Fluid Needs in Hot and Cold Environments

Transcription

1 International Journal of Sport Nutrition, 1995, 5, O 1995 Human Kinetics Publishers, Inc. Fluid Needs in Hot and Cold Environments Robert Murray Of all the physiological perturbations that can cause early fatigue during exercise, dehydration is arguably the most important, if only because the consequences of dehydration are potentially life threatening. The rise in body temperature that normally accompanies exercise stimulates an increase in blood flow to the skin and the onset of sweating. Normal hydration is protective of these thermoregulatory responses, whereas even a slight amount of dehydration results in measurable declines in cardiovascular and thermoregulatory function. Mild to severe dehydration commonly occurs among athletes, even when fluid is readily available. This voluntary dehydration compromises physiological function, impairs exercise performance, and increases the risk of heat illness. Recent research illustrates that maintaining normal hydration (or close to it) during exercise maintains cardiovascular and thermoregulatory responses and improves exercise performance. Consequently, it is in the athlete's best interest to adopt fluid-replacement practices that promote fluid intake in proportion to sweat loss. Body fluid balance during exercise has been studied for decades, resulting in a comprehensive understanding of the physiological consequences of dehydration (for reviews, see 28,4143) and, more recently, of the physiological and performance benefits of euhydration (4, 8, 26). This fluid-balance research has formed the foundation for practical recommendations regarding fluid replacement during physical activity in warm weather (1, 8,9, 15). As a result of such research and related educational efforts, it can be argued that the need to consume fluid during or following physical activity is one of the most widely understood dictums of modem sports medicine. The focus of this brief review is on recent laboratory research that underscores the physiological and performance-related value of ingesting adequate amounts of fluid during exercise, research that also provides the opportunity to refine current recommendations for fluid-replacement practices. Body Fluid Balance The daily intake of fluid is usually closely balanced with the volume of fluid that is lost in urine, feces, sweat, and respiration and via insensible water loss through the skin so that, at rest in thermoneutral conditions, body fluid balance is maintained at f0.2% of total body weight (19). This balance requires the constant integration of input from hypothalamic osmoreceptors and vascular baroreceptors so that drinking behavior closely approximates fluid loss, a minimum of about 2 Llday. Robert Murray is with Gatorade Sports Science Institute, 617 W. Main St., Bamngton, IL

2 Fluid Needs / S63 Maintaining euhydration during and following exercise is more difficult than during rest because of the more rapid loss of body fluid due to sweating. Indeed, sweat loss can be prodigious, rising to rates greater than 3 L/hr in highly fit, well-acclimated athletes (43). This rapid loss of body fluid is often not accompanied by an equivalent volume of ingested fluid, and significant dehydration results. Exercise in the Heat Although there are many anecdotes regarding the travails suffered by those who become dehydrated when exercising in the heat, perhaps one of the most highly publicized and horrifying encounters befell Australian Mark Domty, often referred to as the "Meltdown Man" (25, 35). In the Australian summer of 1988, Mark and a few friends began an 8-km run at 2:30 in the afternoon when the ambient temperature was in excess of 38 OC (100.4 OF). In fact, the fun run in which the men were supposed to take part had been postponed from 2:30 until 530 p.m. in hope of cooler temperatures. Well in the lead near the end of the run, Mr. Domty, 28 years old and in good physical condition, suddenly collapsed and was in such obvious trouble that he was, transported directly to a hospital. Upon admission, his body temperature was 42 "C (107.6 OF). A tracheotomy was immediately performed to ensure the pulmonary ventilation that his impaired respiration system could not maintain. Because his immune system was subdued and his blood-clotting mechanisms were impaired by his high body temperature, an opportunistic infection set in, a result of a scrape sustained on his left leg when he initially collapsed. Over the next few days, the muscles of his left leg turned a khaki color and had the stringy texture of overcooked meat (25). This accelerated rhabdomyolysis-overenthusiastically and inaccurately referred to as meltdown in the Australian press-caused acute renal failure, requiring months of dialysis. Twenty-seven days after admission, the leg was amputated at the hip. Mark remained comatose for 132 days before regaining consciousness and beginning years of convalescence. Although such a case illustrates some of the most extreme complications of dehydration and hyperthermia, the physiological challenge presented by exercise in hot weather can be extraordinary, even under normal circumstances. In fact, Rowel1 (40) has likened the circulatory stress of exercise in the heat to that accompanying hemorrhage because in both instances the cardiovascular system must cope with a diminishing central blood volume and falling arterial pressure. Most people successfully avoid the more severe problems associated with dehydration and hyperthermia by stopping exercise or by reducing exercise intensity, and by eventually rehydrating. It can be argued that even among competitive athletes, the risk of hyperthermia is often reduced by the inevitable decrease in exercise intensity-and heat production-that occurs as a result of dehydration. However, when a highly competitive psyche meets a warm and humid environment, the results can be disastrous for both performance and health. In this regard, it is interesting to note that exercise in the heat, even in the absence of dehydration or impairment in metabolic and cardiovascular response, can have a debilitating effect on performance. For example, Sawka et al. (45) demonstrated that heat stress reduced maximal aerobic power by approximately 7% in euhydrated subjects. Reaching a similar conclusion regarding the relationship between heat stress and performance, Nielsen and colleagues (33) concluded the following:

