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FLUID AND CARBOHYDRATE REPLACEMENT DURING EXERCISE: HOW MUCH AND WHY?

SSE#50, Volume 7 (1994), Number 3


Edward E Coyle, Ph.D.
Director., Human Performance Laboratory
Department of Kinesiology and Health Education,
The University of Texas at Austin
Austin, Texas
Member, Sports Medicine Review Board
Gatorade Sports Science Institute

KEY POINTS

1. During prolonged exercise in the heat, people can become dehydrated at a rate of 1-2 L every hour (about 2-4 lbs of body weight loss per hour). The rate of dehydration can be monitored by recording changes in nude body weight. Each pound of weight loss corresponds to 450 mL (15 fluid ounces) of dehydration.

2. Even a slight amount of dehydration causes physiological consequences. For example, every liter (2.2 lbs) of water lost will cause heart rate to be elevated by about eight beats per minute, cardiac output to decline by 1 L/min, and core temperature to rise by 0.3o C when an individual participates in prolonged exercise in the heat.

3. When it is important to minimize disturbances in cardiovascular function and body temperature and to reduce the perceived difficulty of exercise, people should attempt to drink fluids at close to the same rate that they are losing body water by sweating.

4. Unfortunately, runners generally drink only 300-500 mL of fluids per hour and thus allow themselves to become dehydrated at rates of 500-1,000 mL/h. Dehydration compromises cardiovascular function and places the runner at risk for heat-related injury. The runner must answer the question, Will the time I lose by drinking larger volumes of fluid be compensated for by the physiological benefits the extra fluid produces that may cause me to run faster during the last half of the race?

5. For an exerciser who weighs about 68 kg (150 lb), the requirements for both carbohydrate (i.e., 30-60 g/h) and fluid during prolonged exertion can be met by drinking 625-1,250 mL/h of beverages containing 4-8% carbohydrate. This volume must be adjusted for persons of different body weights. For example, an individual who weighs 50 kg should multiply the above recommendation by 50/68 or 0.74, i.e., 462.5-925 mL/h.

INTRODUCTION

The prevalent thinking from the turn of the century until the 1970's was that participants in endurance sports did not need to replace fluids lost during exercise (Noakes et al., 1991a; Noakes, 1993). This misconception has now given way to the knowledge that drinking fluids reduces the increase in body temperature (hyperthermia) and the amount of stress on the cardiovascular system, especially when exercising in hot environments (Coyle & Montain, 1993). However, the extent to which even a slight degree of dehydration adversely affects bodily function during exercise and the situations in which adding carbohydrate and salt to water provides added benefit are not generally appreciated. The volume of fluid that most athletes choose to drink voluntarily during exercise replaces less than one-half of their body fluid losses (Noakes, 1993). The purpose of this paper is to review the physiology of fluid and carbohydrate replacement during exercise and the likely effects of such replacement on the performance of pro-longed exercise. It is hoped that this knowledge might encourage competitors to drink more during exercise.

FLUID INGESTION DURING PROLONGED EXERCISE

The decision as to how much fluid to ingest during exercise should be based upon a risk-benefit analysis. Undoubtedly, the most serious consequence of inadequate fluid replacement, i.e., dehydration, during exercise is hyperthermia, which when severe will cause heat exhaustion, heat stroke, and even death. The risks of too much fluid ingestion are gastrointestinal discomfort (Rehrer et al., 1990) and a reduced pace during competition associated with the physical difficulty of drinking large volumes of fluid while exercising. The benefits of fluid ingestion are reduced cardiovascular stress and reduced hyperthermia that, by themselves, can probably improve exercise performance.

