What marathoners and ultrarunners need to know about the effects of heat and dehydration on distance-running performance and health.

ith the hot summer months rolling in fast, now is a good time to review Wee single most serious threat to a runner’s life—heat. Thousands of outdoor athletes are sidelined every year with heat illness. First, let’s have a quick look at the physiology and factors that contribute to heat illness,

and then we’ll examine the countermeasures we can take to ensure our safety in heat and humidity and to prevent performance decline.

What causes heat injury

Air temperature, air movement, humidity, exposure to the sun, lack of acclimatization, and the intensity or duration of your running session or competition can combine to cause heat illness.

Acclimatization, or our previous recent exposure to heat, is a major factor in determining how we handle heat. Through training, our thermoregulation mechanism partially adapts to heat, but because people respond very differently to heat, individual adjustments to running outdoors have to be made. Even fit runners can tolerate only a narrow range of internal core temperatures, although they can sustain a high core temperature longer than unfit people. Athletes of all abilities, from the poorly conditioned to the elite, can suffer from dehydration, heat exhaustion, and heat stroke, especially early in the spring when they are unacclimatized and push beyond their limits.

As the intensity and the length of your running sessions increase, they become major contributors to heat stress. We generate tremendous amounts of heat during exercise, and because the human body isn’t very efficient, 75 percent of our energy turns into heat. Thus, the faster and longer we run in high heat and humidity, the

worse the heat load placed on our body. This is exacerbated when we can’t disperse the heat quickly enough. So running marathons and ultras makes us prime candidates for heat problems because of their extended nature.

Generally, we can tolerate temperatures as high as 80 to 90 degrees Fahrenheit ina dry climate because we’re able to sweat as much as 2 liters per hour. In dry air, heat is carried from the body core to the skin, where evaporating sweat cools the blood before its return to the core. But our skin temperature always needs to be at least 2 degrees cooler than the core temperature to permit a cooling heat gradient. In direct sunlight, this gradient is eliminated because our skin absorbs heat faster than evaporation can cool it, thus elevating our skin temperature. Heated blood is then carried back inside the body, causing a rapid rise in core temperature.

The worst combination of factors is high heat and humidity. An air temperature of 60 degrees plus a 95 percent humidity level is more dangerous than a “dry” 85 degrees. Because of the high level of water in the humid atmosphere, little evaporation occurs, making sweat pool on your skin. The blood that has been shunted to your skin to cool by evaporation now returns to the core heated up. The hypothalamus—the body’s thermostat—detects the rapidly mounting heat and responds by dilating the skin’s blood vessels to cool it more. The heart pumps harder and faster to shunt more blood to the surface.

You now have a vicious competition for blood between the working muscles (that need more blood to keep you running but are getting less and less) and the demands of getting blood to the skin and the brain (which always needs 25 percent

Treatment of Heat Exhaustion

Do not try to compete through these symptoms. Stop! Find shade and pour water on the victim.

Seek medical help.

Raise the victim’s legs to get blood to the brain. (Raise heels eight to 12 inches.)

Keep the victim lying down. Give the victim cold water to drink for one hour. Loosen or remove clothing.

Sponge bare skin with cold water or rubbing alcohol.

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Use a fan or an air conditioner to create a draft over the victim’s body.

of heart output to function). It’s at this point that athletes can make an often-fatal mistake—instead of slowing down, they keep pushing the pace. They become more dehydrated, causing their plasma to become thicker and more viscous and making their heart pump even harder. This extra burden on the cardiovascular and thermoregulatory systems often causes people with undetected cardiac problems to collapse at this stage.

But if you continue running in this state, you’ll continue to sweat, and if you aren’t taking in adequate fluids, the demands on your circulatory system will ultimately become intolerable. You’ve now become a prime candidate for heat exhaustion. Watch for these warning signs: dizziness, profuse sweating, weakness, dehydration, parched throat, and hot, red skin.

If you ignore these warning signs and continue to push, you’re heading for heat stroke. Your performance will have deteriorated badly by now, which often pressures overcompetitive athletes to try to pick up their pace. Finally, the competition for blood to maintain running and cooling the body becomes unbearable and your circulatory system and hypothalamus shut down. Blood pressure drops. Unconsciousness. Possible coma or death. Watch for these warning signs of heat stroke: headaches, dizziness, disorientation, nausea, pale dry skin, decrease in sweating, fatigue, blurred vision, pounding head, fainting, and tingling sensation (goose bumps) on the trunk. The box below shows how to treat these symptoms.

