Bob Murray, Ph.D.
Director
Gatorade Exercise Physiology Laboratory
Barrington, IL

 KEY POINTS

1. Recent scientific research has underscored the physiological and performance benefits of remaining well hydrated before, during, and following physical activity.

2. Maintaining euhydration takes a concerted effort on the part of the athlete to modify drinking behavior throughout the training day.

3. The amount of fluid voluntarily ingested during physical activity is affected by the palatability of the beverage, the composition of the beverage, and by ease of use. These factors must be considered when planning a fluid-replacement regimen for athletes.

4. The goal for fluid intake during exercise should be to fully replace sweat losses. The physiological and performance benefits of doing so are well documented.

5. Rapid and complete rehydration following exercise requires the ingestion of sodium chloride to replace that which was lost in sweat and the consumption of a volume of fluid that is greater than that which was lost as sweat.

INTRODUCTION

In a book titled Physiology of Man in the Desert, E.F. Adolph and associates expertly described the negative impact of dehydration upon physiological function, physical performance, and health (Adolph et al., 1947). Their exhaustive research demonstrated that preventing dehydration by regular ingestion of fluids was indispensable in ensuring the physical and mental well-being of their subjects. Unfortunately, more than two decades passed before the value of regular fluid replacement during physical activity was widely recognized and practiced in the athletic setting. During this time, dozens of athletes and military recruits died from hyperthermia complicated by dehydration (Baumann, 1995). Although athletes and others continue to fall prey to exertional heat stroke, the frequency of deaths has been drastically reduced over the years (Bauman, 1995), in large part because the necessity of adequate fluid replacement has become well recognized.

Although information on fluid intake during physical activity eventually found its way into textbooks, classrooms, and onto the practice field, most of these recommendations were fairly general in nature. For example, documents published by the American College of Sports Medicine (1987), the United States military (Marriott & Rosemont, 1991), and the National Institute of Occupational Safety and Health (1986) included information on fluid replacement from which some general guidelines could be drawn. In the case of the American College of Sports Medicine (ACSM), recommendations for fluid replacement were included in a position stand entitled The Prevention of Thermal Injuries During Distance Running (ACSM, 1987). The ACSM article emphasized the need for regular fluid intake during races of 10 km and longer, and encouraged runners to ingest 100 - 200 ml (3 - 6 oz) at every aid station. The public health value of this recommendation was significant because it helped assure that race organizers included fluid stations in their events and that participants were given the opportunity to drink. However, depending upon the speed of the runner, the distance between aid stations, and the volume of fluid ingested at each station, the resulting fluid intake could vary widely, replacing a very large or very small portion of sweat loss.

This uncertainty has been addressed in the most recent position stand published the American College of Sports Medicine. The ACSM position stand on Exercise and Fluid Replacement (ACSM, 1996) provides clear and practical guidelines regarding fluid, carbohydrate, and electrolyte replenishment for athletes. In preparing the recommendations, a panel of experts in fluid homeostasis and related fields completed a comprehensive review of the scientific literature, making certain that each practical recommendation was well substantiated by research. As a result, the ACSM position stand will benefit the lay and scientific communities for years to come.

The ACSM Recommendations

The ACSM position stand contains a summary of practical recommendations supported by four pages of scientific review complemented by 92 references. The document begins by stating that, "It is the position of the American College of Sports Medicine that adequate fluid replacement helps maintain hydration and, therefore, promotes the health, safety, and optimal physical performance of individuals participating in regular physical activity."

The purpose of this Sports Science Exchange is to further underscore the scientific and practical relevance of the ACSM recommendations so that coaches, athletic trainers, physicians, dietitians, and athletes gain an increased appreciation of the value of remaining well hydrated during physical activity. The recommendations found in the ACSM position stand are highlighted below and are supplemented with scientific and practical information related to their content.

Fluid Ingestion Before Exercise

"It is recommended that individuals consume a nutritionally balanced diet and drink adequate fluids during the 24-h period before an event, especially during the period that includes the meal prior to exercise, to promote proper hydration before exercise or competition."

