Fluid Balance and Exercise Performance

Transcription

1 Mal J Nutr 9(1): 53-74, 2003 Fluid Balance and Exercise Performance Rabindarjeet Singh Sports Science Unit, School of Medical Sciences, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia. rabindar@kck.usm.my ABSTRACT Major sporting events in Malaysia are commonly staged in hot environments where the average daytime temperature is generally in the range of 29 to 31 C with the average relative humidity ranging from 80 to 95%. Exercise capacity and exercise performance are reduced when the ambient temperature is high and it has major implications for competitors as well as for spectators and officials. Prolonged exercise leads to progressive water and electrolyte loss from the body as sweat is secreted to promote heat loss. The rate of sweating depends on many factors and increases in proportion to work rate and environmental temperature and humidity. Sweat rates are highly variable and can exceed 2L.h -1 for prolonged periods in high heat. Since dehydration will impair exercise capacity and can pose a risk to health, the intake of fluid during exercise to offset sweat losses is important. Carbohydrate-electrolyte fluid ingestion during exercise has the dual role of providing a source of carbohydrate fuel to supplement the body s limited stores and of supplying water and electrolytes to replace the losses incurred by sweating. The composition of the drinks to be taken will be influenced by the relative importance of the need to supply fuel and water which, in turn depends on the intensity and duration of exercise activity, the ambient temperature, and humidity. Carbohydrate-electrolyte solutions appear to be more effective in improving performance than plain water. There is no advantage to fluid intake during exercise of less than 30-minute duration. Complete restoration of fluid balance after exercise is an important part of the recovery process and becomes even more important in hot, humid conditions. If a second bout of exercise has to be performed after a relatively short interval, the speed of rehydration becomes of crucial importance. Rehydration after exercise requires not only replacement of volume losses, but also replacement of some electrolytes, primarily sodium. Studies show that rehydration after exercise can be achieved only if sweat electrolyte losses as well as water are replaced. Drinks with low sodium content are ineffective at rehydration and they will only reduce the stimulus to drink. Addition of smalls amounts of carbohydrate to the rehydrating drinks may improve the rate of intestinal uptake of sodium and water and will improve palatability. The volume of the rehydration beverage consumed should be greater than the volume of sweat lost to provide the ongoing obligatory urine losses. Palatability of the beverage is a major issue when a large volume of fluid has to be consumed. INTRODUCTION Water balance is critical for the functioning of all organs and for maintenance of health in general (Mack & Nadel, 1996; Sawka, 1988;). Water provides the medium for transport of nutrients, gases and waste products and for all biochemical reactions within cell tissues. Water is essential for maintaining an adequate blood volume and thus the integrity of the cardiovascular

2 Rabindarjeet Singh system. The body s ability to redistribute water within its fluid compartments provides a reservoir to minimise the effects of a water deficit. Each body water compartment contains electrolytes; the concentration and composition of which are critical for moving fluid between intracellular and extracellular compartments and for maintaining membrane electrolyte potentials (Mack & Nadel, 1996; Sawka, 1988; Sawka & Montain, 2000). The physicochemical properties of water also assist thermal homeostasis by thermal conduction and latent heat of vaporisation. During exercise, the heat generated from energy metabolism can easily increase 10-fold in active healthy persons, and up to 20-fold in well-trained athletes (Sparling & Millard-Stafford, 1999). About 80% of this energy is released as heat; only a small proportion is converted to muscular work. This heat will flow down a temperature gradient from the muscle to body core and to the skin; from here heat is dissipated into the environment. To counter heat accumu-lation and a rising core temperature, metabolic heat generated by exercise must be dissipated to the environment to maintain body temperature within narrow physio-logical limits. At rest, overheating is prevented primarily through radiation, convection and conduction and to a lesser degree by sweat evaporation. During exercise, evaporation of sweat becomes the major means of heat transfer (Sparling & Millard-Stafford, 1999). Therefore, when ambient temperature exceeds skin temperature, heat loss can occur only by evaporation of sweat from the skin surface. The evaporation of 1 L of sweat from the skin will release 580 kcal of heat from the body (Maughan & Shirreffs, 1998). Significant rates of sweat production will also occur in a cool environment if the exercise is of high intensity. High rates of sweat secretions are essential during prolonged hard exercise to limit the rise in body temperature which, would otherwise occur. Sweat rates exceeding 2L. h -1 can be maintained for many hours by trained and acclimatised people exercising in warm humid conditions (Costill, 1972). The loss of large amounts of sweat has a disadvantage in that it leads to progressive dehydration. At high sweat rates, a significant proportion of the sweat may not be evaporated, but simply drips from the skin. This part of the water is not effective in cooling the body and therefore increases the risk of dehydration without any benefit to the athlete. FLUID LOSS The rate at which sweat is secreted onto the skin depends on the need to lose heat, which is determined mainly by the rate of heat production and ambient temperature (Maughan & Noakes, 1991). Heat produc-tion is directly proportional to the intensity of exercise where for sports like running and cycling, heat production is therefore a function of speed. For most other sports however, the work intensity is not constant, but consists of rest, low and high intensity exercise periods. In a marathon race the faster runners lose sweat at a higher rate than the slower runners, but on the other hand they are active for a shorter period of time, so the total sweat loss of these marathon runners is unrelated to finishing time. Among competitors in all sporting events, it is obvious that the rate of sweat secretion varies considerably between individuals. Some people sweat profusely, and a large part of the sweat produces drips from the body surface without

