Get in the know about: Dehydration

Dehydration impairs athletic performance whenever body fluid level falls below 98% of normal. The primary cause of dehydration is sweat loss, an essential body process which facilitates the release of body heat into the environment. When athletes don't replace what they lose in sweat, the physiological function of the body's heat management system is compromised, placing both the athletes' performance and physical well-being at risk.

Exercise Intensity:

Since heat is a byproduct of muscular activity, when an athlete's exercise intensity increases, so does internal body temperature. The athlete's body then has to regulate that temperature by sweating more, meaning that the harder athletes work, the more sweat they will lose.

Environment:

Elevated environmental temperatures activate the sweat mechanism and cause sweat rates to increase. The humidity of an environment also plays an important role by restricting the evaporation of sweat, which in turn restricts heat loss from the body.

Clothing & Equipment:

Minimal, loose-fitting clothing helps promote heat loss by exposing sweat-laden skin to the air, allowing the sweat to evaporate more easily. Heavy clothing and equipment (e.g., football pads and helmet) can trap heat and sweat against the skin, forcing the body to produce more sweat to cool itself.

Genetics:

Heredity assures that every athlete sweats differently. Those athletes with a sensitive sweat mechanism experience a more severe response to changes in internal body temperature, which predisposes them to higher sweat rates,and higher risks of dehydration. At the other extreme, some athletes are very efficient sweaters, producing only enough sweat to cool their bodies effectively.

Fitness & Acclimatization:

An athlete who is more fit will sweat sooner,and sweat more than an athlete who is less fit.The same is true for athletes who are accustomed (acclimatized) to exercise in the heat.

The Negative Impact on Performance:

Cardiovascular Response:

Dehydration strains the cardiovascular system by reducing blood volume. For every liter of fluid lost during prolonged exercise, body temperature rises by 0.3'C, heart rate elevates by about eight beats per minute, and cardiac output (the volume of blood pumped by the heart per minute) declines by 1 liter/min.

Dehydration's Warning Signs:

Sweat loss reduces blood volume and increases the concentration of sodium in the blood. This stresses the cardiovascular system and contributes to a faster increase in body temperature. Dehydration results in a number of symptoms that are important to remember.

*NOTICEABLE THIRST
*MUSCLE CRAMPS
*WEAKNESS
*DECREASED PERFORMANCE
*DIFFICULTY PAYING ATTENTION
*HEADACHE
*NAUSEA
*FATIGUE
*LIGHTHEADED FEELING OR DIZZINESS
*DECREASED URINE OUTPUT

Don"t wait for the warning signs!
Dehydration's warning signs only occur when the body is already dehydrated. An athlete's best bet for peak performance is to drink to minimize weight loss during exercise but avoid over-drinking.

How often do you need to rehydrate: Water may not be enough. You will need electrolyte replacement as well. What kind? More on that to come.

How much water do you need to maintain a normal state of hydration at baseline (without your daily period of exercise)? For a 70 kilogram adult, about 2500 to 3000 cc per day. This is about 4% of your body weight or in terms of energy expenditure, about 1 cc required for each Calorie of energy expenditure per day. If your diet is well balanced, about 1000 cc (4 cups) will come from fruits, vegetables, and other foods eaten. Another 1 cup will be from the metabolism of carbohydrates, and the balance, about 7 cups, should be from fluid you drink.

And if you exercise for an hour or two, add in replacement for losses from sweat and respiration. Under normal environmental circumstances, you will lose 1 - 2 liters of sweat per hour, and in the heat this can be as high as 4 - 6 liters per hour. And respiratory fluid losses are not insignificant. Up to 60% of overall fluid loss can be via the lungs - which means that even swimmers can get dehydrated.

Under normal conditions, you should be drinking a minimum of 4 to 5 ounces of fluid every 15 minutes and should aim for 1 to 2 standard water bottles per hour. When extreme conditions of heat and humidity are anticipated, the following strategy may be of additional benefit:

" drink 20 oz of cool water 2 hours before exercise
" 8 to 16 oz 30 minutes before
" and then 4 to 8 oz every 15 minutes on the bike

If you want a simple measure of the effectiveness of your personal hydration program, weigh yourself before and after a long rides (without clothes to avoid inaccurate weights from sweat soaked clothing). A standard water bottle (20 ounces) weighs about 1 1/4 pounds - or a pound of weight equals 16 ounces (1 pint;2 cups) of fluid; a quart (4 cups) is 2 pounds. This will enable you to tailor YOUR OWN replacement program.

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