3 S64 / Murray The ultimate cause for the exhaustion in the severely hyperthermic condition may be due to an effect of heat stress on brain function. The central nervous system and mental functions are susceptible to high temperatures, as can be observed in the dizziness and confused behavior of heat-stressed subjects in long distance sports events.... it may be that core temperatures >39 O C r102.2 OF] reduce the function of motor centers and the ability to recruit motor units required for the activity, perhaps via an effect on the "motivation" for motor performance. (P. 1045) Effect of Heat Exposure and Dehydration on Physical and Mental Performance. It is well established that during continuous exercise, dehydration, beginning with as little as a 1 % decrease in body weight (lo), impairs physiological and performance responses (28,41-43,45,46) (Table I). Dehydration limits a wide range of cardiovascular and thermoregulatory responses (reviewed in 28, 41-43), culminating in an increase in core body temperature of OC ( OF) with every 1 % decrease in body weight (43). The physiological coup de grace of the impairment in cardiovascular and thermoregulatory function is a decrease in skin blood flow and sweating rate, leading to an inevitable rise in heat storage and core body temperature if exercise intensity is maintained. In 1985, Armstrong and colleagues (2) demonstrated that dehydration by 2% of body weight (by furosemide diuresis) reduced running performance in 1,500-, 5,000-, Table 1 Physiological Responses to Dehydration Physiological factor Gastric emptying rate Incidence of gastrointestinal distress Splanchnic and renal blood flow Plasma volume Plasma osmolality Blood viscosity Central blood volume Central venous pressure Cardiac filling pressure Heart rate Stroke volume Cardiac output Sweat rate at a given core temperature Core temperature at which sweating begins Maximal sweat rate Skin blood flow at a given core temperature Core temperature at which skin blood flow increases Maximal skin blood flow Core temperature at a given exercise intensity Performance

4 Fluid Needs / S65 and 10,000-m events by averages of 3.1, 6.7, and 6.3%, respectively. Although use of diuretics tends to produce a greater relative loss of plasma volume (in comparison to total body water) than does exercise-induced dehydration, similar impairments of physical work capacity have been demonstrated by numerous researchers, as noted by Sawka and Pandolf in their review (43). Not surprisingly, the greatest decrements in exercise performance occur during prolonged exercise in hot environments at high levels of dehydration, although even low levels of dehydration (1-2%) can have an ergolytic effect on performance (2, 45). Interestingly, dehydration not only reduces exercise performance in the heat but also induces exhaustion at a lower core temperature (46). During intermittent, short-duration exercise of high intensity (e.g., wrestling), the effects of dehydration on various performance indices (e.g., maximal muscle strength, anaerobic power and capacity) are less clear (21); however, regardless of the degree of influence that dehydration has upon high-intensity exercise pelformance, the chronic dehydration that often accompanies weight-class sports likely has a debilitating effect upon the athlete's ability to optimally train for competition. Mental performance also deteriorates as a result of heat exposure (18,36,39,5 1). Indices of sustained attention, error rate, response time, and task accuracy are all negatively affected during heat exposure, even when subjects are sedentary (and when significant dehydration is unlikely) (36). It should be noted that an increase in the conscious effort associated with performing mental tasks in the heat can maintain performance at levels similar to cooler environments, but this maintenance is only temporary (36). Acclimatization to the heat appears to improve mental performance; the upper limit of optimal mental functioning appears to be about 25 OC (77 OF) in resting, unacclimatized subjects and "C (86-95 OF) in acclimatized subjects (39). Interestingly, Wyon et al. (51) demonstrated that tasks associated with school work (e.g., sentence comprehension, multiplication, and word memory) were impaired in Danish high school students exposed to slowly rising ambient temperatures between 20 and 29 OC (68-84 OF); other performance measures such as spelling, vocabulary, and card-punching tests were not significantly affected by heat exposure. In summary, physical and mental performance is impaired during heat exposure, even in the absence of dehydration. (Evidently, the effect of dehydration on mental performance has not been adequately studied.) The reduction in physical and mental performance is thought to be due to some as yet undetermined effect of hypertherrnia on central nervous system function. Consequently, during warm-weather training and competition, athletes should recognize that reduced performance is an unavoidable consequence of heat exposure and a consequence that is made more severe with dehydration. Perhaps even more important, coaches must lower their expectations of their athletes' capacities for exercise during warm-weather training. Exercise in the Cold While there is a plethora of scientific literature on fluid balance during physical activity in warm environments, considerably less is known about the physiological responses to exercise in the cold, particularly in regard to the effects of dehydration. However, there are excellent reviews of this topic by Toner and McArdle (49) and Freund and Sawka (11). Although not as much may presently be known about the physiological responses to exercise in the cold as to those in the heat, it is known that euhydration can be compromised during exercise in the cold in a number of ways (1 1).

