Are you planning to run a marathon, compete in a triathlon, or take a long bike ride? If you have been told to "drink as much and as often as you can" in order to prevent dehydration during long events like this, then this story is for you.

Pretty much everyone knows that you should drink lots of water when you exercise. And, unless you have been living under a rock, you know that sports drinks like Gatorade® exist specifically for this purpose.

THIRST IS A POOR INDICATOR

The basis for the "drink as much as you can" (ie., during the event) concept comes from several observations. During prolonged exercise, particularly in hot/humid conditions, substantial amounts of fluid are lost due to perspiration and, to a lesser extent, water vapor expired during breathing.

But, by the time an athlete experiences thirst, a significant amount of body fluid has already been lost. Tests conducted by the Gatorade Sports Science Institute showed that the average runner, in conditions of 85 degrees and 40 percent humidity, will lose about 2-4 lbs of body weight per hour through sweat loss (Runner's World, August 2004).

Further, athletes often don't take in enough fluid when left to rely on themselves. Soccer players in Puerto Rico only consumed an average of 2.7 liters per day when allowed to drink as they wish. 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 per day (Rico-Sanz, et al. 1996).

The thirst reflex appears to be a poor signal in children and adolescents, too. In one study, 34 boys (average age 13 yrs) and 24 girls (average age 12 yrs) who participated at a 4-day soccer camp in Pennsylvania during July 2003 were studied. Most of the players were dehydrated by the second day of camp; more than half of the girls were seriously dehydrated at this point. By the last day of camp, 59% of boys and 70% of girls were significantly dehydrated. This occured despite the fact that coolers of fluids were available within 25 meters and coaches encouraged players to drink and take frequent rest breaks (Mamula PW. 2004).

DEHYDRATION OR POSTURAL HYPOTENSION?

The second reason for recommending that endurance athletes drink as much as they can came from observing some competitors who collapsed after crossing the finish line. The impression was that these athletes were severely dehydrated; they were rushed into the locker room and given fluids intravenously.

Thus, based on these scenarios, it makes sense to be aggressive about drinking water during a prolonged event, right? Certainly at one time, the sports medicine community believed it to be the right approach. In 1996, the American College of Sports Medicine officially endorsed this strategy in their guidelines Exercise and Fluid Replacement (Convertino VA, et al. 1996). In this position paper, they urged individuals to "consume the maximal amount that can be tolerated."

Since then, the understanding of fluid replacement during exercise has changed. Read on....

HYPONATREMIA CAUSED BY DRINKING TOO MUCH WATER

Today, the strategy of drink "as much as you can" is no longer recommended. Here's why: it turns out that the overzealous consumption of free water (ie., water without electrolytes) during a sustained period of heavy perspiration will lead to a condition known as "hyponatremia". Hyponatremia occurs when the amount of sodium in the bloodstream is too low. This can lead to confusion, seizures, and even death. More than 70 cases have been reported since it was first described in 1985. Here are data from two of those reports:

• Twenty-six runners were identified in the 1998 and 1999 San Diego Marathon who had developed symptomatic hyponatremia. Many of these runners admitted drinking as much fluid as possible during and immediately after the race (Davis DP, et al. 2001).

• At the 2002 Boston Marathon, 488 runners were studied after the race and it was found that 13% developed hyponatremia (Almond CSD, et al. 2005).

But what about that marathoner or triathlete who collapses after crossing the finish line? Isn't this evidence that fluid consumption should be aggressive?

No. It is now thought that the physiologic explanation for this is due to venous pooling of blood resulting from a slower heart rate and a cessation of the massaging action on venous circulation by leg muscles after the runner stands still. The athlete feels lightheaded, or may even faint, because venous return to the heart has declined abruptly. If the athlete continues to jog or just walk, the tendency to faint will be minimized. Admittedly, these individuals do need fluids to boost their venous return and blood pressure. But, the trick is to ingest fluid in amounts that won't cause hyponatremia.

HOW MUCH WATER IS TOO MUCH?