3 Fluid Balance and Exercise Performance evaporating. This may be considered an inefficient sweating mecha-nism: the rate of sweat secretion is not directly linked to the rate of heat production. The reasons for this excessive sweat secretion in some individuals are not clearly understood. Marathon runners competing under the same conditions and with the same fluid intake at low (10 C) ambient temperature may lose from as little as 1% to as much as 6% of the body weight during a race ( kg of body mass for a 70-kg man) (Maughan & Miller, 1984). At high ambient temperatures, sweat losses equi-valent to 8% body weight may occur in marathon runners: this amounts to 5 6L of water for a 70-kg individual (Maughan & Shirreffs 1998). A loss of body water corresponding to as little as 2% of body weight can result in some impairment of exercise tolerance (Armstrong, Costill & Fink, 1985; Maughan & Shirreffs, 1998; Nielsen, Sjogaard & Boude-Petersen, 1984; Walsh et al., 1994;) and losses in excess of 5% of body weight can decrease the capacity for work by about 30% (Saltin & Costill, 1988). The fluid losses are distributed in varying proportions between the plasma, the extracellular water and the intracellular water. The reduction of plasma volume may be of particular significance and could have a significant impact on perfor-mance. Blood flow to the muscles must be maintained throughout the exercise period to supply oxygen and substrates to the exercising muscles and to remove carbon dioxide and other waste products. In addition to that, blood flow to the skin must also be adequate to carry heat to the body surface where it can be dissipated. If the blood volume decreases due to fluid loss, there may be difficulty in meeting the requirements of a high flow rate to both muscle and skin. Decreases in plasma volume may also result in significant increases in the viscosity of the blood, forcing the heart to work harder to pump it around the circulatory system (Costill & Fink 1974; Fortney et al., 1981; Fortney et al., 1984; Greenleaf et al., 1977; Horstman & Horvath 1972; Nadel, 1980; Nadel, Fortney & Wenger, 1980; Nielsen, 1974, Saltin & Costill, 1988; Saltin & Stenberg 1964). Dehydration may therefore be more important than substrate depletion in causing fatigue during prolonged exercise, particularly in hot weather where fluid losses are high and where it is not possible to replace the losses during the exercise period. Performance in events requiring brief bursts of very high intensity exercise may be affected more than events which, involve prolonged relatively low intensity exercise. When the ambient temperature is high and blood volume has been decreased by sweat loss during prolonged exercise, there may be difficulty in meeting the requirements for a high blood flow to both the working muscles and skin. In this situation also, skin blood flow is likely to be compromised, allowing central venous pressure and muscle blood flow to be maintained but reducing heat loss and causing body temperature to rise (Rowell, 1986) and perhaps contributing to the fatigue. EFFECT OF ENVIRONMENT AND TRAINING STATUS ON FLUID BALANCE The major influence on the physical exchange of heat between the body and the environment is the ambient temperature. When ambient temperature exceeds skin temperature, heat is gained from the environment by physical transfer, leaving evaporative loss as the only mechanism available to prevent or limit a rise in body temperature. Hence, increased sweating rate in the heat will result in an increased requirement for fluid replacement. A precaution which can be taken to prevent sweat loss, and hence reduce the need for fluid replacement is to limit the extent of warm-up prior to the competition or reduce the clothing worn.

4 Rabindarjeet Singh When the humidity is high and particularly when there is no wind, evaporative heat loss will be severely affected which, will then limit exercise tolerance by dehydration and hyperthermia rather than by the limited carbohydrate stores in the body. Suzuki (1980) reported that exercise time at 66% VO 2max was reduced from 91 min when the ambient temperature was 0 C to 19 min when the same exercise was performed at 40 C. It is recognised that training will provide some protection against the development of heat illness during exercise in the heat. This adaptation is most marked when training is carried out in a hot environment (Senay, 1979). One of the benefits of training in a hot environment is the expansion of plasma volume (Hallberg & Magnusson, 1984), which occurs within a few hours of completion of exercise and may persist for several days (Davidson et al., 1987; Robertson, Maughan & Davidson, 1988). This post-exercise hypervolaemia should be regarded as an acute response rather than an adaptation, although it may appear to be one of the first responses to occur when an individual starts a training programme. This increased resting plasma volume in the trained athlete allows the endurance-trained individual to maintain a higher total blood volume during exercise (Convertino, Keil & Greenleaf, 1983) allowing for better maintenance of cardiac output at the cost of a lower circulating haemoglobin concen-tration. In addition, the increased plasma volume is associated with an increased sweating rate which limits the rise in body temperature (Mitchell et al., 1976). The increase in plasma volume increases progressively over the first 6 days, reaching a value of about 23% greater than the control, with little change thereafter (Mitchell et al., 1976; Senay, Mitchell & Wyndham, 1976). The main adaptation in terms of an increased sweating rate and an improved thermoregulatory response occurs slightly later than the cardiovascular adaptations (Wyndham et al., 1976). With training, better maintenance of body temperature is achieved at the expense of an increased sweat rate. Although the increased sweat rate allows for a greater evaporative heat loss, the proportion of the sweat which is not evaporated and which therefore drips wastefully from the skin is also increased (American College of Sports Medicine, 1996). Although a high sweat rate may be necessary to ensure adequate evaporative heat loss, unfortunately many individuals have an inefficient sweating mechanism. EFFECT OF DEHYDRATION ON EXERCISE PERFORMANCE Numerous studies have examined the influence of dehydration on maximal aerobic power and physical work capacity (Sawka, Montain & Ladzka, 1996). The physical work capacity for aerobic exercise of progressive intensity is decreased when a person is dehydrated (Sawka et al., 1996). Physical work capacity has been shown to be decreased even with marginal (1 2% body weight loss) water deficits and the reduction is larger with increasing water deficits (Armstrong et al., 1985; Caldwell, Ahonen & Nousiainen, 1984). Dehydration results in much larger decrements of physical work capacity in hot than in temperate climates (Buskirk, Lampietro & Bass, 1958; Caldwell et al., 1984; Craig & Cummings, 1966; Webster, Rutt & Wettman, 1990). In a temperate environment, less than 3% body weight loss appears not to alter maximal aerobic power. Maximal aerobic power decreased when dehydration equalled or exceeded 3% body weight. In a hot environment on the other hand, Craig and Cummings (1966) demonstrated that small (2% body weight) to moderate (4% body weight) water deficits resulted in large reductions

