly less (P<0.05) than those for the group receiving 250 g of CHO.
SOURCE: Adapted from Moore et al. (1991).
TABLE 3–2 Influence of Carbohydrate on Marksmanship Before and After Load-bearing Work
acute altitude exposure in the altitude chamber, the mean endurance time of the caffeine treated group increased 54 percent relative to that of the placebo group (22.8±6.9 versus 35.0±10.7 min; P<0.01). Following 2 weeks of altitude exposure on Pikes Peak, the mean endurance time of the caffeine-treated group was increased 24 percent compared with that of the placebo group (30.5±14.5 versus 38.7±46.1 min); however, this difference was not significant at P<0.05. Caffeine was effective in significantly reducing the perception of effort at 10 min of exercise during the acute altitude exposure.
King et al. (1993) recently tested the effect of a similar dose of caffeine (4 mg/kg) on the hiking times of eight test subjects hiking the Barr Trail (21.7 km) to the summit of Pikes Peak (4,300 m). They could not find a significant difference between the placebo and the caffeine-treated groups (272±37 versus 264±22 min; P>0.05).
Lieberman et al. (1993) studied the effect of caffeine on vigilance in 20 test subjects receiving doses ranging from 32 to 256 mg of caffeine. The placebo group received no caffeine. All doses of caffeine significantly improved the number of correct detections on a Wilkinson auditory vigilance task Wilkinson (1969). The results obtained after administration of a 256-mg dose of caffeine are shown in Figure 3–1. These scientists also studied the effect of a single 200-mg dose of caffeine or a placebo on the mean number of correct detections in 10-min time blocks over 12 successive 10-min time blocks.
Caffeine significantly increased the number of correct detections throughout the 120-min test period. The results of the work of Lieberman et al. (1993) and Fulco et al. (1989) show that caffeine can increase physical and mental performance under carefully controlled laboratory conditions; they do not, however, show that a similar effect will be present under field conditions.
NUTRITIONAL NEUROTRANSMITTER RESEARCH (TYROSINE)
Tyrosine is a large neutral amino acid and is a precursor of the neurotransmitters norepinephrine and dopamine, which are secreted by catecholaminergic neurons during stressful situations. Some of the behavioral effects of acute stress may result from the depletion of norepinephrine and/or dopamine. Banderet and Lieberman (1989) studied the effect of tyrosine administration (100 mg/kg in two divided doses) on the protection of humans from the adverse consequences of a 4.5 h exposure to cold (15°C) and simultaneous simulated high-altitude chamber exposure (4,700 m). They employed a double-blind, placebo-controlled crossover design and found that
tyrosine significantly decreased some measurements of symptoms, adverse moods, and performance impairments. Since these were a rather specialized set of testing circumstances, the authors cautioned that further research should be accomplished to determine whether tyrosine would be beneficial under other stressful (for example, field) circumstances. Further discussion concerning this particular study can be found in Chapter 15.
CURRENT USARIEM NUTRITION AND PERFORMANCE ENHANCEMENT RESEARCH
The U.S. Army Research Institute of Environmental Medicine is currently engaged in research employing glycerol to achieve hyperhydration at high altitude and in the cold to combat altitude- and cold-induced diuresis and dehydration. Glycerol is also being investigated as an agent for use in the prevention of high-altitude cerebral edema. Future work is planned to determine whether glucose electrolyte beverages may have a beneficial effect on hydration status and performance of military tasks in the heat.
Most of USARIEM’s work on dietary methods to enhance performance has centered around carbohydrate, caffeine, and tyrosine, and recent work has focused on glycerol. Caffeine and tyrosine have shown considerable promise of performance enhancement of military tasks; however, the positive results obtained with these two compounds have come from carefully controlled laboratory tests, not measurements of soldiers performing military field tasks. Indeed, most of the carbohydrate research and the portion of the caffeine research that was done under field conditions has failed to demonstrate a positive significant impact. This is perhaps due to the rather large amount of experimental “noise” associated with conducting performance tests in the field. It appears that field tests of physical performance require more closely controlled experimental testing conditions, larger numbers of test subjects, or both. Most of the USARIEM field studies employ 6–12 test subjects per treatment group. Subject availability, attrition, or manageability (logistical) aspects usually result in this final number of participants in field studies.
As this brief review illustrates USARIEM has been actively exploring the diet-performance interface between soldiers and their environment for the past 7 years and intends to increase its efforts in this direction in the future. The information gained from the chapters in this volume will help investigators at
USARIEM focus their research efforts on these and other nutrients that might prove to be beneficial to soldier performance enhancement.
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Wilkinson, R.T. 1969 Some factors influencing the effect of environmental stressors on performance. Psychol. Bull. 72:260–272.
Young, A.J., M.N.Sawka, P.D.Neufer, S.R.Muza, E.W.Askew, and K.B.Pandolf 1989 Thermoregulation during cold water immersion is unimpaired by low muscle glycogen levels. J. Appl. Physiol. 66:1809–1816.