During exercise, particularly in the heat, sweat output increases tremendously. The amount of fluid lost through perspiration can vary enormously. During heavy exercise in hot conditions, an athlete can lose more than 1 liter of sweat PER HOUR per square meter of body surface area (Wilmore JH, et al. 1999).

At the same time, urine production declines by 20-60% compared to resting values. Hyponatremia can occur any time you drink water faster than your kidneys can eliminate it. At rest, the kidneys can produce about 1000 ml (eg., 1 liter) of urine per hour. Most adults can actually drink more than 1000 ml per hour if they make a conscious effort.

Hyponatremia occurs when water losses (via perspiration) are replaced and sodium losses are not.

The risk for exercise-related hyponatremia seems to be higher during exercise sessions that last longer than 4 hours because you have more time to drink more fluid.

To provide a perspective on how much water consumption is too much, consider the following cases:

• On a single summer day in 1995, nine US Marine recruits developed hyponatremia. Each soldier consumed 10-22 quarts of water over a few hours (Gardner JW. 2002).

• One female competitor developed hyponatremia during a marathon after consuming 10 liters of fluid the night before the race (Eichner ER. 2002).

Today, the concern regarding hyponatremia during distance races is so high that some marathons are reducing the number of water stations. For example, last year, the Houston Marathon reduced the number of water stations from 30 down to 15 and noticed fewer problems attributable to hyponatremia.

WHO IS AT RISK FOR HYPONATREMIA?

In an effort to cool down the body's temperature, logically you need to consume plenty of water. But some types of exercisers are at risk for hyponatremia: for example, cyclists, because it is easier to drink while riding, and slower runners, because they consume fluids over a longer period of time. Also, it appears that females are at higher risk than males (Ayus JC, et al. 2000).

DETERMINING FLUID REQUIREMENTS

So, how much water or sports drink should an athlete consume during a distance event?

In the Fall of 2001, the International Marathon Medical Directors Association (IMMDA) met to develop an advisory statement regarding fluid replacement during marathon running. In this statement, released in 2002, they advise marathoners to drink NO MORE THAN 400-800 ml of fluid per hour. This may not seem like enough, but the document also mentions that some elite-level marathon racers only consume about 200-800 ml per hour during a race and have no problems with dehydration.

Because different athletes have differing sweat rates, it is impossible to provide a single recommendation that applies to all competitors. But, Douglas J. Casa, PhD, ATC, FACSM, at the University of Connecticut has devised a formula whereby each athlete can calculate their fluid requirements based on their own sweat rate:

a) Do a warm-up run to the point where you begin to perspire.

b) Urinate if necessary.

c) Determine your weight (in pounds), naked, after drying off any remaining sweat.

d) Run for 60 minutes at an intensity similar to your target pace. (Note that if you do this test run on a treadmill inside an air-conditioned health club, it will probably not be representative of a run outdoors with different ambient temperature and humidity.)

e) If you drink during the run, keep track of the amount (in ounces). Do not urinate during the run.

f) Weigh yourself again, using the same procedure as in (c).

g) Subtract your post-run weight from your pre-run weight. Multiply that value by 16.

h) If you drank during the run, add the number of ounces to the number obtained in line (g) above.

You now know the number of ounces you need to consume per hour. Divide this amount into thirds and try to consume this amount of fluid every 20 minutes during exercise.

This formula was summarized in the August 2004 issue of Runner's World magazine, and can also be obtained from the USA Track & Field web site at www.usatf.org.

SHOULD I TAKE SALT TABLETS?

In general conversation, "salt" means "sodium chloride". But, from a medical perspective, the term "salts" refers to many electrolytes such as sodium, potassium, and chloride, all of which are lost during the sweating process. To be accurate, most cases of exercise-induced hyponatremia are due to excessive consumption of free water during a period of heavy sweating. The loss of sodium and chloride in sweat is adaptive and variable. For example, untrained males and females lose roughly twice as much as trained athletes. Also, some athletes lose more sodium in their perspiration, thus, generating the label "salty sweater". As mentioned above, the body's ability to retain minerals like sodium and chloride improves as you get fitter.

