March 2008

Fluid Replacement Guidelines for Exercise
By Ellen Coleman, MA, MPH, RD
Today’s Dietitian
Vol. 10 No. 3 P. 10

Suggested CDR Learning Codes: 2110, 3040, 4060; Level 2

The evaporation of sweat is the primary way to dissipate heat during vigorous exercise in warm or hot weather, but sweat losses can be significant. As a result, water and electrolyte deficits can develop and have an adverse effect on a person’s athletic performance and possibly his or her health.1,2 Subsequently, replacement on a timely basis is essential.

In February 2007, the American College of Sports Medicine (ACSM) published an updated position stand on fluid replacement and exercise.1 The ACSM paper provides guidance to maintain an appropriate level of hydration during physical activity. However, due to the considerable variability in sweating rates and sweat electrolyte content between individuals, the ACSM recommends customizing fluid replacement plans.1
This article will present background, current research, and guidelines for dietitians working with physically active individuals.

Fluid and Electrolyte Requirements
Various factors influence sweat loss during exercise, including exercise intensity and duration, environmental conditions (eg, temperature, humidity, sun, and wind), and clothing and equipment.1 There are great differences in sweat rates and total sweat losses between individuals in different sports, as well as in the same sport—eg, sweat rates vary among soccer players according to their position, playing style, and total time spent on the field).3 Also, American football players, who tend to have a large body mass and wear protective clothing, have noticeably higher daily sweat losses (8.8 liters per day) than cross-country runners (3.5 liters per day) who train in the same hot environment for the same duration.4

A comparison of competitors across a wide range of sports during training and competition shows that individuals often reach sweating rates of 0.5 to 2 liters per hour. This wide range demonstrates the difficulty and impracticality of providing universal guidelines for fluid replacement. While the differences in sweating rates between individuals in the same event and environment are reduced when body size (body weight or body surface area) is taken into account, distinct individual differences remain.1

During exercise, metabolic heat is transferred from the active muscles to the blood and then the body core. Subsequent physiologic adjustments facilitate the transfer of heat from the body core to the skin for release into the environment. In mild and cool environments, sweat losses are relatively small due to the high capacity for dry heat loss (radiation and convection). As the heat stress intensifies, there is a greater dependence on sweat for evaporative cooling.

Wearing heavy clothing (eg, a football uniform) greatly increases heat stress and evaporative cooling requirements during exercise in temperate to hot environments.1 Football was once a cold weather sport, but with migration south and west and expansion of the summer training and competition schedule, teams may play in triple-digit temperatures while wearing gear they’d be comfortable in at 40°F.

When sweat is not evaporated and drips from the body, a higher sweating rate is necessary to achieve evaporative cooling. Conversely, increased air motion (wind and speed of movement) facilitates evaporation and minimizes wasted (dripping) sweat.1

Heat acclimatization enhances the ability to achieve higher and more sustained sweating rates. Aerobic training also increases sweating rate, while wet skin (eg, from high humidity) and dehydration can suppress sweating rate.1

The sweat electrolyte losses depend on the total sweat losses and concentrations of electrolytes in sweat. The concentration of sodium in sweat averages approximately 35 milliequivalents per liter, or 805 milligrams per liter (range 10 to 70 milliequivalents per liter) and varies depending on genetic predisposition, diet, sweating rate, and heat acclimatization state.1,5,6 Sweat concentrations of potassium average 5 milliequivalents per liter (range 3 to 15 milliequivalents per liter), concentrations of calcium average 1 milliequivalent per liter (range 0.3 to 2 milliequivalents per liter), concentrations of magnesium average 0.8 milliequivalents per liter (range 0.2 to 1.5 milliequivalents per liter), and concentrations of chloride average 30 milliequivalents per liter (range 5 to 60 milliequivalents per liter).5 Gender, maturation, and aging don’t appear to have discernible effects on sweat electrolyte concentrations.1

Dehydration can increase sweat concentrations of sodium and chloride.7 The concentration of sweat sodium and chloride increases with sweating rate, but the ability of sweat glands to reabsorb these electrolytes does not.8 Heat acclimatization improves the capacity to reabsorb sodium and chloride and generally reduces sweat sodium concentrations by more than 50% for any given sweating rate.8