5 S66 / Murray Cold-Induced Diuresis. Although other physiological mechanisms for cold-induced diuresis have been proposed, one school of thought holds that cold exposure results in peripheral vasoconstriction, a resultant shunting of blood into the central circulation, a rise in arterial pressure, a rise in renal blood pressure, and increased excretion of sodium and water (50). Regardless of the mechanisms involved, coldinduced diuresis represents an avenue for fluid loss. (It is important to note that both exercise and dehydration reduce the magnitude of the diuresis.) Respiratory Water Loss. The partial pressure of water vapor in cold air is considerably less than that of warm air, establishing a pressure gradient for water loss. For example, when cold air is inhaled, it is immediately saturated with water vapor from the body; the lower the partial pressure of water vapor in the inspired air, the greater the water loss from the lungs. Consequently, expired air is maximally saturated with water vapor (44 mmhg at 37 "C; 98.6 OF). By contrast, the partial pressure of water vapor in air at 0 "C (32 OF) is only 5 mmhg. The resulting respiratory water loss can account for a total fluid loss from the body of approximately Lfday, values that can increase with the rise in minute ventilation that accompanies increasingly intense exercise (1 1, 14). Sweating. A sufficient rise in body temperature will result in the onset of sweating, regardless of the ambient temperature. This is particularly true in cold-weather exercise when there is often a mismatch between the insulating properties of clothing and the rate of metabolic heat production. Freund and Sawka (1 1) estimated that sweat loss can reach 2 L/hr under conditions of moderate to heavy exercise (metabolic rate = 600 W) at 0 "C (32 OF) conducted in clothing that provides insulation properties of 4 Clo (1 Clo is the insulation equivalent to a business suit). Unavailability of Fluids. It is probably safe to say that most people do not give much thought to ingesting fluid during exercise in the cold, and those who do often struggle with ways of keeping the fluid from freezing. For these reasons, fluid is often not available during cold-weather exercise. Reduced Fluid Intake. As is true for exercise in the heat, the thirst mechanism is not a good indicator of body fluid needs. Consequently, dehydration during coldweather exercise is not uncommon (1 1). It is also likely that some people reduce fluid intake as a way of avoiding the logistical problems and the discomfort often associated with urinating in the cold. Effect of Dehydration on Mental and Physical Performance in the Cold Independent of dehydration, cooling of core and limb temperatures below normal has performance-limiting consequences. Coordination, fine motor skills, strength, sprint performance, maximal power production, and maximal aerobic power are all impaired during cold exposure (11, 49). Also, dehydration by 2.5% of body weight reduces cognitive performance in a cold environment, as determined via tests of grammatical reasoning, number comparison, and computer interaction (3, 11). Although there is a paucity of research on the effects of dehydration on responses to cold-weather exercise, Freund and Sawka (1 1) postulated that dehydration will have deleterious effects on physiological function and performance in the cold, provided that both work rates and heat storage are high. In other words, the negative ramifications of dehydration are likely to be less evident at low work rates when the rates of heat production are equal to or less than the rates of heat loss. At higher work rates, when heat production is high and heat loss may be impeded by clothing insulation, the deleterious effects of dehydration