CARBOHYDRATE INGESTION DURING PROLONGED INTENSE EXERCISE

The primary purpose of carbohydrate ingestion during strenuous exercise lasting longer than one hour is to maintain a sufficient concentration of blood glucose and to sustain a high rate of energy production from blood glucose and glycogen stored in muscles (Coggan & Coyle, 1991; Coyle et al., 1986), which can allow competitors to exercise longer and sprint faster at the end of exercise (Coggan & Coyle, 1991 ). Most studies demonstrating improved performance with carbohydrate feedings have given subjects 25-60 g of carbohydrate during each hour of exercise (Coggan & Coyle, 1991; Murray et al., 1991). We therefore recommend that individuals consume solutions that provide 30-60 g of carbohydrate per hour in the form of glucose, sucrose, or starch (Coggan & Coyle, 1991 ).

It was previously thought that the addition of carbohydrate to solutions impaired fluid replacement because carbohydrate is known to slow the rate at which fluids empty from the stomach (gastric emptying). However, the most important factor regulating gastric emptying and fluid replacement is the volume of fluid ingested; the carbohydrate concentration of the solution is of secondary importance (Coyle & Montain, 1992a; Coyle & Montain, 1992b, Mitchell et al., 1989; Noakes et al. 1991b; Rehrer et al. 1990). Practically speaking, solutions containing up to 8% carbohydrate appear to have little deleterious influence on the rate of gastric emptying, especially when the drinking schedule adopted maintains a high gastric volume (Coyle & Montain, 1992b; Houmard et al., 1991; Mitchell et al., 1988; Noakes et al., 1991b). Thus, it is quite possible to ingest 30-60 g of carbohydrate per hour and still replace 600-1,250 mL of fluid per hour. Our experience is that cyclists have no difficulty drinking 1,200 mL/h of a 6% carbohydrate solution.

Difficulties in Drinking Large Volumes of Fluids While Running

Large gastric volumes will no doubt cause discomfort in some runners. Therefore, in runners, it remains to be deter-mined if the performance benefits of high rates of fluid replacement outweigh the discomfort it may cause. We suspect that many marathon runners allow themselves to become dehydrated to some extent because they feel their stomachs cannot tolerate the large volumes of fluid that must be drunk to totally offset sweat losses. In general, most runners drink less than about 500 mL of fluid per hour (Noakes et al, 1991 a; Noakes, 1993). Because sweat rates often average 1,000-1,500 mL/h, marathon runners commonly become dehydrated at a rate of 500-1,000 mL/h, although dehydration rates can be much higher when the fastest runners compete in hot environments. Unfortunately, drinking large volumes of fluid cost the runner additional seconds in approaching the aid-station table and in attempting to drink and breathe while running. Furthermore, the added gastrointestinal discomfort may cause the competitor to run at a slower pace until the discomfort subsides. The runner must answer the question of whether the time lost while drinking larger volumes of fluid will be compensated for by the physiological benefits the extra fluid produces that may cause me to run faster during the last half of the race. However, if the goal is safety, which means minimizing hyperthermia, it is clear that the closer that the rate of drinking can match the rate of dehydration, the better.

To our knowledge, no studies have directly compared the effects on running or cycling performance of fluid replacement at rates that prevent dehydration versus rates voluntarily chosen by many endurance athletes (e.g., 500 mL/h) who replace only 30-50% of fluid losses. The cardiovascular benefits of full compared to partial fluid replacement when cycling are discussed below, and it is likely that the same cardiovascular benefits are derived when running.

LOW INTENSITY EXERCISE AND FLUID REPLACEMENT

In experiments conducted about the time of World War II, it was repeatedly found that fluid ingestion during prolonged low-intensity exercise such as walking and stair stepping attenuated deep body (core) temperature and improved exercise performance (Adolph, 1947; Bean & Eichna, 1943; Eichna et al. 1945; Pitts et al., 1944). Fluid ingestion equal to the rate of sweating was more effective than voluntary or partial fluid replacement (Bean & Eichna, 1943; Eichna et al., 1945; Pitts et al., 1944). Furthermore, voluntary fluid ingestion during low-intensity exercise is more effective in attenuating hyperthermia than when fluid intake is totally prohibited or is restricted to small volumes (Eichna et al. 1945; Pitts et al. 1944). Thus, during prolonged, low-intensity, intermittent exercise, the optimal rate of fluid replacement for attenuating hyperthermia appears to be the rate that most closely matches the rate of sweating.