Assessing Hydration Status
Individuals can determine their hydration status by using several simple biological markers (body weight and urine specific gravity [USG] or osmolality [Uosmol]). These measurements have limitations when used separately but can provide valuable information when used together in the proper setting. Combining the first morning body weight measurement (after voiding) with a measure of urine concentration can effectively discern deviations in fluid balance.1

Urine markers of hydration status can help establish whether an individual is normally hydrated or dehydrated. The USG and Uosmol are quantifiable measurements, whereas evaluation of urine color and volume are more subjective. A USG of 1.020 or less indicates normal hydration. The Uosmol measurement is more variable, but values below 700 milliosmoles per kilogram indicate normal hydration.9

Body weight measurements are easy and effective tools to assess fluid balance. A first morning nude body weight measurement after urination fluctuates by approximately 1% in well-hydrated persons who are maintaining their body weight. At least three such measurements should be taken to establish a baseline value for normal hydration in active men. Since the menstrual cycle influences body water status, women may need more body weight measurements to establish a baseline value.1

Acute changes in body weight during exercise can be used to calculate sweating rates. Individuals should monitor changes in body weight during training and competition to establish sweating rates for specific exercise and environmental conditions. This allows the development of customized fluid replacement plans to meet their specific needs.

To determine sweat rate, an individual should weigh in before and after a specified time of exercise (eg, one hour). Nude weights should be used when possible to avoid corrections for sweat trapped in clothing. Then subtract the postworkout weight from the preworkout weight and also subtract any urinary fluid losses. Then add in the amount of fluid consumed during the workout.

Physiology and Performance
During exercise in the heat, dehydration increases physiologic stress as measured by core temperature, heart rate, and perceived exertion.10 Dehydration of more than 2% body weight decreases aerobic exercise and mental performance in temperate, warm, or hot environments.2 The greater the body water shortage, the greater the physiologic strain and impairment of aerobic performance.11 The critical water deficit (greater than 2% of body weight for most people) and degree of performance decline are probably related to the heat stress, exercise task, and an individual’s characteristics (eg, dehydration tolerance). Dehydration of 3% to 5% body weight probably does not decrease either muscular strength or anaerobic performance.1,12

Physiologic factors that contribute to reduced aerobic performance when dehydrated include increased core temperature, increased cardiovascular strain, increased glycogen utilization, altered metabolic function, and possibly altered central nervous system function.11 Dehydration and hyperthermia (elevated body temperature) also decrease cognitive performance and therefore have a negative impact on concentration, skilled tasks, and strategic planning.1

Heat Illnesses
Dehydration increases the risk for heat exhaustion and is a risk factor for heat stroke. It, along with sodium deficits and muscle fatigue, is associated with skeletal muscle cramps. Dehydration can also increase the likelihood or severity of acute renal failure associated with rhabdomyolysis, a syndrome that causes release of skeletal muscle contents. Rhabdomyolysis is most often observed with unusual and strenuous exertion.1

Symptomatic exercise-associated hyponatremia (plasma sodium concentration below 135 millimoles per liter) can occur in endurance events. The lower and more rapidly the blood sodium drops, the greater the risk of dilutional encephalopathy and pulmonary edema. When plasma sodium concentration drops below 120 millimoles per liter, the risk of severe cerebral edema with seizure, coma, brainstem herniation, respiratory arrest, and death increases.1

Exercise-associated hyponatremia is primarily caused by the consumption of a fluid amount that exceeds sweat losses.13,14 In marathon runners, symptomatic hyponatremia is more likely to occur in smaller individuals who run slowly, sweat less, and drink excessively before, during, and after the race.13,14 Large sweat sodium losses can also contribute to hyponatremia in long ultra-endurance events (eg, a triathlon).14

Individuals can experience health problems from either dehydration or overdrinking. Dehydration is more common and can impair exercise performance and contribute to serious heat illness. However, symptomatic hyponatremia is more dangerous and can lead to grave illness or death.1

