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Hydration Research
SSE Roundtable #26: Hydration and Physical Activity: Scientific Concepts and Practical Applications
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Michael F. Bergeron, Ph.D., Gary W. Mack, Ph.D.
ROUNDTABLE
HYDRATION AND PHYSICAL ACTIVITY: SCIENTIFIC CONCEPTS AND PRACTICAL APPLICATIONS
RT# 26 / Volume 7 (1996), Number 4
Introduction
Generally, research that is conducted under controlled laboratory conditions does not have an immediate impact on sports practitioners-coaches, trainers, athletes, etc., who rightly feel that the non-controlled, spontaneous, and somewhat unpredictable aspect of sport warrants field testing under less-controlled conditions. Of course, the most complete answer to a problem can be developed when the theoretical tenets of basic science can be melded with the more practical aspects of applied science.
The issue of hydration and physical performance has been studied for many years by both basic and applied scientists. In this issue of the GSSI Roundtable, we discuss a number of topics pertaining to dehydration and exercise with Gary Mack, Ph.D., a basic scientist, and Michael Bergeron, Ph.D., who has focused much of his research on the effects of dehydration in tennis players. Their responses to our questions follows.
What type of studies have you conducted regarding the effects of dehydration on physical performance?
Mack: Our studies have focused on two aspects of dehydration. First, we have examined the detrimental influence of dehydration on the body’s ability to dissipate heat during a thermal load. These studies have focused on identifying the physiological mechanism by which hypovolemia and hyperosmolality, produced during dehydration, impose limitations in heat transfer from the body core to the skin, and a reduction in heat loss from the skin to the environment. Our studies have also characterized baroreflex modulation of skin blood flow and sweating in response to alterations in central blood volume, and the inhibition of thermal sweating by increases in plasma osmolality. Second, we have examined the phenomenon referred to as “involuntary dehydration.” In these studies we have examined the mechanisms that contribute to a delay in complete restitution of body fluids following a reduction in total body water. Our efforts have been directed to understanding the factors that contribute to this phenomenon so that we can improve rehydration practices.
Bergeron: Most of my recent studies have been more applied in nature. Our work has been directed toward examining fluid balance in tennis. Many of the tennis players that I have worked with have experienced significant performance decrements when they haven’t managed fluid balance well, and more than a few have suffered problems such as heat cramps and heat exhaustion during competition. However, with a sport such as tennis it is somewhat difficult to identify reliable and measurable outcome-related performance variables. Thus, much of my work in this area has been descriptive in nature, in an attempt to determine the extent and rate of fluid loss that players routinely encounter during competition. As a next step, we are developing projects to examine the effects of dehydration on a variety of tennis-specific psychomotor skills.
Dr. Mack, what are the physiological consequences of dehydration on one’s ability to perform physical activity?
Mack: Fluid deficits imposed voluntarily (i.e., by fluid restriction) or by previous thermal and/or exercise stress will impair subsequent work performance. Water losses due to sweating can often exceed 30 g/min. (1.8 kg/h). The consequences of a progressive loss of body water are a decrease in blood volume (hypovolemia) and an increase in the concentration of electrolytes in the body fluids (hypertonicity). Both of these conditions can impair the body’s ability to dissipate heat generated during exercise. The greater level of dehydration, the greater the degree of impairment.
Numerous studies have clearly demonstrated that cardiovascular strain is greater and body core temperature rises faster when a person exercises in a dehydrated condition, regardless of the environmental conditions. Of course, the decrement in performance is exaggerated when exercise is performed in a hot environment. Furthermore, the combined effects of dehydration and exercise in the heat can lead to heat-related disorders ranging from simple heat cramps to life-threatening heat stroke.
Dr. Bergeron, you have focused the majority of your research on tennis players. What is the profile of the athletes who have served as subjects in your studies?
