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Drink Up!

The Science of Hydration

Stover, Beth M.S., CSCS; Murray, Bob Ph.D., FACSM

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ACSM's Health & Fitness Journal: March 2007 - Volume 11 - Issue 2 - p 7-12
doi: 10.1249/01.FIT.0000262486.81937.4b
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Proper hydration is essential for maintaining optimal health. By maintaining body fluid balance, some researchers believe that the risks of certain diseases, infections, kidney stones, and possibly even some cancers (1) can be reduced. The Food and Nutrition Board of the U.S. Institute of Medicine has identified the daily, average, total fluid intake for sedentary male and female adults to be 3.7 L and 2.7 L, respectively. These fluid volumes, if consumed only by drinking, are the equivalent of 16 cups (for men) and 11 cups (for women). However, as these experts point out, there is a wide range of daily fluid intakes required for normal hydration, and the amount needed increases considerably for anyone who is physically active and/or living in a warm environment (1).

Research has shown that the average sedentary adult typically has little problem keeping up with daily fluid needs (1). However, you and your clients are not average sedentary adults. Your involvement in regular physical activity dramatically increases your fluid needs, and that is why you have to be keenly aware of your fluid replacement habits. Although drinking is a simple task, there is a science to optimal hydration. This science often gets confused, sometimes even lost, in the constant information shuffle of scientific and lay publications. The purpose of this article is to help make sense of the hydration messages by relying on current science to separate fact from fiction.

Below are statements or questions that are often brought up by clients or in the media. The responses reflect the current research that has been published in scientific journals or textbooks.

Importance of Hydration

Client: I never drink during exercise, and I feel fine.

Fact: If sweat loss is slight (e.g., less than 16 oz; ~500 mL; for reference 1 oz = approximately 30 mL), dehydration may not be an issue, but at higher sweat losses, dehydration has unavoidable and negative physiological consequences. For example, dehydration reduces blood volume, makes blood more viscous (thicker), increases heart rate, and makes it more difficult for the body to lose heat. In addition, exercise will feel more difficult, causing clients to spontaneously decrease exercise intensity and/or end exercise sessions early. The plain truth is that proper hydration during exercise enables people to work harder and longer, helping them get the most out of each workout (2).


Client: I rarely drink during my workouts, but I am careful to rehydrate afterward. Is that good enough?

Fact: Rehydrating after a workout is important if you fail to replace your fluid losses during exercise. However, it is always better to hydrate during a workout rather than afterward. By keeping up with your fluid losses, you are better able to maintain your exercise intensity as well as prevent the negative physiological consequences of dehydration.

Client: I heard that hyponatremia (from overdrinking) is a bigger threat than dehydration.

Fact: The truth is that both conditions are potentially life threatening; dehydration increases the risk of heat illness (which can lead to heatstroke), and hyponatremia (water intoxication) can cause severe central nervous system dysfunction. In fact, both conditions can result in death in severe cases. However, for most exercisers, heat illness is far more common than hyponatremia. Research conducted by the U.S. Army found that heat illness occurs with much greater frequency (>20 times) than hyponatremia during physical activity (3). Keep in mind that both heat illness and hyponatremia are not common events, and the risks of both can be drastically reduced by drinking the right volumes and types of fluids during exercise.

Hyponatremia occurs when the sodium concentration of the blood is markedly reduced, usually by zealous overdrinking of plain water. Moderate decreases in blood sodium can result in headache and nausea. In more severe cases, disorientation, confusion, seizures, lung congestion, coma, and death can occur because hyponatremia causes the brain to quickly swell (4). Hyponatremia is typically caused by excessive fluid intake. The loss of sodium in sweat can make things worse, especially during endurance exercise. The simple fact of the matter is that humans are capable of drinking at faster rates than the kidneys can process the excess fluid. At rest, the maximum capacity of the kidneys to excrete urine is about 1,000 mL (~32 oz) per hour only; during exercise, this value can decrease by 60% (5) because kidney function is reduced naturally by exercise.