The physiological and performance benefits of entering training and competition well hydrated and with large stores of muscle and liver glycogen are widely accepted from a scientific standpoint. In terms of fluid balance, it is clear that athletes who enter competition in a dehydrated state are at a competitive disadvantage (Sawka, 1992). For example, in a study by Armstrong et al. (1985), subjects performed a 5,000-meter (~ 19 min) and 10,000-meter (~ 40 min) run in either a normally hydrated or dehydrated condition. When dehydrated by ~2% of body weight (by a diuretic given prior to exercise), their running speeds decreased significantly (by 6% - 7%) in both events. To make matters worse, exercise in the heat exacerbates the performance-impairing effects of dehydration (Sawka et al., 1984).

Getting athletes to actually modify their drinking behavior during the training day is arguably a much larger challenge than convincing them about the scientific value of doing so. Dr. Ron Maughan, a sports scientist at the University of Aberdeen and an adviser to the 1996 British Olympic Team, indicated that the British athletes had to be schooled in mealtime drinking behavior during their training camps in Tallahassee, Florida. Unaccustomed to the decorum of buffet-line eating at an American university, the British athletes politely took just one beverage as they passed through the line while their American counterparts loaded up with three or four drinks. The British athletes were losing an important opportunity to rehydrate after hot-weather training. With a little prodding and some reminders, they became more aggressive mealtime drinkers. (R.J. Maughan, personal communication).

"It is recommended that individuals drink about 500 ml (about 17 ounces) of fluid about 2 h before exercise to promote adequate hydration and allow time for excretion of excess ingested water."

Laboratory subjects who ingest fluid in the hour before exercise exhibit lower core temperatures and heart rates during exercise than when no fluid is ingested (Greenleaf & Castle, 1971; Moroff & Bass, 1965). These physiological responses are undoubtedly beneficial as they reduce the strain on the body and lower the perception of exertion at a given workload (Montain & Coyle, 1992). When athletes live and train in warm environments, the value of adequate fluid intake prior to exercise cannot be overemphasized. This is apparent in the results of a study conducted on soccer players in Puerto Rico (Rico-Sanz et al., 1996). The athletes were studied during two weeks of training. When the players were allowed to drink fluids throughout the day as they wished (average intake = 2.7 L/d), their total body water at the end of one week was about 1.1 L lower than when they were mandated to drink 4.6 L of fluid per day. In other words, voluntary fluid consumption was insufficient to meet the players' daily fluid requirements, causing them to enter training and competition already dehydrated.

From a practical standpoint, the frequency of urination and the color and volume of urine can be monitored as a means of helping athletes assess their hydration status. Infrequent urination with a darkly colored urine of relatively small volume can be an indication of dehydration, a signal that the athlete should continue drinking before beginning exercise. Monitoring urine output is a common recommendation in occupational settings such as the mining industry in which the workers are constantly exposed to conditions of high heat and humidity.

Fluid Ingestion During Exercise

"During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating, or consume the maximal amount that can be tolerated."

This is perhaps the most-significant recommendation in the position stand because it clearly identifies that the ideal goal of fluid intake during exercise is to prevent any amount of dehydration, and yet it recognizes that an optimal intake may be difficult under some circumstances. The value of maintaining full hydration is well illustrated by the studies of Montain and Coyle (1992) and Walsh et al. (1994). These researchers demonstrated that cardiovascular, thermoregulatory, and performance responses are optimized by replacing at least 80% of sweat loss during exercise. Montain and Coyle showed that larger volumes of fluid intake during exercise were associated with greater cardiac output, greater skin blood flow, lower core temperature, and a reduced rating of perceived exertion. The data of Walsh et al. reaffirmed that even low amounts of dehydration (1.8% of body weight, in this case) can impair exercise performance.

The dramatic impairment in physiological and performance response that occurs with dehydration is more easily understood when the limitations of the cardiovascular system are considered. In his text on Human Circulation: Regulation During Physical Stress, cardiovascular physiologist L.B. Rowell wrote that, "Perhaps the greatest stress ever imposed on the human cardiovascular system (except for severe hemorrhage) is the combination of exercise and hyperthermia. Together these stresses can present life-threatening challenges, especially in highly motivated athletes who drive themselves to extremes in hot environments. A long history of heat fatalities gives stark testimony to the gravity of the problem and the failure of various organizations to recognize and deal with it effectively." (Rowell, 1986). Rowell's statement is a dramatic but accurate way of explaining that both exercise and hemorrhage require the body to cope with progressively diminishing blood volume and blood pressure. Although the physiological challenge to the body occurs much more quickly and with decidedly deadlier potential in the case of hemorrhage, the slower progression of events that occurs as a result of sweat loss is no less challenging from a physiological standpoint.