More recently, the theory (Tucker 2008) has been proposed that when our core temperature hits 42 degrees Celsius, it triggers a switch of some sort in our brain that makes us lose our will to exercise and causes collapse (that is, heat stroke). Research shows that fatigue and collapse, symptoms of heat stroke, may occur even at lower temperature than this, around 38.5 to 39.5 degrees Celsius if the runner is dehydrated (Montain and Coyle 1992; Coyle 2004; Cheuvront and

Treatment of Heat Stroke 1. Same as 1 to 5 under “Treatment of Heat Exhuastion” on page 97. 2. Treat for shock, but do not cover the victim with blankets.

3. Take immediate measures to cool the body quickly as for steps 7, 8, and 9 in the heat exhaustion sidebar.

4. If the person is not responding to steps 7, 8, and 9 outlined in the heat exhaustion sidebar, apply cold ice packs or ice continuously, or place the victim in tub of cold water. The objective is to get the core temperature down as quickly as possible.

Haymes 2001). This is disconcerting for marathoners because it is quite commonplace for them to have core (rectal) temperatures ranging from 39.0 to 41.7 degrees Celsius at the finish of the 26.2-mile event (Maughan 2007).

How dehydration affects running performance

A mountain of research dating from as far back as World War II shows that the more sweat and weight lost during a race or competition, the more drastically your running performance declines. As you may have found based on your own experience, the rate at which you lose sweat depends on a variety of factors, including individual sweat rate (which is highly variable) and the temperature, humidity, and length and intensity of your running.

How much sweat can we lose during a race or training effort? Lots! Losses of over | liter (34 ounces) of fluid (sweat) per hour per square meter of body surface area have been recorded. On a hot, humid day, an average-sized person (110 to 165 pounds) can lose 1.6 to 2 liters of fluid, or 2.5 percent to 3.5 percent of body weight. Some research has even found fluid losses up to 1.5 to 3.5 liters/hour. Total fluid losses during marathons are reported as high as 7 liters, or 1.0 to 5.3 kilograms. It is here that acclimatization can help. If you adapt to the hot and humid climate by spending two weeks training in it, your sweat rate increases and you start sweating earlier. Your plasma volume increases to support the higher sweat rate and thus better supports cardiac output at higher sweat rates. Your electrolyte losses also decrease by 50 percent, so your body retains them longer.

How much stress does dehydration put on our bodies? A study by Coyle (2004) found that for each liter of sweat lost through dehydration, exercise heart rate increases by eight beats per minute and decreases cardiac output by | liter. Burge’s study (1993) found that a water loss of 4 percent to 5 percent of body weight impairs physical work capacity and function. An army study (Pitts 1944) during World War II found that dehydration of 3 percent to 4 percent of body weight during prolonged marching in the heat impaired performance and caused collapse. This again is disconcerting for marathoners because sweat losses from 4 percent to a staggering 10 percent of body weight have frequently been recorded at the end of the 26.2-mile event (Maughan 2007, Coyle 1992). Even exercise physiologists are puzzled as to how marathoners can finish in this state without a higher incidence of heat injury.

Subjects in Montain’s well-conducted trial (Montain and Coyle 1992) exercised for two hours, losing 4 percent of their body weight. Then, in a series of similar trials, they drank enough to replace 20 percent, 50 percent, or 80 percent of their sweat loss, resulting in dehydration of 3 percent, 2 percent, and 1 percent of body weight. The more they drank, the better they were able to maintain body weight through drinking and the less their performance declined.

A study by Craig (Craig and Cummings 1966) found that walking endurance time was reduced by 48 percent and VO,max was reduced by 22 percent with a fluid loss of 4.3 percent of body mass. Even slight dehydration hurts performance. Craig’s study also found that a reduction in body weight of 2 percent resulted in decreased endurance performance of 22 percent and a reduction in VO,max of 10 percent. A 1994 study by Walsh et al. found similar results when subjects lost 1.8 percent of body weight by cycling for 60 minutes followed by high-intensity cycling to exhaustion at 90 percent of VO,max. When the subjects were hydrated throughout the 60 minutes, they lasted 10 minutes in the high-intensity cycling but only six minutes when they were not rehydrated. No performance decrements have been found with dehydration less than 2 percent.

