Fluid Intake Before Exercise

It is critical for athletes to be in a state of optimal hydration before the initiation of exercise or competition. All the evidence suggests that even a minor level of under-hydration (as little as 2 percent of body weight) can cause a measurable difference in endurance and performance, and the greater the underhydration, the greater the negative impact.^47,48Furthermore, it can take 24 hours or longer to bring a dehydrated athlete back to a well-hydrated state. Therefore, waiting until just before practice or competition to bring an athlete to a well-hydrated state or simply failing to take any steps to make certain the athlete is in an optimally hydrated state will doom that athlete to having a poor practice or competition outcome.

In some sports, athletes try to achieve a particular "look" or try to make a particular weight. The classic body profile in rhythmic gymnastics is long, graceful lines with, essentially, no secondary sexual characteristics. It is common for rhythmic gymnasts to restrict water intake before a competition because they think it will help give them the desired look. Wrestlers have a well-established regimen for fluid restriction to achieve a particular weight class. They then have about 24 hours to rehydrate themselves before the competition. Besides the inherent health dangers (there are well-documented deaths associated with this strategy), it is unlikely that dehydrated wrestlers would be able to adequately rehydrate themselves in just 24 hours. Therefore, performance is likely to be affected.

On the other side of the continuum, some athletes try to superhydrate with fluid before exercise. This is typically a strategy of long-distance runners, whose water loss during competition is likely to be greater than their ability to replace it. The runner with the best hydration state near the end of the competition clearly has a major advantage over less-hydrated competitors. When athletes constantly superhydrate they may develop a greater blood (plasma) volume, with resultant lower core temperatures and heart rates during activity, suggesting the potential for improved endurance and performance.^49,50 Consumption of large fluid volumes is also associated with frequent urination, but this may be somewhat mediated by consumption of sodium-containing fluids.⁵¹ In addition, superhydration is associated with higher sweat rates and a lower heart rate during exercise.⁵²

Glycerol is a simple three-carbon lipid that is metabolized like a carbohydrate. It is occasionally used by endurance athletes to aid superhydration because it acts as a humectant (i.e., it attracts water).

Limited evidence suggests that adding glycerol at the rate of 1 gram per kilogram of body mass to preexercise fluids improves endurance performance in extremely hot and humid environments. This improvement occurs because glycerol enables a retention of more of the consumed fluids.^53,54 However, individual athletes have described the acceptability of this protocol differently. Some athletes find that superhydrating with glycerol makes them feel stiff and uncomfortable, while others are more comfortable with this sensation.⁵⁵Athletes using glycerol should make every effort to maintain electrolyte balance so as not to increase the risk of hyponatremia.^56,57 In general, athletes should follow these hydration guidelines before exercise:

Figure 3.1 Glycerol (also referred to as "glycerine".)

1. The sensation of thirst should not be relied on as an indicator of fluid need. Thirst should be considered an "emergency" sensation that occurs when the body has already lost 1.5 to 2.0 liters of water. Because the thirst sensation is likely to be delayed during exercise, waiting for thirst results in excessive water loss and a downward shift in total body water.
2. Athletes should become accustomed to consuming fluids without the thirst sensation. As a practical matter, this is made easier if athletes carry fluids with them, wherever they are and wherever they go. Fluid consumption is much more likely to occur if the fluid is readily available without the need to go looking for it, especially if the athlete doesn't feel thirsty.
3. Athletes should consume enough fluids before exercise to produce clear urine (a sign the athlete is well hydrated). Dark urine is a sign that the athlete is producing a low-volume, concentrated urine that results from the need to retain as much fluid as possible-a clear sign of underhydration.
4. Approximately 1 to 1.5 hours before exercise, the athlete should consume a large volume of fluid (up to .5 liters) within a relatively short time period to ensure adequate hydration and to improve gastric emptying. After this, athletes should sip on fluids (approximately .5 cups every 10 minutes) to maintain hydration state before exercise or competition begins. Athletes should consume fluid as frequently and in as high a volume as can be tolerated to replace water losses.
5. Athletes seeking to superhydrate should not try this technique without careful monitoring, especially if it is done using glycerol. Superhydration should never be attempted by individuals who have compromised cardiovascular systems. It is also something that shouldn't be tried for the first time just before a competition. As a practical matter, the safest way to superhydrate is to frequently consume fluids.
6. Athletes should avoid foods and drinks that may have a diuretic (water-losing) impact. For instance, caffeine and related substances commonly found in coffee, tea, chocolate, and sodas could increase the rate of urinary water excretion if consumed in large quantities. Therefore, these substances could be counterproductive in terms of optimizing hydration state before exercise.