Fluid Replacement
Before exercise. The purpose of prehydrating is to begin an activity with normal hydration (euhydration) and plasma electrolyte levels. Individuals will generally be normally hydrated when they have consumed ample beverages with meals and had adequate time (eight to 12 hours) to recover from their last exercise session.15 If the person has experienced extensive sweat losses and hasn’t had enough time to reestablish normal hydration, an aggressive prehydration program may be warranted. This will help ensure that fluid and electrolyte deficits are rectified prior to exercising. The individual should slowly drink fluid (roughly 5 to 7 milliliters per kilogram per body weight) at least four hours before activity. (Note: 7 milliliters per kilogram is equivalent to approximately 1 ounce for every 10 pounds of body weight). If the individual does not produce urine or the urine is dark or highly concentrated, he or she should slowly drink more fluid (an additional 3 to 5 milliliters per kilogram) approximately two hours before exercising.

Drinking several hours before exercise allows adequate time for the urine output to return to normal. Drinking beverages that contain sodium (20 to 50 milliequivalents, or 460 to 1,150 milligrams per liter) and/or eating small amounts of salted snacks or sodium-containing foods at meals helps stimulate thirst and promote fluid retention.16

Hyperhydration can be achieved by overdrinking combined with an agent such as glycerol that “binds” water within the body.17 This greatly increases the risk of having to urinate during competition and does not provide a noticeable physiologic or performance benefit over normal hydration.17,18 Hyperhydration can also substantially dilute and lower plasma sodium prior to starting exercise and therefore increase the risk of dilutional hyponatremia if the individual drinks too much during exercise.14,17

Enhancing the fluid’s palatability helps promote consumption before, during, and after exercise. Fluids that are flavored, cooled, and contain sodium can increase voluntary fluid intake, but temperature and flavor preference vary greatly between individuals and cultures.1

During exercise. Drinking during exercise is necessary to prevent the detrimental effects of excessive dehydration (more than 2% body weight loss) and electrolyte loss on exercise performance and health. The amount and rate of fluid replacement depends on the individual’s sweating rate, exercise duration, and opportunities to drink.1

It is impossible to propose a one-size-fits-all fluid and electrolyte replacement schedule due to multiple factors that influence sweating rate and sweat electrolyte concentrations. For example, fluid and electrolyte replacement strategies will be completely different for a 280-pound football player during twice daily summer practices compared with a 110-pound marathoner running at a five-hour pace. Individuals should monitor body weight changes during training and competition in different environmental conditions to estimate their sweat losses. This allows them to develop customized fluid replacement programs for their particular needs.1

Individuals participating in prolonged exercise lasting more than three hours should be meticulous in establishing their fluid replacement schedule. As the exercise duration increases, the cumulative effects of slight disparities between fluid intake and loss can cause extreme dehydration or hyponatremia.1

It has been suggested that marathon runners drink ad libitum from 0.4 to 0.8 liter per hour.19 The higher rates are for faster, heavier individuals competing in warm environments, and the lower rates are for slower, lighter persons competing in cooler environments.19 These guidelines are probably satisfactory for individuals participating in marathon-length events.14 However, it is apparent that longer running distances, different types of activity, more severe weather, and unique populations can have noticeably different fluid replacement requirements. American football players wearing full equipment in hot weather will require much larger fluid volumes to maintain normal hydration on a daily basis.1

Consuming beverages containing carbohydrate and electrolytes can be more beneficial than drinking water, depending on the specific activity conditions (eg, intensity and duration) and weather conditions. Sodium and potassium help replace sweat electrolyte losses. Sodium also helps stimulate thirst and carbohydrate provides energy. Carbohydrate and electrolytes can also be obtained via nonfluid sources such as carbohydrate gels and energy bars.1