Bergeron: Most of the players I have worked with were regionally or nationally ranked juniors, Division I collegiate players, or touring professionals. As a result of their regular training and competition schedules, which typically includes at least 2-3 hours a day on the court, these athletes generally have a high degree of cardiorespiratory fitness, a relatively low amount of body fat, and a unique blend of on-court endurance, speed, agility, and power. They usually train and compete year-round, and often play tennis in places in which they have very little time to adequately acclimatize to new environmental conditions. Their matches generally last from less than one hour to sometimes more than four hours. During tournaments, these players often play multiple, long matches on successive days. Clearly, their schedules can be grueling.
What type of sweat and electrolyte losses have you documented in the players you have studied?
Bergeron: Most of the sweat losses that we have calculated were incurred during matches in fairly hot and humid conditions. The ambient temperature was generally 90°F (32°C) or more and the relative humidity was around 60%. In general, during singles play the boys and girls (12-16 yrs.) and young women (18-22 yrs.) had sweating rates of 0.7-1.4 liters per hour; young men (18-30 yrs.) sweated at a rate of 1.2 to 2.5 liters per hour. Although the highest sweat rates that I have measured in a male and female were 3.4 liters and 2.5 liters per hour, respectively.
In heat-acclimatized young adult tennis players the sweat concentration of sodium has generally been a little above 20 mmol per liter, and sweat potassium losses have approximated 5 mmol per liter. However, in heat-acclimatized boys, the sweat sodium loss tends to be somewhat higher (approximately 40 mmol per liter). Even with a high degree of mineral conservation the on-court hourly loss of sodium for many of these players can easily exceed 1 gram. As we have observed with some players, the combination of very high sweat rates (2.5-3.4 liters per hour) coupled with moderate sweat sodium concentrations (35 to just over 60 mmol/L) can yield rather impressive on-court sweat sodium losses of 2,000 to almost 5,000 mg. per hour of play. Considering that tennis players routinely play multiple or long matches on successive days during tournaments, it is not surprising that many tournament players often begin matches in a dehydrated and sodium-deficient condition.
Dr. Mack, are these values out of line with those that you see in a laboratory setting?
Mack: Answering this question is not as clear-cut as it may seem. Several factors influence whole body sweat rate and the determination of sweat electrolyte composition. First, sweating and sweat composition is not uniform over the entire body. Second, sweat composition is dependent on the local sweat rate. Finally, progressive dehydration associated with prolonged exercise in the heat may modify regional sweat rates and thereby sweat composition. Thus, determination of an average sweat composition during exercise performed in the laboratory or field is not a simple measurement.
In our laboratory we sample sweat from five different skin sites and then use an equation which incorporates factors that account for the regional differences in sweat rate and adjusts for the relative contribution of each region to the total surface area of the body. Using this technique we have determined the average electrolyte composition of sweat in active college aged students under standard exercise protocols. Whole-body sweat rates of ~0.8 L/hr. induced with mild (40% VO2 max.) cycle ergometry in the heat (36°C; 30% RH) produces sweat with an average sodium concentration of 68 mmol/L and a potassium concentration of 4.7 mmol/L. However, these values may vary considerably with a range of 30 to 110 mmol Na/L and 2.5 to 9.3 mmol K/L. During prolonged exercise (up to six hours) in the heat, when sweat rates are maintained by simultaneous fluid replacement, individuals may lose in excess of 5 g of sodium (the equivalent of 12.5 g of table salt). At higher sweat rates (1.4 L/hr.) induced by intense treadmill exercise (70% VO2 max) we have measured an average whole body sodium concentration of 74 mmol/L (range of 40 to 104 mmol/L). Lower values of sweat sodium concentration, such as those in the tennis players described by Dr. Bergeron, are a function of the athletes’ high level of fitness and degree of heat acclimatization.
Dr. Mack, the importance of sodium for rehydration purposes has been outlined in numerous articles. However, is there a downside to giving a healthy athlete “carte blanche” access to sodium?
Mack: During recovery from dehydration, electrolyte replacement ensures complete restoration of the extracellular fluid and a more complete restitution of water balance. The normal range of daily U.S. intake of sodium chloride is 2-9 grams (35-156 mmol sodium), and potassium is 2-4 grams (50-100 mmol). Electrolyte losses in these ranges are generally replenished within 24 hours following exercise if adequate fluid is consumed. In the absence of meals, more complete rehydration can be accomplished with fluids containing sodium than with plain water. The ideal salt concentration in the ingested fluid has not been determined. However, a consensus report sponsored by the National Academy of Sciences recommends that the solution should provide approximately 20-30 mmols of sodium per liter, 2 to 5 mmols of potassium per liter, and chloride as the only anion.