It is wise to advise clients to monitor their body weight during exercise. The goal is to weigh the same or slightly less after exercise as he/she did before beginning the workout. A body weight reduction of 2% or more can impair performance and result in the negative physiological consequences described previously. For example, a 130-lb client should weigh no less than 127.4 lbs after a workout to keep dehydration less than 2% of body weight. However, a marked increase in body weight during exercise is a clear sign of overdrinking, which can lead to hyponatremia, and should be avoided.

Fluid Intake Guidelines

Client: I read that I should drink only when I am thirsty.

Fact: For sedentary individuals, thirst is what maintains fluid balance. But for people who sweat on a regular basis, thirst is less precise. Here is the physiology: body fluid needs are sensed in the hypothalamus in response to changes in the blood. Whenever there is a sufficient increase in the saltiness of the blood (as indexed by an increased plasma osmolality) and a decrease in the volume of the blood, two changes that accompany dehydration, a cascade of events occurs that culminates in the perception of thirst. When adequate fluid and sodium intake restores blood osmolality and volume to normal, thirst is slaked. The challenge during exercise is that by the time we are aware of thirst, we are already 1% to 2% dehydrated, and dehydration will worsen if we continue to rely on thirst, and our capacity to exercise will be reduced (6).

Client: I like to lose weight during my workouts because it means I am closer to achieving my weight-loss goals.

Fact: Weight loss that occurs during exercise is merely water lost in sweat. Weight lost during a workout is not fat loss, it is dehydration. Body weight should be monitored before and after exercise as a means to gauge hydration status. The goal of fluid intake is to drink enough to minimize weight loss (and thereby maintain good hydration) but not to overdrink.

Client: I try to drink as much as I can before my workout and then top that off by drinking every 10 minutes while I am exercising.

Fact: That might be a good idea if you are a heavy sweater. But for most people, a more modest fluid-replacement scheme will do the trick. Fluid intake during exercise is not a one-size-fits-all proposition because everyone needs different amounts, depending on their sweat rates. Guidelines published by various professional organizations recommend a range of appropriate consumption values. These ranges have been determined by using average sweat rates for various activities. For example, some recommend consuming 400 to 600 mL (14 to 20 oz) 2 hours before exercise, 150 to 350 mL (5 to 12 oz) every 15 to 20 minutes during exercise (7), and 600 to 720 mL (20 to 24 oz) of fluid for every remaining pound of weight lost (8). Of course, if you do a good job of replacing your fluid losses during exercise, you won't have to worry about fluid intake after exercise.

Most people lose between 500 to 1,500 mL (17 to 50 oz) of sweat during an hour of vigorous activity. It is likely that the amount of fluids your clients should consume during an hour-long workout will fall within this range. Sweat losses will differ based on the exercise intensity, duration of exercise, and environmental conditions. For example, more intense workouts, longer workouts, and workouts performed in warm and/or humid conditions will result in greater sweat losses. Differences in sweat loss also can be seen with different types of exercise. We have measured the sweat rates of exercisers in a fitness club setting, engaging in various types of exercise for 45 to 90 minutes; their average hourly sweat rates are listed in the Table.

Average Sweat Rates for Various Activities

Before a Workout

Starting a workout well hydrated is a critical first step in staying well hydrated during the workout. Our fitness club research showed that 46% of people arriving for their workouts were not optimally hydrated. A modest amount of fluid (14 to 20 oz) ingested an hour or two before exercise is usually enough to assure good hydration. Monitoring body weight, thirst, and urine color first thing in the morning is a simple way to help gauge hydration status (9).

If the answer to more than one of these questions is "yes," dehydration is likely:

  • "Am I thirsty?"
  • "Is my urine dark yellow?"
  • "Is my body weight noticeably lower (e.g., a pound or more) than it was yesterday morning?"

During a Workout

The amount of fluid to be consumed during exercise depends on sweat loss. A person who sweats lightly (i.e., less than 600 mL/h; 20 oz/h) can stay well hydrated by drinking relatively little, whereas a heavy sweater (i.e., sweats more than 1.5 L/h) will need substantially more. Sweat rate is highly individualized and will vary greatly, even for the same person, based on the type of activity, the exercise intensity, clothing, fitness level, hydration status, acclimation to the heat, and environmental conditions.