5 Fluid Balance and Exercise Performance of maximal aerobic power. Even at low levels of dehydration (1.8%), high-intensity (90% VO 2max ) exercise performance time is reduced (Walsh et al., 1994). It is believed that the thermoregulatory system, via increased body temperature plays an important role in the reduced exercise performance mediated by a body water deficit. Table 1 presents a summary of investigations concerning the influence of dehydration on maximal aerobic power and physical work capacity. On the effects of dehydration on physiologic tolerance to submaximal exercise, Adolph (1947) reported that 16% and 2% of the soldiers suffered exhaustion from heat strain during an endurance (2-23 h) desert walk (at km/h) (T a ~38 C) when they did not drink and did drink water ad libitum, respectively. Ladell (1955) had subjects attempt 140-min walks in a hot (T a ~38 C) environment while ingesting different combinations of salt and water. They reported that exhaustion from heat strain occurred in 75% of their subjects when not receiving water and in 7% of their subjects when receiving water. Clearly, dehydration increases the incidence of exhaustion from heat strain. Sawka et al. (1985) had subjects attempt lengthy treadmill walks (~25% of VO 2max for 140 min) in a hot-dry (T a = 49 C, rh = 20%) environment when euhydrated and when dehydrated by 3%, 5% and 7% of their body weight. All eight subjects completed the euhydration and 3% dehydation experi-ments, and seven subjects completed the 5% dehydration experiments. In the 7% dehydration experiment, six subjects discontinued after completing only 64 min (mean). Clearly dehydration increases the incidence of exhaustion from heat strain. Dehydration also impairs competitive middle distance running performance in athletes. Armstrong et al. (1985) had athletes compete in 1500-, and m races when euhydrated and when dehydrated. Dehydration (2% body weight loss) was achieved by diuretic administration, which decreased plasma volume by 11%. Running performance was impaired at all distances but more in the longer races (~6% for the 5000 and 10000m) than in the shortest race (3% for the 1500m). Burge, Carey & Payne, (1993) who examined simulated 2000-m rowing performance, found that it took the rowers an average of 22s longer to complete the task when they were dehydrated than when they were euhydrated. In addition, dehydration reduced average power by 5%. In more prolonged exercise, sweat rates of 2-3 L h -1 are possible. The combined effects of progressive dehydration and a rising body temperature pose a considerable threat to the runner. During a marathon race even at high ambient temperatures, runners may lose as much as 8% of body weight, corresponding to about 13% of total body water (Costill, 1972). Even in an event as short as 10km, losses corresponding to more than 2% of body weight are possible. Many of the highest values of rectal temperatures in distance runners have been observed after races at distances less than the marathon. Among competitors in a 14-km road race, Sutton (1990) reported more than 30 cases over a period of years where rectal temperatures exceeded 42 C. From these cases, hyperthermia may be more common when the rate of heat production is very high as in races of 10km (England et al., 1982). At such high exercise intensities, skin blood flow is likely to be reduced with a larger fraction of the cardiac output being directed to the working muscles, and so heat loss will be reduced.

6 Rabindarjeet Singh For an extensive review on the factors influencing fluid loss during exercise, and the effects of dehydration on exercise performance, see Lamb and Brodowicz (1986) and Maughan and Shirreffs (1998). FLUID REPLACEMENT DURING EXERCISE Physical working capacity is impaired when fluid losses cause even small reductions in plasma volume (Saltin, 1964). In prolonged exercise, the maintenance of plasma volume may be of major importance. A progressive rise in heart rate normally occurs during exercise at a constant work load: this reflects the increased cardiac output necessary to meet the requirements for an increased skin blood flow to promote heat loss while maintaining the supply of oxygen and substrates to the working muscles (Rowell, 1974). The redistribution of regional blood flow includes a decreased flow from the intestines to the liver, which may reduce the rate of gastric emptying and intestinal absorption. The maximum rate at which water can be absorbed from the gut during strenuous exercise may therefore become inadequate to match the high rates of sweat loss which can occur (Costill, Kammer & Fisher, 1970). Table 1. Hypohydration effects on maximal aerobic power and physical work capacity Study Dehydration Procedure WT Test Environment Exercise Mode Maximal Aerobic Power Physical Work Capacity Armstrong et al. (1985) Diuretics -1% Neutral TM ND 6% Caldwell et al. (1984) Exercise -2% Neutral CY ND 7W diuretics -3% Neutral CY 8% 21W sauna -4% Neutral CY 4% 23W Saltin (1964) Sauna, heat, exercise, diuretics -4% Neutral CY ND (?) Pichan et al (1988) Fluid restriction -1% Hot CY - 6% -2% Hot CY - 8% -3% Hot CY - 20% Craig and Cummings Heat -2% Hot TM 10% 22% (1966) -4% Hot TM 27% 48% Buskirk, Lampietro & Exercise, heat -5% Neutral TM Bass (1958) L.min -1 Webster et al. (1990) Exercise in heat, sauna -5% Neutral TM 7% 12% Herbert and Ribisl (1971)? -5% Neutral CY - 12% Houston et al. (1981) Fluid restriction -8% Neutral TM ND 17% TM = treadmill; CY = cycling

