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FLUID REPLACEMENT AND HEAT STRESS Part I Executive Summary PART I IS THE EXECUTIVE SUMMARY OF THE REPORT. The Executive Summary comprises Chapter 1 of the report. It describes the task presented to the Committee on Military Nutrition Research (CMNR) by the Military Nutrition Division, U.S. Army Institute for Environmental Medicine (USARIEM), U.S. Army Medical Research and Development Command; summarizes the relevant background material; and presents the committee's findings. The Executive Summary also includes specific and general recommendations developed by the CMNR.
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FLUID REPLACEMENT AND HEAT STRESS This page in the original is blank.
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FLUID REPLACEMENT AND HEAT STRESS 1 Committee Summary and Recommendations INTRODUCTION Advances in our understanding of the value of carbohydrate-electrolyte solutions have come from information derived from two major fields of study--exercise physiology and sports nutrition--and from research on diarrheal diseases. Research in the first area has been concerned with physical performance, primarily of athletes. Research results have demonstrated that even small fluid deficits have adverse effects on performance through elevated heart rates, reduced sweat rates, and elevated body temperature. Glucose-electrolyte solutions have been found useful in rehydration and in preventing dehydration. Carbohydrate is needed to facilitate sodium and water absorption. Other ions may or may not be needed, depending on sweat losses or losses from the gastrointestinal tract. Advances in exercise physiology also have demonstrated the value of carbohydrate solutions in providing energy for muscular activity in endurance events that last at least 60 minutes and involve vigorous exercise. Diarrhea is a major, perhaps the most important, contributor to death of infants and preschool children in less-developed countries. Death rates are being reduced around the world through the use of oral rehydration therapy (ORT), which involves the use of carbohydrate-electrolyte solutions and is based on the same basic physiologic mechanism as the rehydration solutions given to athletes, i.e., the provision of glucose to promote the absorption of sodium and potassium ions and of water. Both these established uses for carbohydrate-electrolyte beverages have potential military applications. Military personnel are often called upon to perform heavy physical activity during training or combat conditions in very
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FLUID REPLACEMENT AND HEAT STRESS hot environments--either dry climates, as in Middle-Eastern deserts, or under humid tropical conditions. The resultant high sweat rates can lead to dehydration. In some cases, the subjects may be acclimated to heat, but in others (for example, in basic training, or in emergency troop deployment to the tropics) they may not, and may thus be vulnerable to extensive electrolyte losses. This problem could be accentuated when personnel have been given garrison or field rations with reduced sodium to meet prudent dietary goals established for the general population in 1989 by the Diet and Health Committee of the Food and Nutrition Board, National Academy of Sciences. A carbohydrate-electrolyte beverage could be useful in providing glucose to sustain muscular activity in troops involved in heavy physical activity for long periods. Recognizing that the maintenance of an adequate hydration status is dependent on an adequate fluid intake, the military has for a long time instructed troops on ways to maintain a safe supply of drinking water under field conditions. Carbohydrate-electrolyte solutions are useful in rehydration during episodes of diarrhea, especially to counteract acute dehydration that results when diarrhea occurs in conjunction with heavy sweat losses. FINDINGS FROM THE WORKSHOP PRESENTATIONS Maintaining an adequate state of hydration is important for the maintenance of high levels of physical performance by soldiers in the field. At a 3% decrease in body weight due to dehydration, there is a substantial decrease in physical working capacity. The maintenance of adequate fluid intake is of primary importance in the prevention of hypohydration that may otherwise occur under such conditions as prolonged air travel, extended working hours, wearing of chemical protective clothing, missed meals, or working in mountainous areas or in hot or extremely cold environments. Increased psychological stress associated with basic or field training exercises or anticipation of combat or actual combat may lead to extreme hypohydration due to decreased voluntary fluid intake. Conscious efforts to increase fluid intake before and during such situations could prevent this condition. Training and the initiation of disciplined programs to increase both voluntary and programmed fluid intake are important preventive actions. Heavy physical activity, especially in hot environments, and wearing of protective clothing promote sweating and will lead not only to excessive fluid losses but also to associated electrolyte losses. Sodium, potassium, and chloride losses in sweat are affected by temperature, humidity, and state of acclimatization. Febrile conditions or gastrointestinal disturbances, parti-
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FLUID REPLACEMENT AND HEAT STRESS cularly those associated with vomiting and diarrhea, may result in significant fluid and electrolyte losses and require replacement of electrolytes in addition to fluid. Gastrointestinal losses may also include hydrogen ion, bicarbonate, magnesium, and other cations and anions, depending on the cause of the losses and the severity of the disturbance. Glycogen depletion from muscle and liver may result from prolonged physical exercise--more than 60 or 90 minutes at 60% to 70% of exercise capacity or several hours at lower exercise intensities. Such depletion may be aggravated by poor nutritional intake of carbohydrates, inadequate periods of recovery from previous glycogen-depleting exercise, and sustained negative calorie balance. Under these conditions, soldiers may benefit from consuming fluid replacement beverages containing carbohydrates. This is particularly true if food intake is inadequate, resulting in significant caloric deficit or limited carbohydrate intake. The resultant reduced muscle and liver glycogen content will result in earlier fatigue and slower recovery. It is evident from the research reported at this workshop that a fluid replacement solution may play an important role in preventing fluid, electrolyte, and glycogen depletion, thereby maintaining or improving a soldier's performance. It is also evident that the composition of the replacement fluid might well vary, depending on the physical demands of the military activity and the environmental conditions under which the activity is undertaken. Water intake is the primary requirement to ensure adequate hydration during psychological and environmental stress not associated with intense physical activity and during sedentary activity at high altitudes. If a normal meal pattern is established and fluid is consumed, the body's balance is restored. Palatability of the fluid replacement solution is important to ensure compliance. This may be enhanced by appropriate coloring and flavoring. The solution should also be compatible with halogens to make it possible to use halogen-treated water in the preparation of the solutions. AREAS FOR FUTURE RESEARCH The participants whose papers appear in this volume provided an excellent review of the current state of knowledge on fluid replacement and stress. These proceedings will provide investigators and product formulators with important guidance in the development and testing of electrolyte-carbohydrate-containing fluid replacement products for use by the military. Continued research is needed on energy, electrolyte, and fluid requirements in different environmental and operational conditions that require different