Clients should be educated to pay close attention to their sweat losses. If you desire more information, you can calculate hourly sweat rate by weighing before and after exercise (nude body weights are best) and then adding in the amount of fluid consumed during the exercise period.


During exercise, your clients should drink enough water or sports drink to minimize weight loss. The amount should be divided into 10- to 20-minute intervals. In the example given, Anne should drink roughly 8 oz every 20 minutes to remain well hydrated.

After a Workout

Drinking after exercise is important to replace fluid losses for those who cannot ingest enough fluid during exercise. If you weigh less after exercise, you should consume 20 to 24 oz per pound of remaining weight deficit (7).

For example, Alvin weighed 198 lbs before his workout. He did a kickboxing class and did not drink anything during the workout. When he weighed himself after the class, he was 196 lbs; 2 lbs (32 oz) less than when he started. To regain fluid balance, Alvin would have to drink 40 to 48 oz. The additional fluid is required to compensate for obligatory urine production (the volume urine naturally produced to rid the body of waste).

Type of Fluid

Client: I prefer water during my workout because sports drinks have calories and sodium, and I am trying to lose weight.

Fact: For many exercisers, especially those who do not lose a lot of sweat, water can be an effective fluid replacement beverage. But for heavier sweaters and for clients who want to get the most out of every workout, a properly formulated sports drink can provide benefits that plain water cannot. The carbohydrates and electrolytes in a sports drink provide functional benefits that aid in hydration, maintain exercise capacity, and reduce the perceived effort of the activity.

You can guide your clients to the beverage that is right for them by understanding the pros and cons of the beverage options that are available to them.


Water is usually readily available (sometimes even free) and can help replace sweat losses, provided enough is consumed. However, research has shown that when provided with plain water, most exercisers only replace a fraction of their fluid losses because water turns off thirst too soon (10).

Fitness Waters

Fitness waters are typically lightly flavored waters with few calories (e.g., often <25 Kcal/serving) and can help encourage hydration by stimulating drinking. Research indicates that exercisers will do a better job at replacing their fluid losses when provided with a flavored beverage (10). However, not all fitness waters are low in calories, so instruct your clients to read the labels. Most fitness waters also contain vitamins. Although there is no role for these vitamins in aiding hydration, they do help support overall health and can help supplement what your client might be lacking in his/her diet.

Sports Drinks

For clients who want to get the most out of every workout, sports drinks are the most efficacious choice. Under the right circumstances, the carbohydrates and electrolytes in sports drinks provide important functional benefits. On those occasions when your clients are sweating a lot and working intensely, consuming a sports drink will benefit their exercise capacity in ways that plain water and fitness waters cannot. These benefits are highlighted below:

  1. Flavor. The flavor in a sports drink helps to stimulate drinking. Studies have shown that exercisers replace their fluid losses best when drinking a carbohydrate-electrolyte solution (i.e., sports drink) during exercise (10). To help stimulate drinking behavior, the flavor needs to be appealing to your client. Encourage your clients to try a variety of flavors to find ones they like best.
  2. Carbohydrate concentration and type. The carbohydrates in a sports drink are important in providing energy to the working muscles. Research shows that the body can only oxidize, on average, about 60 to 90 g of exogenous (ingested) carbohydrate per hour, so it makes little sense to ingest more. In addition, when a beverage contains too much carbohydrate, gastric emptying and fluid absorption are slowed down, leading to stomachaches and nausea (10). For these reasons, properly formulated sports drinks contain approximately only half the carbohydrate of fruit juices and regular soft drinks. Look for a sports drink containing 14 to 17 g of carbohydrate per 8-oz (236-mL) serving.
  3. Electrolyte concentration. Sodium is the electrolyte lost in the greatest quantity in sweat. The sodium in a sports drink helps replace what is lost in sweat and helps to maintain the osmotic drive to drink (10). In addition, the sodium in a sports drink helps the body retain more of the fluid that is consumed, allowing for better hydration. If your client is losing large amounts of sweat (i.e., is a heavy sweater and/or is working out for long durations), he/she should consider a sports drink that contains greater amounts of sodium than the typical sports drink (i.e., more than 100 mg/8-oz serving) and other electrolytes, such as calcium and magnesium, to replace his/her losses more fully.