While otherwise healthy adults should take in some sodium (in the form of salt) if they are perspiring heavily, using salt tablets is generally not recommended. Salty foods, such as chips or pretzels, or, sports drinks are preferred. Even though sports drinks contain a relatively small amount of sodium (typically, only about 50-200 mg sodium per 8-oz.serving), they are still preferred over plain water because drinking plain water tends to shut down the sensation of thirst prematurely. Athletes will consume more fluids if the drink is flavored.

The sodium content of some popular sports drinks (per 8-oz serving) are listed below:

In general, most sports drinks don't provide much sodium.  By comparison, PediaLyte contains 248 mg sodium and Red Bull contains 207 mg sodium per 8-oz serving.

NOTE: Potassium supplements, however, should NEVER be used except under the supervision of a physician. There is a reason why potassium supplements are only available via a prescription and that's because too much potassium in the body can cause fatal cardiac arrhythmias. The amount of potassium lost via sweat does not differ between trained vs. untrained subjects (Wilmore JH, et al. 1999).

RECOMMENDATIONS FROM USA TRACK & FIELD

Prior to a distance race, or other type of prolonged physical exertion in the heat, the following guidelines are recommended by USA Track & Field:

• for several days prior to race day: add salt to your foods (unless you have a medical condition like high blood pressure)
• 2 hours prior to race time: drink 16-oz
• 1 hour prior to race time: drink another 16-oz
• during the race: drink no more than 8-10 oz every 15-20 minutes
• last half of the race: consume salty pretzels

HEAT-RELATED ILLNESS

A related concept to be discussed here is the issue of heat-illness (heat exhaustion, heat stroke). It turns out that heat-related illness is more likely in shorter races, such as a 10-K, as opposed to marathons. Why? Because elite racers maintain a higher per-mile pace during the shorter race, and thereby generate higher levels of body heat. Heavier athletes are also at higher risk since a larger runner will generate more heat per mile. The use of caffeine with ephedra supplements is also a risk factor for heat illness. Caffeine increases thermogenesis while ephedra causes vasoconstriction, thereby interfering with the body's ability to radiate heat. Finally, weather conditions must be considered since higher temperature and humidity will put all athletes at risk.

QUESTIONS

Q: Should athletes avoid caffeine and alcohol?

ANSWER: If you don't drink caffeine-containing beverages regularly, then it's best to avoid them before, during, or after a lengthy race or event. While caffeine is a known diuretic, its diuretic effects are relatively mild. Also, it seems as though the diuretic effects of caffeine diminish with daily consumption. During exercise, it appears that the kidney can easily reverse the diuretic effect of caffeine once exercise begins (Graham TE, et al. 1998; Wemple RD, et al. 1997).

While caffeine has been shown to enhance athletic performance, alcohol is clearly detrimental. Alcohol is a stronger diuretic than caffeine. In addition to the fact that alcohol is a banned substance, it has negative effects on performance and thus should be avoided prior to, and during, competition.

Q: Can glycerol help me stay hydrated?

ANSWER: Glycerol has been investigated as an agent for promoting a state of "hyper-hydration". The theory is that glycerol helps to "load" the circulatory system with an abnormally high amount of free water, and, also, reduces the rate at which the kidneys eliminate this excess. This, then, would help the athlete stay hydrated longer.

Studies have been conducted on glycerol in athletes for over 15 years. Most of these studies do show that it lives up to its promise: Performance was enhanced during cycling (Anderson MJ, et al. 2001; Montner P, et al. 1996) and in triathletes (Coutts A, et al. 2002). The dose was generally 1.0 - 1.2 g/kg body weight and was consumed roughly 2 hours prior to exercise testing.

FOR MORE INFORMATION

The following resources provide additional information on proper hydration for endurance athletes:

• USA Track & Field (www.usatf.org) issued new hydration guidelines for athletes in April 2003. Their web site contains several papers on the topic of fluid replacement during exercise.

• Sports medicine professionals may wish to look at the web site for the Association of International Marathons and Road Races (AIMS) (www.aims-association.org). Dr. Timothy Noakes has written guidelines for evaluating and treating athletes with heatstroke.