7 Fluid Balance and Exercise Performance The ability to perform prolonged exercise in the heat requires replacement of water losses to prevent dehydration. Numerous studies have shown that water intake during prolonged exercise is effective in improving performance and in delaying the onset of fatigue (Armstrong et al., 1985, Below et al., 1995; Costill et al., 1970; Montain & Coyle, 1992; Sawka & Pandolf, 1990; Sawka et al., 1984; Sawka, 1992). To sustain a high rate of work output or exercise performance in heat requires replacement of water losses to prevent dehydration, but exercise performance may also be limited by the availability of carbohydrate as fuel for the working muscles. Carbohydrate-electrolyte fluid ingestion during exercise therefore, has dual functions of supplying water to replace the losses incurred by sweating and of providing a source of carbohydrate fuel to supplement the body s limited stores (Lamb & Brodwicz, 1986; Coyle & Coggan, 1984; Maughan & Shireffs, 1998; Murray, 1987; Singh et al., 1997; Singh et al., 1995; Tsintzas et al., 1995). Below et al. (1995) have shown that ingestion of water and carbohydrate have independent and additive effects on exercise performance. However, in one study, water replacement at a rate of 100ml every 10 min did not improve endurance during cycle exercise at 70%VO 2max, whereas ingestion of 4% glucose-electrolyte solution did significantly extend further the exercise time (Maughan, Fenn & Leiper, 1989). The rates at which substrate and water can be supplied during exercise are limited by the rates of gastric emptying and intestinal absorption. It is not clear which of these processes is limiting. It is commonly assumed that rate of gastric emptying is the limiting factor and that it determines the maximum rates of fluid and substrate availability (Lamb & Brodwicz 1986; Murray, 1987). Increasing the carbohydrate content of drinks will slow the rate of gastric emptying (Costill & Saltin, 1974), thereby decreasing the rate at which fluid loss can be replaced. The presence of glucose and sodium in the lumen of the small intestine stimulates water absorption, provided that the osmolality of the solution is not high (Leiper & Maughan, 1988). On the other hand, increasing the carbohydrate content of drinks will increase the amount of fuel which can be supplied, but will also tend to decrease the rate at which water can be made available. Where provision of water is the first priority, the carbohydrate content of drinks will be low, thus restricting the rate at which substrate is provided. The composition of drinks to be taken will thus be influenced by the relative importance of the need to supply fuel and water; this in turn depends on the intensity and duration of the exercise task, the ambient temperature and humidity and the physiological and biochemical characteristics of the individual athlete. When water replacement is the first priority, an isotonic or moderately hypotonic solution containing glucose and sodium will be most effective (Farthing, 1988). Most commercial sports drinks contain 6-8% carbohydrate, about mmol.l -1 sodium and a low (4 5 mmol.l -1 ) concentration of potassium. These formulations represent a compromise between that which will give the highest rates of fluid replenishment and that which can provide most carbohydrate. From a summary of all these studies, it has been reported that similar rates of gastric emptying are seen for beverages in which the carbohydrate content ranges from 0 to 10% (Maughan, 1997). Using a wide variety of experimental models, the effects of feeding different types and amounts of beverages have been extensively investigated. Most have shown a positive effect of fluid ingestion on performance. Coyle et al., (1986) demons-trated that a fixed workload can be sustained 30% longer (from 3 to 4h) when carbo-hydrate solutions are given during exercise. Similarly, five other well controlled trials where continuous cycling exercise for 2h or more was performed, showed improvements in exercise with administration of carbohydrate-containing

8 Rabindarjeet Singh drinks (Björkman et al., 1984; Coggan & Coyle 1988; Coggan & Coyle, 1989; Coyle et al., 1983; Ivy et al., 1979). However, three studies showed no significant effects of administration of carbohydrate-containing drinks (Brookes, Davies & Green, 1975; Felig et al., 1982; Flynn et al., 1987) and in none of these studies was performance adversely affected. In all the studies where positive effects were seen, the total volume of fluid consumed varied from 0.4 to 2.7L and the carbohydrate intake ranged from 120 to 410g. In studies employing prolonged intermittent cycle exercise followed by brief high intensity sprint, performance of sprint lasting 12 to 14min was enhanced if solutions containing 50, 60 or 75g/L of various sugars were given at a rate of 8.5 ml/kg/hr (Mitchell et al., 1988) or when a 6% sucrose solution (42g in 692ml) was given (Murray et al., 1989b). Similarly, 47g glucosesucrose mixture with added electrolyte or 55g glucose polymer-fructose mixture with added electrolytes also improved performances of a sprint ride lasting about 6 minutes (Murray, 1987). However, when low glucose polymer, fructose (76g in 1.27L) or when 8 10% carbohydrate polymer solutions were given, no improvements in performances were seen (Kingwell et al., 1989; Murray, 1987; Murray et al., 1989a). As with cycling, running performance too improved when carbohydrate-containing beverages were given (90g sucrose in 500ml (Sasaki et al., 1987); 7% carbohydrate- electrolyte solution (Macaraeg, 1983). In a study involving prolonged walking, walking time increased when 120g glucose polymer was given in 1.5L compared with a placebo (Ivy et al., 1983). In another experimental model where subjects are able to adjust the treadmill speed while running either to cover the maximum distance possible in a fixed time or complete a fixed distance in the fastest time possible, Williams and coworkers, showed an increased running speed in the closing stages of the trails when carbohydrate-containing drinks were given (Williams, 1989; Williams et al., 1990). In field studies, despite the many practical difficulties associated with the conduct of such studies and the lack of well-controlled studies, studies have shown the ergogenic effects of administration of fluids or glucose-electrolytes solutions. A 7 mile (11.2km) walk/run course was completed when given glucose-electrolyte solution as compared to no ingestion of fluids (mean 7.5 km) (Cade et al., 1972). In a study where matched groups of marathon runners consumed 1.4L of either water or a glucose-electrolyte solution, 60% of the competitors were able to run faster than expected when drinking carbohydrate-electrolyte solution compared with 40% when water was given (Maughan & Whiting, 1985). Similarly, soccer players utilised 31% less glycogen when 7% glucose polymer was given during a practice match compared to the placebo group (Leatt, 1986). This reduced utilisation translates to beneficial effect in the latter stages of the game by players taking the glucose polymer, where it has been shown that soccer players beginning the game with low muscle glycogen concentration covered less distance during the game, particularly during the second half, and spent more time walking and running at low speed (Saltin & Karlsson, 1977). Despite the clear evidence demons-trating the negative effects of dehydration on exercise performance and the positive effects of fluid ingestion (Herbert, 1983), most athletes still do not drink enough to match fluid losses during exercise (Nokes, 1992). This may in part be due to the relative insensitivity of the thirst mechanism in humans, and some degree of dehydration, as