Some sports drinks contain other ingredients (such as protein, caffeine, glucosamine, etc.). To date, there is no strong scientific evidence that supports an added benefit of including any additional ingredients to a sports drink beyond carbohydrate and electrolytes.

Soft Drinks, Fruit Juices, Beer, and Other Alcoholic Beverages

These beverages are fine to drink during the day but should be restricted or avoided during the active occasion or any time when rapid and effective hydration is important. Too much carbohydrate slows down absorption, and alcohol and caffeine can promote urine loss (diuresis) and are not conducive to optimizing hydration.


The simple fact is that drinking the right volume and type of fluids is key to maintaining optimal hydration, especially during exercise. Here are three tips to help your clients improve their hydration habits:

  1. Everyone's fluid needs are different. Know your sweat rate to help you consume the right volume of fluids during exercise.
  2. Not all beverages are created equal when it comes to hydration. Help your clients choose the right beverage (using the hydration continuum in the Figure) to match their hydration needs with their workout goals.
  3. Monitor your hydration status. The goal is to have urine that is pale like lemonade not dark like apple juice (need to drink more) or clear like water (should back off on fluid intake).
Hydration continuum for the active occasion.

Condensed Version and Bottom Line

Hydration needs vary greatly among individuals. It is important to understand the proper volume and types of fluids to ingest to help clients get the most out of their workouts.


1. Food and Nutrition Board. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington: National Academies Press, 2004, p. 73-185.
2. Sawka, M.N., and K.B. Pandolf. Effects of body water loss on physiological function and exercise performance. In: Perspectives in Exercise Science and Sports Medicine: Fluid Homeostasis During Exercise. Lamb, D.R., and C.V. Gisolfi (Editors), Indianapolis: Benchmark Press, 1990, p. 1-38.
3. Carter, R., S.N. Cheuvront, J.O. Williams, et al. Epidemiology of hospitalizations and deaths from heat illness in soldiers. Medicine & Science in Sports & Exercise® 37:1338-1344, 2005.
4. Montain, S.J., M.N. Sawka, and C.B. Wenger. Hyponatremia associated with exercise: risk factors and pathogenesis. Exercise and Sport Sciences Reviews 29:113-117, 2001.
5. Guyton, A.C., and J.E. Hall. Textbook of Medical Physiology. Philadelphia: W.B. Saunders, 2000.
6. Hubbard, R.W., P.C. Szlyk, and L.E. Armstrong. Influence of thirst and fluid palatability on fluid ingestion during exercise. In: Perspectives in Exercise Science and Sports Medicine: Fluid Homeostasis During Exercise. Lamb, D.R., and C.V. Gisolfi (Editors), Indianapolis: Benchmark Press, 1990, p. 39-95.
7. Sawka, M.N., L.M. Burke, E.R. Eichner, R.J. Maughan, S.J. Montain, and N.S. Stachenfeld. Exercise and Fluid Replacement. Medicine & Science in Sports & Exercise® 39(2):377-390, 2007.
8. Casa, D.J., P.M. Clarkson, and W.O. Roberts. American College of Sports Medicine roundtable on hydration and physical activity: consensus statements. Current Sports Medicine Reports 4:115-127, 2005.
9. Cheuvront, S.N., and M.N. Sawka. Hydration assessment of athletes. Sports Science Exchange 18:1-5, 2005.
10. Maughan, R.J., and R. Murray. Sports Drinks. Basic Science and Practical Aspects. Boca Raton: CRC Press, 2001.

Hydration Needs; Sports Drinks; Water; Sweat Rate; Drinking Guidelines

© 2007 American College of Sports Medicine