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FLUID REPLACEMENT AND HEAT STRESS types of physical activity. More studies are also needed to provide us with a better understanding of (1) the factors affecting liver and muscle metabolism and injury during heat stress, and (2) the factors that are important in preventing muscle injury during heat stress and in enhancing muscle recovery. The following issues raised at the workshop could lead to a better understanding of the appropriate composition and usage of a fluid replacement beverage: What are the effects of food in the small intestine on fluid and electrolyte absorption? How are fluid and electrolyte absorption affected relative to timing of meals? What are the effects of hypohydration on the absorption of electrolyte-carbohydrate solutions? What factors regulate depletion of muscle and liver glycogen stores during negative caloric balance or prolonged physical activity? What is the role of glycogen depletion in the fatigue of different muscle groups? What other factors related to beverage composition determine muscular fatigue? What factors determine the rate of glycogen depletion and resynthesis? There is a need to obtain quantitative data on the effects of feeding and the provision of electrolyte-carbohydrate solutions in maintaining glycogen stores and enhancing replenishment of glycogen stores following glycogen-depleting physical activity. What are the effects of fluid and electrolyte deficits combined with elevations in body temperature on cognitive and mental function? What factors need to be considered in product development and water purification techniques to provide compatible systems for field use under a variety of environmental and operational conditions? Factors such as halogen or other purification requirements and the composition of local water supplies need to be considered in relation to formulation of practical electrolyte-carbohydrate mixtures. What effect would result from the provision of an electrolyte-carbohydrate replacement solution on soldiers who previously consumed a low-sodium diet? Will the addition of specific amino acids such as glycine be beneficial in enhancing sodium and water absorption?
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FLUID REPLACEMENT AND HEAT STRESS RECOMMENDATIONS When used appropriately, electrolyte-carbohydrate-containing beverages appear to have the potential not only for maintaining but also, possibly, for enhancing performance and endurance in a variety of military situations. The specific needs for water, electrolytes, and carbohydrate may vary somewhat depending on the specific circumstances in which the solution is used. The ideal solution would be one that could be diluted in different ways to meet the relative specific needs of the personnel. The goal of using such a solution should be to maximize fluid intake, replace electrolyte losses, and provide a carbohydrate source for energy and rapid replenishment of muscle and liver glycogen stores during and following physical activity. The use of an electrolyte-carbohydrate-containing beverage may be applicable to a number of circumstances in the military such as the following: Maintaining adequate fluid intake prior to military operations during which voluntary dehydration is probable. Providing fluid, electrolyte, and carbohydrate replacement during physical work in a variety of environmental conditions, including high temperatures, humidities, or wearing of chemical protective clothing. In such situations, sweat rates are high and account for large fluid and electrolyte losses. Providing rapid rehydration following heavy or prolonged physical work, thereby facilitating recovery from heat injury. Providing carbohydrate during and following physical activity to maintain plasma glucose concentrations, furnishing carbohydrates for energy, and enhancing replenishment of glycogen stores during postoperational recovery. Replacing gastrointestinal losses due to vomiting or diarrheal diseases. The committee recommends that the Surgeon General of the Army evaluate the use of electrolyte-carbohydrate fluid replacement products as an aid to maintaining proper hydration of soldiers during periods involving psychological and environmental stress and also assess the effectiveness of these products in maintaining or enhancing both physical and cognitive performance during training activities and field operations. Physical demands and adverse environmental conditions that occur during military training and operations may lead to any one or all the conditions summarized above. In view of this, the committee concludes that there are circumstances in which the performance of military personnel
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FLUID REPLACEMENT AND HEAT STRESS would be improved by appropriate use of electrolyte-carbohydrate solutions under field conditions. Below are the committee's recommendations developed following the workshop: The solutions should provide approximately 20 to 30 meq of sodium per liter, 2 to 5 meq of potassium per liter, and chloride as the only anion. The carbohydrate content should be provided as glucose or sucrose, malto-dextrin, or other complex carbohydrate in a concentration of 5% to 10%. The value of additional magnesium, bicarbonate, and phosphate to compensate for gastrointestinal losses due to diarrhea or other gastrointestinal disturbances should be determined. The promotion of fluid intake with such palatability and psychogenic aids as flavorings and colorings should be evaluated with respect to the promotion of fluid intake. The components of the solution must be compatible with halogens or other water purifiers. A variety of training and field operations should be considered as a means for evaluating the effectiveness of prototype electrolyte-carbohydrate-containing solutions under the following conditions: When soldiers are in significant negative caloric balance. Under conditions of hypohydration. When the solution is the principal beverage available. Under conditions of environmental extremes, especially those conducive to stress. Interventions for prevention and therapy of heat-related disorders should be evaluated. When used by soldiers previously on a low sodium diet (less than 3 g/day) who are suddenly exposed to hot or humid environments and who are performing heavy physical activity. Under field conditions when halogen-treated water is likely to be available. Do any of the components in the prepared solution interfere with purification of the water? Is the resulting beverage sufficiently palatable to ensure an intake adequate to prevent significant hypohydration?
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