CARDIOVASCULAR AND THERMOREGULATORY BENEFITS OF HIGH RATES OF FLUID REPLACEMENT DURING INTENSE CYCLING IN THE HEAT

To gain some insight into the effects of various fluid replenishment schemes on exercise at the high intensities typically experienced in sport competition, we deter-mined the effect of different rates of fluid replacement during prolonged intense cycling on hyperthermia, cardiac output, and heart rate (Coyle & Montain, 1992a). On four different occasions endurance-trained cyclists exercised in a warm environment (33 _ C, 50% relative humidity) at 62-67% VO2max, which was the highest intensity that could be maintained for 2 h when no fluid was ingested. During 2 h of exercise, the cyclists randomly received either no fluid or drank small (300 mL/h), moderate (700 mL/h), or large (1,200 mL/h) volumes of a sport drink containing 6% carbohydrate and low concentrations of electrolytes. These fluid volumes replaced approximately 20%, 50%, and 80%, respectively, of the fluid lost in sweat during exercise. The protocol resulted in graded magnitudes of dehydration; body weight declined 4%, 3%, 2% and 1%, respectively, when drinking either no fluid or small, moderate, or large volumes of fluid. The increases in core temperature, heart rate, and perceived exertion during the 2 h of exercise were progressively diminished as more and more fluid was consumed (Figure 1 ). The magnitude of dehydration accrued after 2 h of exercise in the four trials was the major factor associated with hyperthermia and cardiovascular stress. Figure 2 demonstrates that the rise in core temperature, the rise in heart rate, and the fall in cardiac output observed after 2 h of exercise were inversely related to the rate of fluid ingestion and directly related to the extent of dehydration experienced. Specifically, every 1 L loss of sweat (2.2 lb of body weight) caused heart rate to increase by eight beats per minute, cardiac output to decline by 1 L/min, and core temperature to increase by 0.3o C. Therefore, we maintain that there is no acceptable amount of dehydration that can be tolerated before cardiovascular function and thermoregulation are impaired. Drinking 1,200 mL/h was better than drinking 700 mL, which in turn was better than drinking 300 mL/h.

Perception of Effort

Although performance was not actually measured in the study just described, sever-al subjects were barely able to complete 2 h of exercise without fluid ingestion (Montain & Coyle, 1992). Drinking progressively larger volumes of fluid reduced the subjective rating of perceived exertion, as shown in Figure 1. After 2 h of exercise, these cyclists rated the exercise as being "very hard" when no fluid was ingested and "hard" when only 300 mL/h of fluid was ingested. (Competitors often drink only a small volume of fluid (e.g., 300 mL/h), which may give a false sense of security by somewhat reducing their sense of perceived exertion while providing only minimal physiological benefit.) However, when fluid was consumed at a rate of 700 mL/h or 1,200 mL/h, the exercise never was rated "hard." It is likely that these perceptions of effort provide indirect information about performance ability after 2 h of cycling with different amounts of fluid replacement. Additionally, none of the cyclists complained of gastrointestinal discomfort or of difficulty drinking 1,200 mL/h. We therefore conclude that this rate of fluid replacement is tolerable during cycling, but we do not know whether it is acceptable when running.

Figure 1. Core temperature (esophageal temperature), heart rate, and perceived exertion during 120 min of exercise when ingesting no fluid, or small (300 mL/h), moderate (700 mL/h) and large (1,200 mL/h) 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 < 0.05. Significantly lower than small