I don’t think there is a documented downside to ad libitum sodium intake in healthy adults. Sodium intake must vary in proportion to the deficit in total body sodium content. Normal healthy adults have several sophisticated regulatory systems that act to regulate sodium intake and retention. In healthy individuals, when all these mechanisms are working properly, sodium balance is achieved without the need to restrict sodium intake.
Dr. Bergeron, are there other nutritional issues besides hydration status that you see in the athletes you work with?
Bergeron: It’s clear that any time there is extensive and repetitive sweating, there is potential for developing a sodium deficit. This condition is often exacerbated when a susceptible athlete limits his or her salt intake. We are now in the process of looking more closely at other potential mineral imbalances that might develop in athletes during long periods of extensive sweating.
A tennis player’s blood glucose level and carbohydrate stores are also a concern. Therefore, we always stress a high-carbohydrate diet, and we encourage players to consume a carbohydrate-electrolyte drink during and after matches, particularly if they are going to play again soon.
I also find that the daily caloric intake of many athletes is often inadequate. Unfortunately, the high dietary bulk associated with a high-calorie, high-carbohydrate diet is unappealing to some athletes. In these cases, high-carbohydrate, high-calorie drinks or snacks can be beneficial.
Do you see any carryover from your studies to other groups of athletes? To the “average” person who trains and competes in the heat?
Bergeron: Many of the college athletes that I have worked with, including swimmers, basketball players, and soccer players, tend to function in a chronically dehydrated condition, as evidenced by their high urine specific gravities or their inability to urinate prior to practices or games. I don’t think that the typical athlete or the average recreational exerciser appreciates the extent of fluid and electrolyte losses that readily and routinely occur during most forms of physical activity. Generally, athletes should be able to urinate before and after they train or compete. If they are unable to do so, they likely have not consumed enough fluid. For those people who lose considerable sodium from extensive sweating, consuming more sodium-rich foods or adding salt to foods and fluids may be appropriate.
Mack: As I stated earlier, our studies have demonstrated that complete restoration of the extracellular fluid compartment (and blood volume) cannot be attained without replacement of the lost sodium. Furthermore, during prolonged exercise, a combination of sodium loss and the ingestion of large quantities of fluids with little or no electrolytes can lead to low plasma sodium. In ultraendurance events, hyponatremia (blood sodium concentrations of less than 130 mmol/L) has been observed at the end of competition and is associated with problems of disorientation, confusion and, in some cases, grand-mal seizures. To prevent the development of hyponatremia or related conditions, sufficient electrolytes should be provided in fluid replacement beverages. This would certainly constitute a practical application of our research.
Selected Readings:
American College of Sports Medicine (1996). Position stand on exercise and fluid replacement. Med. Sci. Sport Exerc. 28:i-vii
Bergeron, M.F., C.M. Maresh. L.E. Armstrong, J.F. Signorile, et al. (1995). Fluid-electrolyte balance associated with tennis match play in a hot environment. Int. J. Sport Nutr. 5:180-193
Bergeron, M.F., L.E. Armstrong,, & C.M. Maresh (1995). Fluid and electrolyte losses during tennis in the heat. Clin. Sports Med. 14:23-3
Maughan, R.J., J.B. Leiper, & S.M. Shirreffs (1996). Rehydration and recovery after exercise. Sport Sci. Exch. 9(62):1-5
Mack, G.W., H. Nose, & E.R. Nadel. (1988). Role of cardiopulmonary baroreflexes during dynamic exercise. J. Appl. Physiol. 65:1827-1832.
Nose, H., G.W. Mack, X. Shi, & E.R. Nadel (1988). Role of osmolality and plasma volume during rehydration in humans.
J. Appl. Physiol. 65:325-331
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