6 Fluid Needs / S67 are likely to be similar to those seen during exercise in warm environments. Finally, dehydration has been identified as one of the predisposing factors to increased risk of frostbite (12), an effect attributable to the decrease in skin blood flow with dehydration. However, Freund and Sawka (1 1) pointed out the need for further research in this area. Drinking to Improve Bodily Function and Exercise Performance As pointed out by Coyle and Montain (8), the current recommendations of the American College of Sports Medicine (1) provide only very general guidelines regarding the volume and frequency of fluid intake during exercise. The ACSM recommends that ml of fluid be ingested every 2-3 km during running, a fluid intake of 330 ml/hr for slow runners or 2,000 rnl/hr for fast runners. Both rates of intake would be inappropriate for a runner losing sweat at the rate of 1,000 ml/hr, a very typical sweat rate for many athletes (43). When fluid is available during exercise, athletes voluntarily ingest approximately 50% of their sweat loss (34). In other words, an athlete losing sweat at the rate of 1,000 ml/hr will typically ingest fluid at the rate of 500 mllhr. This imbalance between fluid loss and fluid intake will result in substantial dehydration over time. As discussed subsequently, there are well-established physiological and performance benefits (4,26) associated with matching fluid intake to fluid loss as closely as is practically possible during exercise. Factors That Influence Fluid Replacement Palatability. During both rest and exercise, people tend to drink more of what tastes good to them. This commonsensible conclusion is particularly important during exercise, because voluntary fluid intake increases with the perceived palatability of the beverage (5). Gastric Emptying. Dehydration reduces gastric emptying rate (32) and increases the risk of gastrointestinal distress (37), two additional reasons to avoid dehydration. Gastric emptying can be maintained at high levels by beginning exercise with a comfortably large volume of fluid in the stomach (e.g., 400 ml) and by ingesting fluid at frequent intervals (e.g., min) during exercise. This practice maintains a high gastric volume, the primary driver of rapid gastric emptying (24, 29). intestinal Absorption. Although the influence of dehydration on intestinal absorption is not known, it is well recognized that ingestion of a carbohydrate-electrolyte beverage stimulates significantly greater fluid absorption, and reduces the extent of diuresis, compared to plain water (13, 47). For example, in just 40 cm of duojejunum, fluid absorption rates averaged 480 mlbr with a 6% carbohydrate-electrolyte beverage and 324 ml/hr with water (13). Acclimation. Interestingly, in addition to the improvements in cardiovascular and thermoregulatory function that accompany acclimation to the heat (4.9, another important benefit is an increase in voluntary fluid intake (20,43). Although it is unclear why acclimation to the heat makes us better drinkers, this favorable adaptation reduces the extent of voluntary dehydration that often goes hand in hand with exercise in the heat. However, as Sawka and Pandolf (43) pointed out, while acclimation improves the relationship between thirst and body fluid needs, voluntary dehydration still occurs during physical activity in the heat.