POSSIBLE REASONS FOR THE CARDIOVASCULAR BENEFIT OF FLUID REPLACEMENT DURING EXERCISE

The most serious consequence of exercise-induced dehydration is hyperthermia, which places added stress on the cardiovascular system and creates a vicious cycle. Dehydration during exercise causes fluid to be lost throughout the body. As a result, dehydration increases the concentration of dissolved particles in bodily fluids (osmolality), including the concentration of sodium in the blood serum. These increases in osmolality and in sodium concentration seem to play some role in slowing heat loss by reducing blood flow to the skin and by reducing the rate of sweating. An addition-al important effect of dehydration-induced hyperthermia is a large decline in cardiac output, a measure of total blood flow throughout the body. This exacerbates the hyperthermia by further reducing the transfer of heat from the body core to the cooler periphery (Montain & Coyle, 1992a). The most dramatic consequence of dehydration-induced hyperthermia during exercise is a 25-30% reduction in stroke volume that is not generally met with a proportional increase in heart rate; this results in a decline in cardiac output and in arterial blood pressure (Gonzalez-Alonso et al., 1994; Montain & Coyle, 1992a).

The primary benefit of sufficient fluid replacement during exercise is that it helps to maintain cardiac output and allows blood flow to the skin to increase to high levels so as to promote heat dissipation from the skin, thereby preventing excessive storage of body heat (Montain & Coyle, 1992a). The exact mechanism by which fluid replacement pro-motes a high skin blood flow during exercise is not clear. Fluid replacement does help to prevent loss of water from the blood plasma, but in endurance-trained athletes, this improved maintenance of plasma volume apparently does not by itself increase blood flow to the skin to reduce core temperature (Montain & Coyle, 199b). It seems more likely that fluid replacement prevents skin blood flow from declining by preventing dehydration-induced impairments in the neural control of skin blood flow, by preventing declines in blood pressure, and/or by minimizing the exercise/dehydration-induced increases in the blood concentrations of catecholamines, sodium, and other osmotically active particles.

PERFORMANCE BENEFITS OF INGESTING FLUIDS AND CARBOHYDRATE

As discussed previously, fluid replacement during exercise improves work time in subjects walking in the desert, but surprisingly few studies have documented performance benefits of fluid replacement during more intense exercise in laboratory studies or during competitive athletic events (Armstrong et al., 1985; Costill et al., 1970). It is to be expected that fluid replacement would be most beneficial during more prolonged exercise that accentuates the amount of dehydration. As shown in Figure 1, during 2 h of exercise in the heat (33 _ C) at 65% VO2max, the physiological benefits of fluid replacement began to emerge after 1 h of exercise (Montain & Coyle, 1992b). This prompted us to con-duct a performance study during more intense cycling in the heat, performed for a duration of approximately 1 h (Below et al., In Press). After 50 min of exercise at 80% VO2max, heart rate and core temperature were lower by four beats/min and 0.33o C, respectively (P<O.05), when a large volume (1300 mL) compared to a small volume of fluid (200 mL) was ingested during the first 35 min of exercise. Performance was then measured as the number of minutes required to complete a set amount of work, so as to simulate the closing stages of a race. Performance time for this last stage of cycling was 6% faster when the large volume of fluid was ingested.

Carbohydrate ingestion clearly improves performance in events lasting longer than 90 min and in which fatigue is associated with reduced bodily stores of carbohydrate ( Coggan & Coyle, 1991), but little is known about the influence of carbohydrate feedings on shorter duration exercise that is more typical of most sport events. Therefore, in the previously cited study (Below et al., In Press), we also determined if ingestion of 70 g of carbohydrate might improve performance of a brief, high-power cycling test following 50 min of cycling at 80% VO2max. Indeed, performance was also improved 6% by carbohydrate ingestion. Therefore, both fluid replacement and carbohydrate ingestion equally improved high-intensity cycling performance, each by 6%. Furthermore, their beneficial effects were additive, i.e., there was a 12% improvement in performance when both fluid and carbohydrate were administered, and these effects apparently operate through independent mechanisms (Below et al., In Press).