Another study concluded that if the temperature is above 20 degrees Celsius (68 degrees Fahrenheit) and exercise is longer than 90 minutes, performance is impaired (Cheuvront and Haymes 2001). If the temperature is 31 degrees Celsius (87.8 degrees Fahrenheit) or higher, our sweating rate is much higher than at 20 degrees Celsius, so just 60 minutes of running in this heat is enough to cause your body a sweat loss of 2 percent of body weight, enough to impair performance (Below et al. 1995; Walsh et al. 1994).

DeFreitas (1984) also discovered a fascinating fact that I’ve often wondered about while running distance races. When you run in close proximity to other runners, according to his study results, you undergo three times as much heat stress as if you were running solo under the same climatic conditions. Apparently, all that energy being liberated from runners around you has a heating effect on immediate surroundings.

From this small sampling of the hundreds of research papers, it’s clear that running performance declines dramatically with dehydration of 2 percent of body weight or more (and we already know that dehydration is the prime cause for heat injury). This performance decline is caused by a rather formidable combination of deleterious effects on the thermal, cardiovascular, metabolic, and central nervous systems (Maughan 2007). We experience a drop in blood flow to the muscle, skin, and brain; a decrease in blood pressure; a concurrent decrease in oxygen delivery to the working muscles; earlier onset of anaerobic metabolism; and hyperthermia-induced fatigue, any of which will severely slow you down in a marathon or ultra.

The following chart summarizes the chain of events caused by excessive fluid loss during strenuous exercise in hot and humid conditions (Grandjean and Ruud 1994).

Rehydration: How effective are sports drinks?

The most-researched topic in sports science is probably the effects of carbohydrate solutions on prolonged performance. Almost all of the hundreds of studies

Percent Water Loss From the Body

Expected Symptoms 0% No performance decline. Optimal performance. Normal heat regulation. 1% Thirst stimulated. Heat regulation altered. Performance declines very slightly. 2% Further decrease in ability to regulate heat. Performance hindered. Increased thirst.

3% Symptoms above continue. Performance continues to decline. 4% Exercise performance declines by 20% to 30%.

5% Headache. Irritability. Spacey feeling. Fatigue.

6% Weakness. Severe loss of ability to disperse heat.

7% Collapse unless you stop running. 10% Coma likely.

11% Death likely.

investigating the effectiveness of sports drinks show that they have an ergogenic, performance-enhancing effect, allowing us to maintain a high level of performance for a longer time. A study by Millard-Stafford (1992) found that a 7 percent carbohydrate-electrolyte drink increased 5K running performance in the heat compared with an artificially sweetened placebo. A 1995 study by Tsintzas et al. concluded that a 5.5 percent carbohydrate solution improves marathon-running performance. The seven runners in this study deserve special mention (even medals) for running three marathon time trials on a treadmill, a week apart!

A1995 study by Below etal. showed the importance of preventing dehydration and providing carbohydrate during exercise. Subjects cycled for 50 minutes at 80 percent of VO,max, then ended with a sprint finish. Performance was improved by 6 percent when 80 percent of their sweat loss was replaced and by 6 percent when they drank 79 grams of carbohydrate, compared with when they drank fluids with no carbohydrate, for a total improvement in performance of 12 percent.

A 2007 study by Byars et al. found that carbohydrate feeding improves performance when exercise lasts longer than 90 minutes at 70 percent of VO,max. Desbrow et al. (2004) concluded that exercise duration needs to be longer than two hours at an intensity of 70 percent of VO,max for carbohydrate solutions to be effective.

Rehydration: What should we be drinking?

Hundreds of studies have found that carbohydrate and sodium in sports drinks have beneficial effects. Carbohydrate solutions help maintain energy levels for racing by replacing depleted muscle-glycogen stores, and sodium helps retain water, stimulates thirst, and prevents low plasma sodium. However, the problem with some commercially available sports drinks is that they are too salty (high osmolality) or too sugary for some people, causing a delay in gastric emptying and absorption, so they don’t get to where they are needed quickly enough.

The main carbohydrates used in sports energy drinks are glucose, fructose, and maltodextrin. A higher incidence of gastric upset has been reported with fructose, so most of the sports drinks tend to use glucose now. Research shows we need only 30 grams (120 calories) to 60 grams (240 calories) of carbohydrate per hour, as we can’t absorb more than this while exercising.