It is recommended that ingested fluids be cooler than ambient temperature [between 15°and 22°C (59° and 72°F)] and flavored to enhance palatability and promote fluid replacement. Fluids should be readily available and served in containers that allow adequate volumes to be ingested with ease and with minimal interruption of exercise."

It is certainly no surprise that humans are inclined to drink more of beverages that are flavored and sweetened (Greenleaf, 1991) but the practical ramifications of this common-sense knowledge are important in the exercise setting. Any step that can be taken to increase voluntary fluid intake will help decrease the extent of dehydration and reduce the risk of health problems associated with dehydration and heat stress. In addition to having palatable beverages available for athletes to drink, a number of other practical steps should be taken. These include educating coaches, trainers, parents, and athletes about the benefits of proper hydration, making certain that fluids are easily available at all times, encouraging athletes to follow an organized regimen for fluid replacement, and weighing athletes before and after practice as a way to assess the effectiveness of their fluid intake (Broad, 1996).

The composition of beverages can also have a substantial effect on voluntary fluid intake, as illustrated by the research of Wilk and Bar-Or (1996). Young boys were studied during 3 h of intermittent exercise in the heat, during which time the subjects could drink ad libitum. The boys completed this protocol on three occasions; the beverages tested included water, a sports drink, and a flavored, artificially sweetened replica of the sports drink. The results showed that the boys ingested almost twice as much sports drink as they did water. Consumption of the placebo fell in between. Not only did flavoring and sweetness increase voluntary fluid intake, but the presence of sodium chloride in the sports drink further increased consumption (i.e., the subjects drank more sports drink than placebo).

The human thirst mechanism is sensitive to changes in plasma sodium concentration (and plasma osmolality) and to changes in blood volume (Hubbard et al., 1990). The increase in sodium concentration and the decrease in blood volume that accompany exercise result in an increased perception of thirst. Drinking plain water quickly removes the osmotic drive to drink and reduces the volume-dependent drive, causing the satiation of thirst. The resulting decrease in fluid intake occurs prematurely, occurring before adequate fluid has been ingested. The presence of low levels of sodium chloride in a beverage help maintain the osmotic drive for drinking, and assure greater fluid intake (Nose et al. 1988), a physiological certainty well understood by bartenders who make certain that their customers have easy access to salty snack foods.

"Addition of proper amounts of carbohydrates and/or electrolytes to a fluid replacement solution is recommended for exercise events of duration greater than 1 h since it does not significantly impair water delivery to the body and may enhance performance."

The ergogenic effect of carbohydrate feeding during exercise has been extensively confirmed by research, much of which has been conducted using exercise bouts lasting from one to four-or-more hours (Coggan & Coyle, 1991). Ingestion of carbohydrate solutions containing combinations of sucrose, glucose, fructose, and maltodextrins results in improved exercise performance provided that at least 45 g of carbohydrate are ingested each hour (Coggan & Coyle, 1991). It should be noted that some researchers (Murray et al., 1991) have reported performance improvements even when subjects have ingested as little as 20-25 g/h, although a higher rate of carbohydrate intake is more advisable. However, the maximal rate at which exogenous carbohydrate can be utilized appears to be in the range of 60-75 g/h (ie, 1.0 - 1.5 g/min). No additional performance benefit is realized when subjects are fed greater amounts of carbohydrate (Murray et al., 1991).

The specific mechanism(s) by which performance is improved by carbohydrate feeding is still a matter of scientific inquiry, but there is general agreement that the improvement in performance is linked to an increased reliance on carbohydrate as fuel for active muscles (Coggan & Coyle, 1991). During intense physical activity, the metabolic demand for carbohydrate is high; carbohydrate ingestion satisfies part of that demand, helping assure the maintenance of carbohydrate oxidation.

"During exercise lasting less than 1 h, there is little evidence of physiological or physical performance differences between consuming a carbohydrate-electrolyte drink and plain water."