The Institute of Medicine has provided general guidance for the composition of so-called sports beverages for persons performing prolonged physical activity in hot weather. The institute recommends that these types of fluid replacement beverages contain 20 to 30 milliequivalents per liter of sodium (chloride as the anion), 2 to 5 milliequivalents per liter of potassium, and 5% to 10% carbohydrate.20 Consuming carbohydrate can sustain performance during high-intensity exercise lasting approximately one hour or longer and intermittent exercise such as basketball and soccer.21,22 Consuming carbohydrate also maintains endurance performance and blood glucose levels.23

Carbohydrate-based sports beverages are often used to meet carbohydrate needs and replace water and electrolyte losses. Drinking between one half and one of a conventional sports drink (6% to 8% carbohydrate) each hour provides approximately 30 to 80 grams of carbohydrate per hour and sufficient water to avoid excessive dehydration for many activities.23 When the same beverage is used to meet fluid and carbohydrate requirements, it should provide less than 8% carbohydrate, as concentrated carbohydrate beverages reduce gastric emptying.1

The greatest rates of carbohydrate delivery are achieved with a mixture of sugars (eg, glucose, sucrose, fructose, maltodextrin). The maximum amount of carbohydrate oxidized during exercise from a single carbohydrate source (eg, glucose) is approximately 1 gram per minute (60 grams per hour). Adding an additional carbohydrate (eg, fructose) uses a different intestinal transporter for absorption and increases the amount of carbohydrate absorbed. To maintain performance, individuals can consume 1 gram of carbohydrate per kilogram per hour from fluids or foods providing a mixture of carbohydrates.24

Caffeine is unlikely to elevate urine output or cause dehydration if consumed in moderation during exercise.15,25 Judicious use of caffeine may help improve exercise performance.26

After exercise. Following exercise, the individual should replace fluid and electrolyte losses. Consuming regular meals and beverages will restore normal hydration over 24 hours, provided the food contains enough sodium to replace sweat losses.15 If the individual is significantly dehydrated and has a short period (less than 12 hours) in which to recover before exercise, an aggressive rehydration program may be necessary.16,27

Inadequate replacement of sodium losses prevents the return of normal hydration and stimulates excessive urine production.28 Consuming sodium during recovery promotes fluid retention and stimulates thirst. Sodium losses are harder to determine than water losses because individuals have vastly different rates of sweat electrolyte losses. Although drinks containing sodium (eg, sports drinks) may be beneficial, many foods can supply the needed electrolytes. Extra salt (1/2 teaspoon supplies 1,000 milligrams of sodium) can be added to meals and recovery fluids when sweat sodium losses are high.1

Individuals should drink 1.5 liters of fluid for each kilogram lost (24 ounces for each pound lost) to achieve rapid and total recovery from dehydration.16 The additional volume (150% of sweat losses) is required to compensate for the increased urine production that goes along with the rapid intake of large volumes of fluid.1 When possible, fluids should be consumed over time and with ample electrolytes to maximize fluid retention.29

Intravenous fluid replacement after exercise may be indicated in individuals with severe dehydration (more than 7% body weight loss) who experience nausea, vomiting, or diarrhea or who cannot drink fluids. In most circumstances, intravenous fluid replacement does not confer an advantage over oral rehydration to replace fluid and electrolyte deficits.30

Alcohol acts as a diuretic (especially at high doses) and increases urine output. It should be consumed in moderation, particularly following exercise when rehydration is desired.31

Summary
Physical activity in warm or hot weather can cause substantial water and electrolyte losses. If not replaced, the individual will dehydrate. Excessive dehydration (more than 2% body weight loss) impairs performance and increases the risk of heat illness. Overdrinking (consuming an amount of fluid that exceeds sweat losses) can cause symptomatic hyponatremia.1

The goal of prehydrating is to begin physical activity with normal hydration and electrolyte status. Individuals should prehydrate several hours before exercise to facilitate fluid absorption and allow urine output to return to normal levels. Drinking during exercise is essential to prevent the harmful effects of excessive dehydration and electrolyte loss on exercise performance and health. Individualized fluid replacement plans are recommended due to the substantial differences in sweating rates and sweat composition between individuals.1

Measuring preexercise and postexercise body weight to calculate sweating rate is a straightforward and valid method to estimate sweat losses. Consuming beverages containing carbohydrates and electrolytes during exercise can provide benefits over drinking water in certain situations. After exercise, fluid and electrolyte deficits should be replaced. The length of recovery time between exercise sessions and extent of fluid/electrolyte losses will determine whether an aggressive replacement program is warranted.1

— Ellen Coleman, MA, MPH, RD, is a nutrition consultant at The Sport Clinic in Riverside, Calif.