9 Fluid Balance and Exercise Performance much as 2% of body weight is normally incurred before the stimulus to drink is initiated (Adolph, 1947). However, this level of dehydration is sufficient to impair exercise performance and thermoregulatory capacity. Drinking can be stimulated to some extent by improving beverage palatability (Hubbard, Szylk & Armstrong, 1990) or by educating the athletes through increasing the awareness of the need for a conscious effort to increase fluid intake in situations where dehydration is likely to occur. POST-EXERCISE REHYDRATION Replacement of water and electrolyte losses in the post-exercise period may be of great importance for maintenance of exercise capacity when repeated bouts of exercise have to be performed. The need for replacement will obviously depend on the extent of losses incurred during exercise, on non exercise losses and also on the time and nature of subsequent exercise bouts. Athletes living in a hot environment will experience substantially increased fluid losses even when not exercising. The primary factors influencing post-exercise rehydration process are composition and volume of the fluid consumed. Plain water is not the ideal post-exercise rehydration beverage when rapid and complete restoration of fluid balance is necessary. Ingestion of plain water in the immediate post-exercise period results in a rapid fall in plasma sodium concentration and in plasma osmolality and with a subsequent diuresis (Costill & Sparks, 1973; Nose et al., 1988a). These changes have the effect of reducing the stimulus to drink (thirst) and of stimulating urine output, both of which will delay the rehydration process. However, when an electrolytecontaining solution is ingested, urine output is less and net water balance is closer to the preexercise level (Gonzalez-Alonso, Heaps & Coyle, 1992). Nielsen et al. (1986) and others (Costill & Sparks, 1973; Carter & Gisolfi, 1989; Nose et al., 1988a; Wong et al., 1998) showed differences in the rate and extent of changes in the plasma volume with recovery from exerciseinduced dehydration when different carbohydrate-electrolyte solutions were consumed: the plasma volume increase was greater when drinks with sodium, as the only added cation, were consumed compared with drinks containing less electrolytes and more carbohydrate or drinks containing additional potassium. In the study of Nose et al. (1988a), plasma volume was not restored until after 60 min when plain water was ingested together with a placebo (sucrose) capsule. In contrast, when sodium chloride capsules were ingested with water to give a saline solution with an effective concentration of 0.45% (77mmol L -1 ), plasma volume was restored within 20 min. In the sodium chloride trial, voluntary fluid intake was higher and urine output was less, 29% of the water intake being lost as urine within 3h compared with 49% in the plain water trial. The delayed rehydration in the water trial was the result of a loss of water as urine, caused by a rapid return to control levels of plasma renin activity and aldosterone levels (Nose et al., 1988b). The results of these studies showed that the volume of urine produced in the few hours after exercise was influenced by the quantity of sodium consumed. Therefore, the addition of sodium to rehydration beverage can be justified on two accounts. First, sodium stimulates glucose absorption in the small intestine: water absorption from the intestinal lumen is a purely passive process that is determined largely by local osmotic gradients. The active co-transport of glucose