• The Gatorade Sports Science Institute publication #88 (2003, vol. 16, no. 1) titled Hyponatremia in Athletes is a comprehensive summary, though difficult reading for the average weekend warrior. Their web site (www.gssiweb.com) contains a lot of useful information on this topic.

Chris Carmichael's newest book Food For Fitness - Eat Right to Train Right, published in 2004, has a good discussion of fluid requirements in athletes. The book also summarizes sports drinks and energy drinks. Carmichael is, for those who don't know, Lance Armstrong's coach and a former Olympic cyclist himself. (See Book Reviews for a more detailed summary of this book.)

The latest (2005) Dietary Recommendations for Americans contains guidelines for sodium and water intake. However, these limits don't apply to marathon runners and other endurance athletes. For a thorough discussion of this issue, see our related story "Why Sports Nutrition Is Different".

Readers may also be interested in these stories:

• Fluid Requirements for Healthy Adults
• Sports Drinks and Recovery Drinks

PERSONALIZED FITNESS and WELLNESS COUNSELING

If you'd like some help designing your personal fitness program, or have questions regarding exercise, or simply want some regular motivation, sign-up for our Fitness Counseling Service. It may be the best investment in your health that you'll ever make.

REFERENCES

Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med 2005;352:1550-1556. (Abstract)

Anderson MJ, Cotter JD, Garnham AP, et al. Effect of glycerol-induced hyperhydration on thermoregulation and metabolism during exercise in heat. Int J Sport Nutr Exerc Metab 2001;11:315-333. (Abstract)

Ayus JC, Varon J, Arieff AI. Hyponatremia, cerebral edema, and noncardiogenic pulmonary edema in marathon runners. Ann Intern Med 2000;132:711-714. (Abstract)

Convertino VA, Armstrong LE, Coyle EF, et al. Exercise and fluid replacement. Med Sci Sports Exerc 1996;28:i-vii. (Abstract)

Coutts A, Reaburn P, Mummery K, et al. The effect of glycerol hyperhydration on olympic distance triathlon performance in high ambient temperatures. Int J Sport Nutr Exerc Metab 2002;12:105-119. (Abstract)

Davis DP, Videen JS, Marino A, et al. Exercise-associated hyponatremia in marathon runners: a two-year experience. J Emerg Med 2001;21:47-57. (Abstract)

Eichner ER. Exertional hyponatremia: why so many women? Sports Med Digest 2002;24:54-56. (no abstract)

Gardner JW. Death by water intoxication. Mil Med 2002;5:432-434. (Abstract)

Graham TE, Hibbert E, Sathasivam P. Metabolic and exercise endurance effects of coffee and caffeine ingestion. J Appl Physiol 1998;85:883-889. (Abstract)

Mamula PW. Dehydration risk during summer youth sports camps. Phys Sportsmed 2004;32:15. (no abstract)

Montner P, Stark DM, Reidesel ML, et al. Pre-exercise glycerol hydration improves cycling endurance time. Int J Sports Med 1996;17:27-33. (Abstract)

Rico-Sanz J, Frontera WR, Rivera MA, et al. Effects of hyperhydration on total body water, temperature regulation and performance of elite young soccer players in a warm climate. Int J Sports Med 1996;17:85-91. (Abstract)

Wemple RD, Lamb DR, McKeever KH. Caffeine vs. caffeine-free sports drink: Effects on urine production at rest and during prolonged exercise. Int J Sports Med 1997;18:40-46. (Abstract)

Wilmore JH, Costill DL. Physiology of Sport and Exercise 2nd ed., p. 323, Human Kinetics Publishers, Champaign, IL, 1999.

ABOUT THE AUTHOR

Stan Reents, PharmD, is a former healthcare professional. He holds Personal Trainer and Lifestyle Counselor certifications from the American Council on Exercise and has been certified as a tennis coach by USTA. He is the author of Sport and Exercise Pharmacology (published by Human Kinetics).  He can be reached at: Editor@athleteinme.com.