During long-duration exercise (i.e., > 1 h), carbohydrate oxidation normally declines as muscle and liver glycogen stores fall to low levels. Considering these responses, it is not surprising that exercise scientists initially relied upon bouts of long-duration cycling or running to determine if carbohydrate feeding improved performance. Not until recently have scientists turned their attention to studying shorter-duration, intermittent exercise protocols lasting one h or less to determine if carbohydrate feeding elicits a similar ergogenic effect. At the time of the 1996 ACSM position stand, very few such studies had been published. Although much more research needs to be completed, the growing body of evidence (Ball et al., 1995; Below et al., 1995; Wagenmakers et al., 1996; Walsh et al., 1994) indicates that carbohydrate ingestion may indeed benefit performance during shorter duration exercise (i.e., 1 h or less) and during intermittent exercise such as high-intensity running (Nicholas et al., 1996), cycling (Jackson et al., 1995), and tennis play (Vergauwen et al., 1996).

An excellent comparison of the benefits of ingesting water or a sports drink during shorter-duration exercise was conducted by Below et al. (1994) who had subjects cycle for 50 min at 80%VO2max and then complete a "sprint to the finish" requiring 9-12 min. The subjects experienced a 6% improvement in performance when they consumed enough water to replace about 80% of their sweat loss (1330 ml) compared to when they ingested only 200 ml of water. However, when the subjects ingested 1330 ml of a sports drink, their performance improved by 12%, leading the authors to conclude that the benefits of hydration and carbohydrate feeding were additive.

The benefits of proper hydration and carbohydrate feeding that have been illustrated by numerous laboratory studies are often echoed by the experiences of the subjects in the studies. Dr. Edward Coyle of The University of Texas noted that the competitive cyclists who participate in his experiments "know that drinking is critical to surviving the Texas heat. What they usually don't appreciate is that being well hydrated can help them thrive rather than just survive. After they learn how to fully replace fluids in our studies, they are amazed at how much better they feel as far as being cooler, having a lower heart rate, and generating more power." (E.F. Coyle, personal communication)

"During intense exercise lasting longer than 1 h, it is recommended that carbohydrates be ingested at a rate of 30-60 grams per hour to maintain oxidation of carbohydrates and delay fatigue. This rate of carbohydrate intake can be achieved without compromising fluid delivery by drinking 600-1200 ml per hour of solutions containing 4%-8% carbohydrates (grams per 100 ml). The carbohydrates can be sugars (glucose or sucrose) or starch (e.g., maltodextrin)."

As previously indicated, ingesting carbohydrate at the rate of about 60 g/h during exercise is associated with improved physical performance. Considering that most sports drinks contain 6% to 7% carbohydrate (i.e., 60 - 70 g carbohydrate per liter), the consumption of one L (~ one qt) of sports drink per hour will provide the needed amount of carbohydrate. However, many athletes sweat at rates substantially greater than one L/h (Broad et al., 1996) and so should drink more than 1 L/h. Consuming carbohydrate in excess of 60 g/h will not be detrimental to gastrointestinal comfort, physiological function, or performance provided that the carbohydrate concentration of the ingested beverage is not too high. Beverages containing greater than 7% carbohydrate (i.e., > 17 g carbohydrate per 236 ml [8 oz]) are associated with slower rates of intestinal absorption (Shi et al., 1995), which increases the risk of gastrointestinal distress (Davis et al., 1988; Peters et al., 1995).

Sports drinks usually contain more than one type of carbohydrate, most often combinations of sucrose, glucose, fructose, and maltodextrin. Such combinations are acceptable from both a sensory and a physiological perspective. Beverages containing mostly or solely fructose are not optimal because fructose is absorbed slowly by the intestine (Shi et al., 1995) and requires conversion to glucose by the liver before it can be metabolized by skeletal muscle, making fructose an ineffective fuel for improving performance (Murray et al., 1989). Research subjects who have had the unpleasant experience of participating in studies requiring the ingestion of fructose-only beverages can attest to the gastrointestinal limitations of fructose as the sole source of carbohydrate because vomiting and diarrhea are two common side effects when large amounts of fructose are ingested.