10 Rabindarjeet Singh and sodium creates an osmotic gradient that acts to promote net water absorption, and the rate of rehydration is therefore greater when glucose-sodium chloride solutions are consumed than when plain water is ingested. Second, replacement of sweat losses with plain water will lead to hemodilution if the volume ingested is sufficiently large. The fall in plasma osmolality and sodium concentration that occurs in this condition will reduce the drive to drink and will stimulate urine output (Nose et al., 1988a) and has a potentially more serious consequence, such as hyponatremia (Noakes, 1992). Through a systematic investigation of the relationship between whole-body sweat sodium losses and the effectiveness of beverages with different sodium concen-trations in restoring fluid balance, Shireffs and Maughan (1998) have showed that, provided that an adequate volume is consumed, euhydration is achieved when the sodium intake is greater than sweat sodium loss. It is therefore proposed that drinks used for post-exercise rehydration should have a sodium concentration similar to that of sweat. The requirement for sodium replace-ment stems from its role as the major ion in the extracellular fluid. It has been speculated that inclusion of potassium, the major cation in the intracellular space would enhance the replacement of intracellular water after exercise and thus promote rehydration (Nadel et al., 1990: Maughan et al., 1994). Nielsen et al., (1986) did find some evidence that restoration of plasma volume in the 2h after dehydration was more rapid when solutions with high sodium content were given but the intracellular rehydration was favoured by drinks with high concentration of potassium. Yawata (1990) found that there was a tendency for a greater restoration of the intracellular fluid space with potassium chloride solution than with the sodium chloride solution in thermal dehydrated rats despite ingestion of a smaller volume of potassium chloride solution. In the study on the effects of electrolyte addition to ingested fluids, Maughan et al. (1994) found that a smaller volume of urine was excreted after rehydration when sodium or potassium containing beverage were ingested com-pared with the electrolyte-free beverage. An estimated plasma volume decrease of 4.4% was observed with dehydration with all trials but rate of plasma volume recovery was slowest when potassium chloride beverage was consumed. However, there was no difference in the fraction of ingested fluid retained 6h after drinking the fluids that contained electrolytes. It appears that inclusion of potassium is as effective as sodium in retaining water ingested after exercise-induced dehydration: addition of either ion will significantly increase the fraction of the ingested fluid which is retained, but when the volume of fluid ingested is equal to that lost during an exercise period there is no additive effect of including both ions as would be expected if they acted independently on different body fluid compartments. (Leiper, Owen & Maugham, 1993). Using a natural fruit drink high in potassium and low in sodium, Singh and coworkers (Singh et al., 2001) and Saat et al. (2002) found that blood and plasma volume restoration was similar with coconut water and carbohydrate-electrolyte beverage during the 2-h rehydration period after exercise-induced dehydration indicating the role of potassium in enhancing rehydration by aiding intracellular rehydration. In many studies (Carter & Gisolfi, 1989; Costill & Sparks, 1973; Gonzalez-Alonso et al., 1992; Greenleaf, 1992; Nielsen et al., 1986; Wong et al., 1998), it has been found that incomplete rehydration or involuntary dehydration is often observed. One reason for this phenomenon is that normal dipsogenic is not strong enough to fully replace fluid that is lost during the preceding

11 Fluid Balance and Exercise Performance exercise (Gisolfi & Duchman, 1992; Greenleaf, 1992;). In addition, obligatory urine losses persist even in the dehydrated state, because of the need for elimination of metabolic waste products. The volume of fluid consumed after exercise-induced or thermal sweating must therefore be greater than the volume of sweat lost if effective rehydration is to be achieved. This was investigated by Shirreffs et al. (1996) who looked at the influence of drink volume on rehydration effectiveness after exercise-induced dehydration equivalent to approximately 2% of body weight. Drink volumes equivalent to 50, 100, 150 and 200% of the sweat loss were consumed after exercise. To investigate the possible interaction between beverage volume and its sodium content, a drink with relatively low sodium (23 mol. L -1 ) and one with moderately high sodium (61mol. L -1 ) were compared. With both beverages, the urine volume produced was related to the beverage volume consumed; the smallest volumes were produced when 50% of the loss was consumed and the greatest when 200% of the loss was consumed. Subjects could not return to euhydration when they consumed a volume equivalent to, or less than, their sweat loss, irrespective of the drink composition. When a drink volume equal to 150% of the sweat loss was consumed, subjects were slightly hypohydrated 6h after drinking if the test drink had a low sodium concentration, and they were in a similar condition when they drank the same beverage in a volume of twice their sweat loss. With the high-sodium drink, enough fluid was retained to keep the subjects in a state of hyperhydration 6h after drink ingestion when they consumed either 150% or 200% of their sweat loss. The excess would eventually be lost by urine production or by further sweat loss if the person resumed exercise or moved to a warm environment. In this study, plasma volume was estimated to have decreased by approximately 5.3% with dehydration. Six hours after finishing drinking, the general pattern in plasma volume, irrespective of which drink had been consumed, was for the increase to be a direct function of the drink volume consumed; also, the increase tended to be greater for those individuals who ingested the high-sodium drink. In examining the effect of exercise-induced dehydration and rehydration on a subsequent exercise bout, Burge et al. (1993) found that the efficacy of rehydrating with water following 24h of dehydration reduced the subsequent maximal rowing perfor-mance. However, Fallowfield, Williams & Singh (1995) and Wong et al. (1998) found that ingestion of carbohydrate-electrolyte beverage during the 4-h recovery improved and restored subsequent endurance capacity respectively. Similarly, a 2-h rehydration period with 120% of fluid loss using high Na + -carbohydrate-electrolyte solution resulted in improved rehydration efficacy and cycling performance when compared to water placebo (Singh et al., 1996), but in a latter study despite the effective rehydration with high sodium carbohydrate-electrolyte solution after exercise-induced dehydration, the ingestion of this solution did not improve the time-trial cycling performance as a result of hyperinsulinemia which counter-affected the rehydration benefit (Singh, Brouns & Kovasc, 2002). BEVERAGE PALATABILITY AND VOLUNTARY FLUID INTAKE In most of the studies cited, a fixed volume of fluid was consumed in all trials. In everyday situations, intake will be determined by the interaction of physio-logical and psychological factors. In a study to examine the effect of palatability and solute content of beverages in