References
1. Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand: Exercise and fluid replacement. Med Sci Sports Exerc. 2007;2:377-390.

2. Casa DJ, Clarkson PM, Roberts WO. American College of Sports Medicine roundtable on hydration and physical activity: Consensus statements. Curr Sports Med Rep. 2005;4(3):115–127.

3. Shirreffs SM, Ragon-Vargas LF, Chamorro M, et al. The sweating response of elite professional soccer players to training in the heat. Int J Sports Med. 2005;26:90–95.

4. Godek SF, Bartolozzi AR, Godek JJ. Sweat rate and fluid turnover in American football players compared with runners in a hot and humid environment. Br J Sports Med. 2005;39(4):205–211.

5. Brouns F. Heat-sweat-dehydration-rehydration: A praxis oriented approach. J Sports Sci. 1991;9:143–152.

6. Verde TR, Sheppard RJ, Corey P, et al. Sweat composition in exercise and in heat. J Appl Physiol. 1982;53:1540–1545.

7. Morgan RM, Patterson MJ, Nimmo MA. Acute effects of dehydration on sweat composition in men during prolonged exercise in the heat. Acta Physiol Scand. 2004;182(1):37–43.

8. Allan JR, Wilson CG. Influence of acclimatization on sweat sodium concentration. J Appl Physiol. 1971;30(5):708–712.

9. Armstrong S, Maresh CM, Castellani JW, et al. Urinary indices of hydration status. Int J Sports Nutr. 1994;4(4):265–279.

10. Sawka NM, Coyle EF. Influence of body water and blood volume on thermoregulation and exercise performance in the heat. Exerc Sport Sci Rev. 1999;27:167–218.

11. Montain SJ, Coyle EF. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J Appl Physiol. 1992;73(4):1340-1350.

12. Cheuvront SN, Carter R, Haymes EM, et al. No effect of moderate hypohydration or hyperthermia on anaerobic exercise performance. Med Sci Sports Exerc. 2006;38(6):1093–1097.

13. Hew TD, Chorley JN, Cianca JC, et al. The incidence, risk factors, and clinical manifestations of hyponatremia in marathon runners. Clin J Sports Med. 2003;13(1):41-47.

14. Montain SJ, Cheuvront SN, Sawka NM. Exercise associated hyponatremia: quantitative analysis to understand the aetiology. Br J Sports Med. 2006;40(2):98–105.

15. Institute of Medicine. Water. In: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, Sulfate. Washington, D.C.: National Academies Press; 2005.

16. Shirreffs SM, Maughan RJ. Volume repletion after exercise-induced volume depletion in humans: Replacement of water and sodium losses. Am J Physiol. 1998;274:F868–F875.

17. Kavouras SA, Armstrong LE, Maresh CM, et al. Rehydration with glycerol: Endocrine, cardiovascular and thermoregulatory responses during exercise in heat. J Appl Physiol. 2006;100(2):442-450.

18. O’Brien C, Freund BJ, Young AJ, et al. Glycerol hyperhydration: Physiological responses during cold-air exposure. J Appl Physiol. 2005;99:515–521.

19. Noakes T. Fluid replacement during marathon running. Clin J Sport Med. 2003;13(5):309–318.

20. Institute of Medicine. Fluid Replacement and Heat Stress. Washington, D.C.: National Academies Press; 1994.

21. Below PR, Mora-Rodriguez R, Gonzalez-Alonso J, et al. Fluid and carbohydrate ingestion independently improve performance during 1 h of intense exercise. Med Sci Sports Exerc. 1995;27(2): 200-210.

22. Welsh RS, Davis JM, Burke JR, et al. Carbohydrates and physical/mental performance during intermittent exercise to fatigue. Med Sci Sports Exerc. 2002;34(4):723–731.