7 S68 / Murray Responses to Ingesting Varying Volumes and Timing of Fluid During Exercise Physiologiical Response to Varying Volumes of Fluid Intake. Data from Montain and Coyle (26) clearly illustrate the deleterious effects of dehydration-and the benefits of fluid intakrluring cycling exercise (Figure 1). Trained cyclists exercised in the heat for 2 hr at about 65% V02max, during which time they ingested either no fluid or volumes of fluid equivalent to 20,50, or 80% of their predicted sweat losses. These drink volumes resulted in dehydration of 4, 3, 2, and 1% of body weight, respectively. The investigators measured changes in blood volume, stroke volume, heart rate, cardiac output, forearm blood flow, and esophageal temperature. With each measure, the more fluid the subjects ingested during exercise, the better the physiological response. The authors concluded that there is not a critical level of dehydration that can be tolerated before cardiovascular and thermoregulatory functions are impaired, and they recommended that people attempt to drink fluids at the same rate that they are dehydrating due to sweating (or at least drink at a rate that is close to 80% of sweating rate; see Table 2) (8). Perceptual Response to Varying Volumes of Fluid Intake. Interestingly, the perception of exercise intensity is also influenced by dehydration. The ratings of perceived exertion among the subjects in the study by Montain and Coyle (26) were significantly lower at the end of 2 hr of exercise when fluid intake was 50 and 80% of sweat loss (700 to 1,200 ml/hr) than with an intake of zero or 300 ml/hr. Physiological Responses to the Timing of Fluid Intake. Montain and Coyle (27) also studied how the timing of fluid ingestion during 140 min of cycling exercise in the heat influenced cardiovascular and thermoregulatory responses. Subjects drank 1,183 ml of a sports drink (-43% of predicted sweat rate) at the onset of exercise, or at 40 or 80 min of exercise, or at 15-min intervals throughout exercise. In all trials, the extent of dehydration at the end of exercise was similar (-2.9% BW). As demonstrated in previous research (26), drinking attenuated the increase in serum osmolality and sodium concentration, increased forearm blood flow, maintained blood volume, and reduced the rate of heat storage. When fluid was ingested in one large bolus at 0, 40, or 80 min, the aforementioned changes were transient, lasting about 40 min after fluid ingestion. There were no differences in ratings of perceived exertion among the 0-, 40-, and 80- min trials. When fluid was ingested at regular intervals throughout exercise, rectal temperature at the end of exercise was significantly lower than when fluid was ingested at 80 min. Brown (6) reported similar findings for heart rate and rectal temperature when subjects ingested water at regular intervals throughout 165 min of exercise rather than waiting until 135 min to drink. A possible advantage of drinking at regular intervals is that the act of drinking stimulates heat loss by maintaining sweat rate. For example, sweating is known to increase almost immediately following drinking in dehydrated subjects (48). From a practical standpoint, ingesting ample volumes of fluid at regular intervals during exercise appears to confer "optimal" physiological response, although similarly positive responses can be provoked by ingesting a relatively large bolus of fluid during exercise. This information may be valuable for those circumstances (e.g., soccer, rugby, etc.) when it is not possible to ingest fluid at regular intervals during physical activity. Miscellaneous Topics in Fluid Replacement The performance benefits of adequate fluid replacement were studied by Below and Coyle (4), who had subjects cycle for 50 min at 80% V0,max under four conditions.

8 h ,b, my $g r tii 3k, 1-0- No Fluid -m- Small Fluid A- Moderate Fluid ; 37.0: n t Large Fluid.L ' Time (min) *t *t Fluid Needs / S Figure 1 - Core temperature (esophageal temperature), heart rate, and perceived exertion during 120 min of exercise when ingesting no fluid, or small (300 mar), moderate *t (700 mar), and large (1,200 mllhr) *I volumes of fluid. A rating of 17 for perceived exertion corresponds to very hard, 15 is hard, and 13 is somewhat hard. Values are means + SE. *Significantly lower than no fluid, p. i - l. l. l. i ' <.05. Significantly lower than small fluid, p < Significantly lower than moderate fluid,p <.05. Redrawn r - 1. r r ' with permission from E.F. Coyle and S.J. Montain. Benefits of fluid replacement with carbohydrate during * exercise. Med. Sci. Sports Exerc. 24: S324-S330,1992. * t *t On one trial, the subjects were allowed to ingest only 200 mi of water. On another trial, the subjects ingested 1,330 ml of water (about 100% of sweat loss). A third trial provided the subjects 200 ml of water with 80 g of carbohydrate. In the fourth trial, 1,330 ml of fluid was given with 80 g of carbohydrate. Following the 50-min exercise period, the subjects completed a performance task requiring them to complete a set amount of work in the shortest period of time. When ample fluid and carbohydrate were ingested, performance was significantly improved by an average of 12.4% (over the 200 ml water trial); water ingestion alone (1,330 ml) improved performance by 6%, and a like