12 Rabindarjeet Singh promoting rehydration after sweat loss, eight males exercised in the heat to lose 2.1% of their body mass (Maughan & Leiper, 1993). Over a 2-h period following exercise, subjects were allowed to drink as much as they wished of each of the test drinks; the drinks they received, each on a separate occasion, were an oral rehydration solution, carbonated water, a commercial sports drink and an orange juice/lemonade mixture. Subjects drank greater volumes of the sports drink and of the orange juice/lemonade mixture, and this reflected the preference that subjects expressed for the taste of these drinks. After exercise, the subjects were in negative fluid balance and by drinking they moved into positive fluid balance on all trials. Urine output was greatest with the low-electrolyte drinks that were consumed in the largest volumes and was smallest after drinking the oral rehydration solution. These results demonstrated the importance of palatability for promoting consumption, but also confirm the earlier results, which showed that a moderately high electrolyte content is essential if the ingested fluid is to be retained in the body. The benefits of the higher intake with the more palatable drinks were lost because of the higher urine output. Water was the least effective beverage, with low intake and a relatively high loss in urine. Water consumption causes a fall in plasma osmolality and sodium concentration, which reduce the circulating concentrations of vasopressin and aldosterone. This in turn results in less renal reabsorption of water and increased urine production. BEVERAGE TEMPERATURE The palatability of a drink is also influenced by the temperature of the ingesting fluid. The ACSM position paper (1996) recommends that during exercise when large volumes of fluid are consumed, beverages at temperature ranging from 15 to 20 C are most acceptable, even though colder beverages at 5 C may be preferred. Although there are limited number of studies (Sandick, Engell & Maller, 1984; Szlyk et al., 1989) on the ideal temperature to maximise fluid intake, the temperature identified in the ACSM position paper should be viewed as a guideline. Although it is believed that chilled drinks accelerate gastric emptying (Costill & Saltin, 1974), a recent study which used tracer methodology for gastric emptying and intestinal absorption showed that absorption rates were similar when the temperature of the beverage was 4 C or 40 C suggesting that absorption was not affected by fluid temperature (Lambert & Maughan 1992). Nevertheless, it is still encouraged that athletes be provided with cool drinks which will maximise palatability and encourage fluid ingestion. PRACTICAL CONSIDERATIONS FOR FLUID REPLACEMENTS FOR THE THLETE Many factors will affect the athlete s need for fluid replacement during training and competition. Factors such as composition of fluids, volume of drink and frequency of drink will depend on individual circumstances. In addition physiological variables such as large inter-individual variability in the rates of sweating, rates of gastric emptying and intestinal absorption of any ingested beverage will also affect the ingestion of fluids during exercise. Sweat rates under controlled conditions during 1h of exercise at a workload of 70% VO 2max and an ambient temperature of 23 C ranged from 426 to 1665 g/h (Greenhaff & Clough, 1989). From this result,

13 Fluid Balance and Exercise Performance it would seem logical that the need for fluid replacement is greatest in the individual with highest sweat rate and any guidelines as to the rate of fluid ingestion and composition of fluids to be taken must be viewed with caution when applied to the individual athlete. Many organisations (American College of Sports Medicine, 1975, 1984, 1987, 1996; National Institute for Occupational Safety and Health, 1986; United States Military, 1991) have issued recommendations as to the most appropriate fluid replacement regime. In 1975 the American College of Sports Medicine (1975) suggested an intake of 400 to 500ml of fluid 10 to 15min before exercise and that runners ingest fluids frequently during competition and that the sugar and electrolyte content of drinks should be low (2.5% glucose and 10 mmol/l sodium respectively) so as not to delay gastric emptying. This was later revised to intake of about 500ml of fluid about 2 h before exercise to promote adequate hydration and allow time for excretion of excess ingested water (American College of Sports Medicine, 1996). In the 1984 version (American College of Sports Medicine, 1984) it was recommended hyperhydration prior to exercise by ingestion of 400 to 600ml of cold water 15 to 20 min before the event and an intake of 100 to 200ml every 2 to 3km was suggested, giving a total intake of 1400 to 4200ml at the extremes. Taking these extreme values, it is unlikely that elite runners could tolerate a rate of intake of about 2L/h. This has now been revised to start drinking early and at regular intervals in attempts to consume fluid at a rate sufficient to replace all the water lost through sweating or consume the maximal amount that can be tolerated (American College of Sports Medicine, 1996). It is also now recommended that ingested fluid be cooler than ambient temperature (between 15 and 22 C) and flavoured to enhance palatability and promote fluid replacement (American College of Sports Medicine, 1996). Exercise intensity and duration The metabolic heat production rate during exercise is dependent on the exercise intensity and body mass; which is a direct function of speed in activities such as running and cycling. The rate of rise of body temperature in the early stages of exercise and the steady state level which is eventually reached are both proportional to the metabolic rate. As such the sweat rate production is closely related to the absolute workload. However, in many sports including most ball games, short bursts of high intensity activity are separated by variable periods of rest and low intensity activities. As time for sustaining high intensity activities is rather short with the duration being in the range of 10-60min, it appears that fluid and substrate availability are not normally limiting and that performance of continuous activities during these time periods will not be improved by the ingestion of carbohydrate-containing beverages during exercise. Even though the sweat rates may be high, the total amount of water lost by sweating is likely to be rather small. Accordingly, there is generally no need for fluid replacement during very high intensity exercise lasting less than 60min. In addition there are also real problems associated with replacement of fluids during very intense exercise bouts. The rate of gastric emptying, which is probably the most important factor in determining the fate of ingested fluid, is impaired when exercise intensity is high. To achieve a high rate of fluid delivery from the stomach, it is necessary to ingest a large volume and any attempt to do so when the exercise intensity exceeds 80% of VO 2max would almost certainly cause nausea and vomiting.