23. Coyle EF. Fluid and fuel intake during exercise. J Sports Sci. 2004;22(1):39–55.

24. Jentjens RL, Achten J, Jeukendrup AE. High oxidation rates from combined carbohydrates ingested during exercise. Med Sci Sports Exerc. 2004;36(4):1551-1558.

25. Armstrong LE, Pumerantz AC, Roti MW, et al. Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption. Int J Sport Nutr Exerc Metab. 2005;15(3):252–265.

26. Cox GR, Desbrow B, Montgomery PG, et al. Effect of different protocols of caffeine intake on metabolism and endurance performance. J Appl Physiol. 2002;93(3):990–999.

27. Maughan RJ, Leiper JB, Shirreffs SM. Restoration of fluid balance after exercise-induced dehydration: effects of food and fluid intake. Eur J Appl Physiol. 1996;73(3-4):317–325.

28. Nose H, Mack WG, Shi XR, et al. Involvement of sodium retention hormones during rehydration in humans. J Appl Physiol. 1988;65:332–336.

29. Kovacs EM, Schmahl RM, Senden JM, et al. Effect of high and low rates of fluid intake on post-exercise rehydration. Int J Sport Nutr Exerc Metab. 2002;12(1):14–23.

30. Casa DJ, Maresh CM, Armstrong LE, et al. Intravenous versus oral rehydration during a brief period: Responses to subsequent exercise in the heat. Med Sci Sports Exerc. 2000;32(1):124–133.

31. Shirreffs SM, Maughan RJ. Restoration of fluid balance after exercise-induced dehydration: effects of alcohol consumption. J Appl Physiol. 1997;83:1152–1158.

Examination
1. The American College of Sports Medicine recommends:
a. drinking according to thirst.
b. drinking 0.4 to 0.8 liters per hour.
c. drinking 0.5 to 2.0 liters per hour.
d. drinking as often as possible.
e. customizing fluid replacement plans.

2. Heat acclimatization:
a. increases sweating rate.
b. decreases sweating rate.
c. reduces sweat sodium concentrations.
d. a and c
e. b and c

3. Exercise-associated hyponatremia is primarily caused by:
a. high sweat sodium losses.
b. competing at a slow pace in an endurance event.
c. consumption of an amount of fluid that exceeds sweat losses.
d. customizing fluid replacement based on sweating rate.
e. being a small person competing in an endurance event.

4. Using the following information, determine the hourly sweating rate:
Weight before exercise: 190 pounds
Weight after one hour of exercise: 188 pounds
Fluid consumed during exercise: 16 ounces
a. 16 ounces per hour
b. 32 ounces per hour
c. 48 ounces per hour
d. 64 ounces per hour
e. None of the above

5. An individual has lost 2 pounds after a training session. How much should the person drink to achieve rapid and total recovery from dehydration?
a. 16 ounces
b. 24 ounces
c. 32 ounces
d. 40 ounces
e. 48 ounces

6. What provide(s) an effective tool to determine fluid balance?
a. Body weight measurements
b. Urine color and volume
c. Urine specific gravity and osmolality
d. a and c
e. a and b

7. The critical water deficit for most people is:
a. 1% of body weight.
b. 2% of body weight.
c. 3% of body weight.
d. 4% of body weight.
e. 5% of body weight.

8. At least four hours prior to exercise, the individual should slowly drink approximately:
a. 5 to 7 milliliters per kilogram per body weight.
b. 3 to 5 milliliters per kilogram per body weight.
c. 1/2 ounce for every 10 pound.
d. a and c
e. b and c

9. It is not possible to propose a universal fluid and electrolyte replacement schedule due to the multiple factors that influence sweating rate and sweat electrolyte concentrations.
a. True
b. False

10. Consuming carbohydrate:
a. sustains performance during high-intensity exercise lasting approximately one hour or longer.
b. maintains endurance performance and blood glucose levels.
c. does not improve performance during intermittent exercise such as basketball and soccer.
d. a and b
e. b and c