Preservation of blood-plasma volume is a key factor in sustaining optimal endurance exercise, especially for events lasting longer than two hours. It’s possible for elite athletes to lose as much as 4,800 milligrams of sodium per hour in extreme heat and humidity, while a sodium loss of 1,100 to 1,900 mg/hour is expected under normal exercise conditions. Fortunately, a good amount of the sodium present in our body is “recycled” from our sweat glands, so high sodium levels are not necessary in sports drinks. However, excessive fluid consumption should also be avoided because of the potential for hyponatremia (dangerously low levels of sodium in the blood), leg cramping, and even death. One way to boost plasma volume is by loading up with sodium, causing water retention. The sodium concentration in electrolyte replacement drinks ranges from 232 to 845 mg/deciliter. Most electrolyte drinks also have some potassium, calcium, and magnesium for maintenance of muscle contraction and to facilitate other metabolic processes.

Sports drinks containing carbohydrates and electrolytes have much slower emptying rates depending on their concentrations. One study (Coyle 1978) that looked at Gatorade, with a 4.5 percent sugar solution of glucose and fructose, found it took 35 percent longer to clear the GI tract compared with drinks containing 2.5 percent sugar solution and 40 percent longer than plain water. In addition, reports of stomach cramping have been noted when salt or sugar concentrations are too high. Some sports drinks with lower sodium content have even been offered as an alternative for people who cannot tolerate the higher concentrations.

Much research has been done to find the optimal combined concentration of carbohydrates and electrolytes—and it appears to be less than 8 percent. Millard-Stafford’s research (2005) found that carbohydrate-electrolyte solutions of 6 percent and 8 percent both elicited similar performance enhancements (8 percent improvement) in a 32-kilometer run. This equates to 30 to 60 grams per hour during exercise.

If you notice that sports drinks cause gastric distress to you, water may be your best option. It empties at the rate of about 800 ml/hour and has the fastest absorption rate of all fluids. However, before you ditch sports drinks completely, try watering down your favorite sports drinks to a concentration that works for you. This is often all that is needed to make it tolerable to your system. Some research indicates that the fluid should be cooled for maximal absorption.

Rehydration: How much should we be drinking?

Generally, exercise scientists agree that we need to rehydrate at a rate where we do not lose more than 2 percent of our body weight in marathons and ultramarathons. In practice, this is extremely difficult because we simply can’t ingest water quickly enough to replace our sweat losses. Our stomach can hold only 900 to 1,300 milliliters, or 30 to 44 ounces, and we can empty only about 800 milliliters per hour while exercising. Large volumes of excess water pool in our gastrointestinal tract, sloshing around and causing gastric discomfort, nausea, and vomiting. Yet research also tells us that it’s crucial for us to drink large volumes of water early in our marathons and ultras, because there is a 40- to 60-minute lag time for it to clear the gastrointestinal tract and get to where it is needed in the body’s cells and plasma. Ingesting from 400 to 800 milliliters of fluid early in marathons and ultramarathons has been shown to reduce core temperature by 0.3 degrees Celsius, lowering heart rate by eight beats per minute and raising cardiac output 1 liter/minute (Montain and Coyle 1992).

This practice is not necessary for shorter events like the 10K because it takes only an hour for the majority of runners to finish this event. This was confirmed by a 2004 study by Desbrow et al. involving a one-hour time trial where the subjects ingested carbohydrate solutions while cycling. The message here for 10K runners is to hydrate 40 to 60 minutes before your race. However, if you feel thirsty during a 10K, by all means drink—I’m not recommending abstinence from drinking even in shorter events. Heat exhaustion and heat stroke have happened in races as short as 10K if climatic conditions are wicked enough. So make sure that you are adequately hydrated (called euhydration) at the start line of your 10K.

Rehydration: How frequently should we be drinking?

It’s also important to keep hydrating at regular intervals during the race at a rate that replaces fluid loss, about 400 to 800 milliliters of fluid per hour. This averages out at roughly 5 to 7 ounces, or 250 milliliters of fluid every 15 to 20 minutes.