14 Rabindarjeet Singh At lower intensities of exercise, the duration of exercise is inversely related to the intensity. As the distance of a race increases, the pace that an individual can sustain decreases (Davies & Thompson, 1979); equally, in an event such as a marathon race where all runners complete the same distance, the slower runners are generally exercising at a lower relative (as a percentage of VO 2max ) and absolute work intensity (Maughan & Leiper, 1983). As the faster runners are exercising at a higher workload, in both absolute as well as relative terms, the sweat rates of these runners are higher. But because they are active for a shorter period of time, the total sweat rate loss during a marathon race is similar to slower runners and is unrelated to the finishing time (Maughan, 1985). Therefore, the need for fluid replacement is much the same in terms of total volume required, irrespective of running speed. In slower marathon runners, the exercise intensities does not exceed 60% of VO 2max and gastrointestinal function is unlikely to be impaired and maybe equals the resting gastric emptying rates of 40ml/min (Costill & Saltin 1974; Mitchell et al., 1988; Duchman et al., 1990; Rehrer et al., 1989). Hence in theory, it should be possible to meet the fluid loss by oral intake. As gastric emptying rates of fluids are commonly much lower than the maximum rate of 40ml/min, it is inevitable that most athletes who exercise hard in the heat will incur a fluid deficit. Composition of drinks In addition to the recommendation of ingesting cool fluids as the optimum fluid during endurance exercise (American College of Sports Medicine, 1996) there is evidence, as presented earlier in this review, indicating that there are good reasons for taking drinks containing added substrate and electrolytes. Prolonged exercise performances are improved by the addition of an energy source in the form of carbohydrate; the type of carbohydrate does not appear to be critical, and glucose, sucrose and oligosaccharides have all been shown to be effective in improving endurance capacity. Some recent studies have suggested that long-chain glucose polymer solutions are more readily used by the muscles during exercise than are glucose or fructose solutions (Noakes, 1990), but others have found no differences in the oxidation rates of ingested glucose or glucose polymer (Massicote et al., 1989; Rehrer, 1990). However, ingested fructose is less readily oxidised than glucose or glucose polymers (Massicote et al., 1989). Fructose in high concentrations is best avoided on account of the risk of gastrointestinal upset. The amount of carbohydrate in a drink will depend on circumstances. High carbohydrate concentrations will delay gastric emptying, thus reducing the amount of fluid that is available for absorption: very high concentrations will result in secretion of water into the intestine and thus actually increase the danger of dehydration. However, where there is a need to supply an energy source during exercise, increasing the carbohydrate content of drinks will increase the delivery of carbohydrate to the site of absorption in the small intestine. As carbohydrate concentration increases, the volume of fluid emptied from the stomach is reduced but the amount of carbohydrate available for absorption is increased. Available evidence indicates that the only electrolyte that should be added to drinks consumed during exercise is sodium, which is usually added in the form of sodium chloride. Sodium will stimulate sugar and water uptake in the small intestine and will help to maintain extracellular fluid volume. Most fizzy soft drinks (cola or orange variety) contain very little sodium (1 2 mmol/l) whereas sports drinks commonly contain 10 25mmol/L. Although a high sodium

15 Fluid Balance and Exercise Performance content, may stimulate jejunal absorption of glucose and water, it however, tends to make the drinks unpalatable. It is important that drinks intended for ingestion during or after exercise should have a pleasant taste in order to stimulate consumption. Therefore most designed sports drinks are generally formulated to strike a balance between the twin aims of efficacy and palatability. When the exercise duration exceeds 3 to 4h, there may be advantages in taking drinks with sodium to avoid the danger of hyponatraemia, which has been reported to occur when excessively large volumes of low-sodium drinks are taken (Frizell et al., 1986; Noakes et al., 1985; Noakes et al., 1990; Saltin & Costill 1988). Sodium is also necessary for post-event rehydration, which may be particularly important when the exercise has to be repeated within a few hours: if drinks containing little or no sodium are taken, plasma osmolality will fall, urine production will be stimulated and most of the ingested fluid will not be retained. When there is a longer rest period between exercise sessions, it is possible to replace sodium and other electrolytes as a result of intake from the diet without additional supplementation. It is advantageous to take chilled (4 C) drinks as this will accelerate gastric emptying and thus improve the availability of the ingested fluids (Costill & Saltin, 1974). In addition the palatability of most carbohydrate-electrolyte drinks improves at low temperatures. CONCLUSION Prolonged exercise leads to dehydration due to thermoregulatory sweating. Reduction in work capacity is related both to the volume and osmolality changes caused by the loss of sweat. The better the water balance that can be maintained through drinking during the prolonged exercise, the less the impairment in function. Restoration of water balance takes time due to limitations in the rates of uptake and redistribution of fluid to the various body water compartments. As thirst sensation is insufficient in restoring water balance, athletes must be encouraged to drink more than their urge and equivalent to their weight loss. After exercise, complete restoration of fluid balance is an important part of the recovery process and becomes more important in hot, humid conditions. Rehydration after exercise requires not only replacement of volume losses, but also replacement of the electrolytes, primarily sodium. It is clear from many studies that rehydration after exercise can be achieved only if sweat electrolyte losses as well as water are replaced. Drinks with a low sodium content are ineffective at rehydration and they will only reduce the stimulus to drink. Addition of a small amount of carbohydrate to the rehydrating drinks may improve the rate of intestinal uptake of sodium and water and will improve palatability. The volume of the rehydration beverage consumed should be greater than the volume of sweat lost to provide the ongoing obligatory urine losses. Palatability of the beverage is a major issue when a large volume of fluid has to be consumed. REFERENCES Adolph EF (1947). Physiology of man in the desert. New York: Interscience.