EXERCISE/DEHYDRATION-INDUCED HYPERTHERMIA IN VARIOUS ENVIRONMENTS

The exercise intensity and environmental conditions determine the extent to which dehydration causes hyperthermia during exercise and the extent to which fluid replacement can prevent the hyperthermia. When an individual exercises at a moderate intensity, e.g., 60-70% VO2max, in a warm/hot environment (20-35o C) with moderate humidity (<50% rh), heat is dissipated primarily by evaporative heat loss. This heat dissipation is impaired by dehydration, which decreases skin blood flow and sweating rate. In these environments, % loss of body weight due to dehydration causes core temperature to increase by about 0.15-0.30o C (Coyle & Montain, 1993). However, during exercise m a cool environment (0-10o C), dehydration appears to cause a relatively small degree of hyperthermia, probably because convective heat loss is sufficiently large to compensate for reduced skin blood flow and reduced evaporation of sweat.

Body core temperature is the balance between heat production and heat dissipation, and fluid replacement has its limitations if this balance is skewed. For example, in very hot and humid environments in which heat dissipation by evaporation and convection is minimal, fluid replacement will reduce cardiovascular stress and may improve performance, but it will have little effect on body temperature. Likewise, when the exercise intensity is great enough to cause a very high rate of heat production, it will not be possible for even well-hydrated people to increase heat dissipation enough to prevent excessive hyperthermia. In these situations, the only safe option is for individuals to reduce their heat production by lowering exercise intensity.

Figure 2. The influence of dehydration, as assessed by the percent reduction in body weight after 2 h of exercise, on the change in rectal temperature, cardiac output, and heart rate. (Reprinted with permission from Coyle & Montain 1992b).

TIMING OF FLUID REPLACEMENT DURING EXERCISE

Is there a time interval during prolonged exercise that is most advantageous for fluid replacement? Should one drink early in exercise, throughout exercise, or wait until near the end of the exercise? In an attempt to answer these questions, we studied cyclists who drank about 1 L of fluid at various times during 140 min of exercise (Montain & Coyle, 1993). They drank after either O, 40, or 80 min of exercise or drank the 1 L intermittently throughout exercise. In all cases, they incurred the same amount of dehydration after 140 min, and they were not different in any of their cardiovascular or THERMOREGULATORY responses. During the 40 min period immediately after drinking fluid, regardless of the time of drinking, the subjects stabilized their heart rates and core temperatures. During the periods without fluid ingestion, there was progressive hyperthermia and increased cardiovascular strain. These observations suggest that the volume of fluid ingested is most important, and the timing of ingestion is secondary.

Individual Trial and Error

Although we generally recommend that people drink large volumes of fluid and attempt to totally offset dehydration, we realize that individuals differ tremendously in their rates of gastric emptying and, there-fore, in their tolerances of large fluid volumes. Each person must devise an individualized drinking schedule that appears optimal and should become accustomed to this schedule during practice (Rehrer et al., 1989 ).

increases in body core temperature, heart rate, and ratings of the perceived difficulty of exercise. This same phenomenon probably also applies to running and argues against the notion that a certain amount of dehydration (e.g., up to 3% of body weight) is permissible and without cardiovascular consequences (Noakes et al., 1991 a). However, runners generally drink only 500 mL/h and thus allow themselves to become dehydrated at rates of 500-1,000 mL/h. Runners must compare the benefits of drinking large volumes of fluid during competition, i.e., the physiological improvements and the likely improvement in running speed during the late stages of the race, with the drawbacks of having to slow down while drinking and while suffering gastrointestinal discomfort. If the primary goal is safety, which means minimizing hyperthermia, there is no question that the runner should attempt to match the rate of drinking to the rate of dehydration.


SUMMARY

Ingestion of approximately 30-60 g of carbohydrate during each hour of exercise will generally be sufficient to maintain high rates of oxidation of blood glucose late in exercise and to delay fatigue. Because the average rates of gastric emptying and intestinal absorption exceed 1,250 mL/h for water and for solutions containing up to 8% carbohydrate, exercising sports competitors can be supplemented with both carbohydrate and fluids at relatively high rates.

When cyclists exercise at competitive intensities for 2 h in the heat with sweat rates of 1,400 mL/h, it is clear that the closer that fluid consumption matches sweating rate (at least up to 80% of sweating rate), the better. When fluid consumption is inadequate, increasing dehydration directly impairs stroke volume, cardiac output, and skin blood flow, resulting in progressive

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