Palatability of sports drinks

Studies of the contents in sports drinks that make us want to drink more have looked at sweetness, saltiness, tartness, and overall palatability. A 2004 study by

Passe et al. found that carbohydrate-electrolyte sports drinks were most favored, although any fluid, including fruit juices, can be employed if they enhance drinking. Popular fruit juice flavors are grape and apple juice, when diluted by 50 percent. Of course, the flavor and dilution should be a matter of preference for the runner. You should experiment with them during long training efforts, rather than have nasty surprises during a marathon.

Postrace and posttraining rehydration

Many studies show that carbohydrates consumed immediately after and two hours after exercise enhance muscle glycogen restoration. This enhancement is most effective if the carbohydrates are ingested from fluid, as fluid-absorption rate is faster. Many studies also show that electrolyte balance is restored well, almost to preexercise levels when an electrolyte beverage is drunk immediately after exercise (Gonzalez-Alonso, Heaps, and Coyle 1992).

Robert Murray, one of the foremost researchers in this area, summarizes the role of sports drinks nicely with, “The consumption of ~1 gram of carbohydrate per kilogram of body weight per hour appears sufficient to improve performance in prolonged exercise. Research also indicates that ~450 milligrams of sodium per hour is the minimum amount required to maintain plasma volume and slow the decline in plasma-sodium concentration that can accompany prolonged exercise in some runners. Adequate carbohydrate and electrolyte intake can be achieved by consuming a well-formulated sports drink at regular intervals during exercise, in volumes designed to minimize dehydration. For marathon runners, this could range from ~400 milliliters to >1.5 liter per hour, depending upon individual sweating rates” (Murray 2007).

Now that we’ve had a look at the causes of heat injury and what the research shows about rehydrating, here is some practical take-home advice that you can implement immediately to plan ahead for the hot summer months.

Tips for surviving high heat and humidity and maintaining performance intensity

Training advice

¢ Drink lots of cold water before, during, and after your training efforts. Select running routes that have water fountains along the way. Drink 200 to 500 milliliters 15 to 20 minutes before starting, and drink at least one cup of water every 20 minutes during your activity. Although carrying a water bottle weighs you down at the beginning, it weighs nothing at the end of your run.

¢ Train to acclimatize to the heat you’ll be racing in. Start by short, slow training efforts in the heat and humidity, in mornings and afternoons. Find courses that are mostly in the shade. Shady forest trails are perfect for this. Gradually increase the length and intensity over a week or two, and try running in the hotter times of the day when you feel acclimated.

¢ On hot, muggy days, don’t try to stick to your planned training distance and course. Be prepared to cut back if conditions are dangerous, or try to run in cooler shaded areas. On some days, you may have to shorten your run or completely bag it—so work out in your local fitness club. There is a treadmill waiting for you that will give you a great workout.

¢ Indoor cross-training in the summer may be a good alternative in states where the days can get so hot that you just want to crawl under a rock, or run very early in the morning.

¢ Jam against the idea of wearing extra layers of clothing to simulate higher temperatures and promote sweating. I’ve seen people running in plastic garbage bags get into heat-exhaustion territory in a very short time. It may seem like a good way to speed up the acclimatization process, but the reality is that it may cause heat injury—the very thing you’re trying to avoid.

¢ There is nothing macho or intelligent about the archaic practice of shunning water on your training efforts, thinking it will toughen you up. This practice is highly detrimental to your performance and often leads to heat injury.

¢ Posttraining or postrace rehydration: Weigh yourself before and after your race or training effort. Make sure you drink that weight back on within an hour or two of finishing. Choose carbohydrate-rich fluids such as juices that replace both water losses and muscle glycogen. Juices contain more carbohydrates than sports drinks, so drink your fill of your favorite fruit juices.

Racing advice ¢ Know your current state of fitness and be properly conditioned for running. If you aren’t, don’t compete or go out in the blazing sun.

¢ On hot, humid days, don’t push your pace beyond your current level of fitness. Above all, do not be tempted to go out fast from the start. Early dehydration invites disaster. Realize that your performance in heat and humidity will be substandard, and accept this.

¢ Never drink sports drinks in a race without having experimented with them previously on your longer training runs.

¢ Stop for a minute at drink stations on your marathon or ultra. This brief break, combined with drinking cold fluids and taking a sponge bath, provides great relief in the middle of the race. The cooling effect more than compensates in terms of recovery time for the 60 seconds you have taken to do this. Use water sprayers set up for competitors. I’ve been in marathons where the runners diverted through someone’s garden sprayer to cool off.