9 S70 / Murray Table 2 Strategies for Matching Fluid Intake With Sweat Loss Sweat rate Fluid intake Frequency (ml/hr) (Ibhr) (ml) (02) (min) improvement was noted when carbohydrate was ingested with the small volume of water. The investigators concluded that adequate fluid replacement and carbohydrate feeding result in additive benefits to exercise performance. For athletes, military personnel, physical laborers, or anyone who is physically active in a warm environment, dehydration is a constant threat to optimal physical and mental function. This may be particularly true during twice-daily training sessions, during day-long athletic competitions, and even as a result of working outdoors. Under all such circumstances, the effects of dehydration can be exacerbated if adequate amounts of fluid and sodium chloride are not consumed between exercise sessions. Complete rehydration is dependent upon the ingestion of adequate fluid and sodium chloride. For example, an individual who finishes a training session in a state of modest dehydration (e.g., 2% of body weight, 1.6 kg for an 80-kg athlete) will require in excess of 1.6 L of fluid (the excess is needed to allow for obligatory urine loss) and approximately 1.8 g of sodium (1.6 L of sweat x 50 mrnol Na+/L x 23 mg/mmol Na+) to ensure complete rehydration prior to the next training session. Sodium chloride plays a critical role in rehydration because the osmotic properties of the sodium and chloride ions serve to retain fluid in the extracellular space and to reduce urine production (31). For these reasons, ingestion of a carbohydrate-electrolyte beverage containing -20 rnrnol/l Na+ following dehydrating exercise provided more complete rehydration than did the ingestion of plain water or diet cola (17). Glycerol, the molecular backbone of mono-, di-, and triglycerides, has received recent attention in the popular press (7, 22) as a possible method of "hyperhydrating" in an attempt to provide a cardiovascular and thermoregulatory advantage during exercise in the heat. Indeed, ingestion of glycerol solutions prior to exercise decreases urine production, with retention of substantial volumes of fluid ( kg; lb) (23). This transient state of hyperhydration is provoked by the osmotic effect of the glycerol molecules, which are only slowly cleared by metabolism from the body water. Glycerol ingestion increases the osmolality of the blood and most other body fluid compartments (the aqueous humor and the cerebrospinal fluid being two notable exceptions), temporarily reducing urine production.

10 Fluid Needs / S71 Some researchers have reported positive physiological responses (e.g., expanded plasma volume, lower heart rate, lower rectal temperature) to glycerol ingestion prior to exercise (23,38), but the physiological and performance implications of these responses should be studied further to more fully elucidate the potential of glycerol as a hyperhydrating agent. Furthermore, it is unlikely that glycerol solutions ingested during exercise will promote demonstrable improvements in cardiovascular or thermoregulatory function, if only because ingestion of glycerol during exercise does not allow time for the glycerol to be distributed throughout the body fluid compartments (30). Teleologically, it seems that glycerol-induced hyperhydration may have potential application for individuals who must exercise or labor in extremely hot environments and who have no access to fluid during that time. Examples include firefighters of all types (communities, forests, oil wells, etc.), military personnel during desert operations, hazardous-waste workers who must labor in fully confined protective suits, and industrial workers who are exposed to high-heat operations. However, more research is required to fully assess the range of physiological responses to glycerol ingestion and to determine the cost:benefit ratio associated with the body-weight gain that goes hand in hand with glycerol ingestion. The latter response may be particularly problematic in runners and cyclists, who pay a metabolic-and perhaps performance--cost for carrying extra body weight. Until more research has been conducted on the effects of glycerol ingestion, it is unwise to recommend this practice to athletes, in part because the side effects of ingesting glycerol can range from mild sensations of bloating and lightheadedness to more severe symptoms of headaches, dizziness, nausea, and vomiting (16, 30). Summary Exercise during both the heat and cold poses severe physiological challenges for the human body, not the least of which is dehydration. The body fluid loss that naturally accompanies most types of physical activity impairs both physiological function and performance, an impairment that is even greater when exercise is conducted in the heat. Anyone who is physically active in a warm environment can benefit by attempting to match sweat loss with fluid intake. The maintenance of euhydration (or as close to it as is practically possible) is a clear physiological and performance advantage for anyone who is physically active. References 1. American College of Sports Medicine. Position stand on the prevention of thermal injuries during distance running. Med. Sci. Sports Exerc. 17:ix-xiv, Armstrong, L.E., D.L. Costill, and W.J. Fink. Influence of diuretic-induced dehydration on competitive mnning performance. Med. Sci. Sports Exerc. 17: , Banderet, L.E., D.M. MacDougall, D.E. Roberts, D. Tappan, M. Jacey, and P. Gray. Effects of dehydration or cold exposure and restricted fluid intake upon cognitive performance (Technical Note T15/86). Natick, MA: U.S. Army Research Institute of Ealvironmental Medicine, Below, P.R., R. Mora-Rodriguez, J. Gonzalez-Alonso, and E.F. Coyle. Fluid and carbohydrate ingestion independently improve performance during Ih of intense exercise. Med. Sci. Sports Exerc. 27: , Boulze, D., P. Monstruc, and M. Cabanao. Water intake, pleasure and water temperature in humans. Physiol. Behav. 30:97-102, Brown, A.H. Water storage in the desert. In Physiology of Man in the Desert, E.F. Adolph (Ed.). New York: Interscience, 1947, pp Burke, E.R. Super-hydration drinks. Winning March 1994, p. 72.

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