"Inclusion of sodium (0.5-0.7 grams per liter of water) in the rehydration solution ingested during exercise lasting longer than 1 h is recommended since it may be advantageous in enhancing palatability, promoting fluid retention, and possibly preventing hyponatremia in certain individuals who drink excessive quantities of fluid. There is little physiological basis for the presence of sodium in an oral rehydration solution for enhancing intestinal water absorption as long as sodium is sufficiently available from the previous meal."

Sweat contains more sodium and chloride than other minerals and, although sweat electrolyte values are normally substantially lower than plasma values (plasma = 138 - 142 mmol/L; sweat = 25 - 75 mmol/L), athletes who exercise in excess of two h each day can lose considerable amounts of sodium chloride. Consider, for example, a football lineman during two-a-day summer practices in which a total of 5 L of sweat is lost. If each liter of sweat contained 50 mmol sodium, the total sodium loss would be 5,750 mg, the equivalent of over 14 g of NaCl.

Food intake is usually accompanied by sodium chloride intake, and most research indicates that sodium deficits are rare among athletes or military personnel (Armstrong et al., 1987). However, there are occasions when sodium losses can present problems, as illustrated by Bergeron (1996) in a case study of a nationally ranked tennis player who suffered from frequent heat cramps. This player had both a high sweat rate (2.5 L/h) and higher-than-normal sweat sodium concentration (90 mmol/h). The muscle cramps were eliminated when he increased his daily dietary intake of sodium chloride from 5 - 10 g/day to 15 - 20 g/day, and increased his fluid intake to assure adequate hydration.

The ACSM position stand also indicates that ingesting sodium chloride in a beverage consumed during exercise can help ensure adequate fluid intake (Wilk & Bar-Or, 1996) and stimulate more-complete rehydration following exercise (Maughan et al., 1996). Both of these responses underscore the important role that sodium plays in maintaining the osmotic drive to drink and in providing an osmotic stimulus to retain fluid in the extracellular space.

It is true that the sodium content of a fluid-replacement beverage does not directly affect the rate of fluid absorption, as demonstrated by recent research (Gisolfi et al., 1995). This is because the amount of sodium that can be provided to the intestine by a beverage is miniscule compared to the amount of sodium that can be provided from the bloodstream. Plasma sodium freely diffuses into the gut following fluid intake because the concentration gradient for sodium between plasma and the contents of the intestine strongly favors sodium influx. The sodium content of the previous meal or of pancreatic secretions is of little importance in the fluid absorption process. That said, sodium chloride remains a critical ingredient in a properly formulated sports drink because it improves beverage palatability, helps maintain the osmotic drive for thirst, reduces the contribution of plasma sodium required in the gut prior to absorption, helps maintain plasma volume during exercise, and serves as the primary osmotic impetus for restoring extracellular fluid volume following exercise (Maughan et al., 1996; Nose et al., 1988).

Fluid Ingestion Following Exercise

Fluid intake following physical activity can be a critical factor in helping athletes recovery quickly between bouts of training and competition. Many athletes train more than once each day, making rapid rehydration an important consideration, particularly during training in warm weather. The ACSM position stand did not elaborate on recommendations for fluid intake after exercise, but in a recent Sports Science Exchange article, Maughan et al. (1996) provided a comprehensive review of this topic. The authors concluded that ingesting plain water is ineffective at restoring euhydration because water absorption causes plasma osmolality to fall, suppressing thirst and increasing urine output. When sodium is provided in fluids or foods, the osmotic drive to drink is maintained (Gonzalez-Alonso et al., 1992; Nose et al., 1988), and urine production is decreased. There are many occasions during training and competition when it is either difficult or unwise to ingest food, making it all the more important that athletes have access to fluid containing sodium chloride and other electrolytes.

Maughan et al. (1996) also emphasized the importance of ingesting fluid in excess of the deficit in body weight to account for obligatory urine losses. In other words, the advice normally given athletes -- "drink a pint of fluid for every pound of body weight deficit" -- must be amended to "drink at least a pint of fluid for every pound of body weight deficit". More-precise recommendations for how much fluid athletes should ingest to assure rapid and complete rehydration will evolve from future research; existing data indicate that ingestion of 150% or more of weight loss may be required to achieve normal hydration within six h following exercise (Shirreffs et al., 1996).

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