¢ Keep your body wet. An old trick used by seasoned marathoners is to put ice under their caps. This technique lowers body temperature.

¢ Ifyou start feeling any of the symptoms of heat exhaustion or heat stroke listed above, stop immediately, get in a cool place, and consume cool fluids. A big problem here is that people suffering from these symptoms may not be thinking clearly and thus not be making smart decisions.

General advice

¢ For good acclimatization, exercise at least three days a week in conditions similar to those you’ll participate in. You’ll need to allow 10 to 14 days of slowly progressive running to adjust to the heat.

¢ Males tend to handle heat less efficiently then females, as do larger people who have less cooling surface per pound of body weight than slim people.

¢ Food digestion interferes with the blood flow to the working muscles, so avoid large meals before running in the heat. Likewise, simple sugars require more fluids to help digest them, so go easy on the sweets.

¢ Wear opaque, light-colored, sun-protective clothing that breathes well and repels the sun’s rays. I’ve read that cotton is no longer considered a good material for evaporation because sweat tends to accumulate in it. Moremodern materials are supposed to provide superior wicking of moisture away from your body. I find that these newer materials make me feel hotter and end up going back to the much-maligned cotton T-shirts. Experiment and find out what you are comfortable in.

¢ Other acceptable sources of water are juice, lemonade, vegetables, soups, yogurt, milk, and juicy fruits.

¢ Electrolyte sports drinks with sodium have been found to help endurance athletes retain water in their system, so you’re well advised to try them. But watch out for the imposters that are loaded with sugar and no better for you than soft drinks. If electrolyte drinks make you feel nauseated, they’re too concentrated, so dilute them 100 percent or more. Avoid alcohol—its diuretic effect causes you to dehydrate quicker.

* You’ll be able to tell whether you are hydrating adequately by the color of your urine. Dark yellow indicates low hydration, and pale to light yellow is good. The old adage of eight glasses of fluid a day is not exact. You should ingest one-half ounce of fluid per pound of body weight if you are running in hot conditions. Eighty to 100 ounces of fluid covers almost every runner.

¢ Recognizing the symptoms and treatments of heat exhaustion and heat stroke and knowing how to treat them are half the battle in dealing with heat injury. Be familiar with these guidelines to prevent hyperthermia—it could save your life.

References

Below, P., R. Mora-Rodriguez, J. Gonzalez-Alonso, and E. Coyle. 1995. Fluid and carbohydrate ingestion independently improve performance during | hour of intense exercise. Medicine and Science in Sports and Exercise 27(2):200-210.

Burge C., et al. 1993. Rowing performance, fluid balance, and metabolic function following dehydration and rehydration. Medicine and Science in Sports and Exercise 25(2):1358-1364.

Byars, A., K. Schneider, M. Hesseltine, W. Simpson, and M. Greenwood. 2007. Sports nutrition: Comparing two sports drinks effects on aerobic performance. Applied Research in Coaching and Athletics Journal 22:226-240.

Cheuvront, S., and E. Haymes. 2001. Thermoregulation and marathon running: Biological and environmental influences. Sports Medicine 31(10):743-762.

Coyle, E. 2004. Fluid and fuel intake during exercise. Journal of Sports Sciences 22(1):39-55.

Coyle, E., et al. 1978. Gastric emptying rates for selected athletic drinks. Research Quarterly 49(2):119-124.

Coyle E., and S. Montain. 1992. Benefits of fluid replacement with carbohydrate during exercise. Medicine and Science in Sports and Exercise 24(9):324-330.

Craig F., and E. Cummings. 1966. Dehydration and muscular work. Journal of Applied Physiology 21:670.

DeFreitas, C, N. Dawson, A. Young, and W. Mackey. 1984. Microclimate and heat stress of runners in mass participation events. Journal of Applied Meteorology. 24:184-191.

Desbrow, B., S. Anderson, J. Barrett, E. Rao, and M. Hargreaves. 2004. Carbohydrateelectrolyte feedings and | hour time trial cycling performance. /nternational Journal of Sport Nutrition and Exercise Metabolism 14(5):541-549.

Gonzalez-Alonso, J., C. Heaps, and E. Coyle. 1992. Rehydration after exercise with common beverages and water. International Journal of Sports Medicine 13(5):399-406.