The Dangers of Dehydration

Summer is here, along with the twin menaces of heat and humidity. Running in the heat can quickly lead to dehydration, which ranks up there with dobermans among runners' worst enemies. Dehydration hurts your performance, and slows your ability to recover for the next workout. Continuing to run when dehydrated can lead to heat stroke and death.

To better understand the dangers of dehydration, let's take a look at what happens in the body when you run on a warm day. First, your body automatically sends more blood to the skin for evaporative cooling, leaving less oxygen-rich blood going to your leg muscles. 

Second, the warmer it is, the more you sweat, and the more your blood volume decreases. Less blood returns to your heart, so it pumps less blood per contraction. Your heart rate must increase, therefore, to pump the same amount of blood. The result is that you cannot maintain as fast a pace on a warm day.

Worst of all, dehydration tends to catch you unawares. If you replace a little less fluid than you lose each day, after a few days you will run poorly but may not know why. Exercise physiologist and marathoner Larry Armstrong, Ph.D., induced dehydration equal to 2% of body weight in runners and observed a 6% decrease in speed over 5K or 10K. That's a 3% decline in performance for each 1% decrease in bodyweight due to dehydration.

It is not unusual to lose 3-4 pounds of water per hour when running on a warm day. At this rate, after 2 hours a 150 pound runner would lose 6-8 pounds, representing a 4-5% loss in bodyweight and a 10-15% decrement in performance. That's about an extra 1 minute per mile. Losing more than 4-5% of your bodyweight, however, could do even more serious damage to your body.


Preventing Dehydration

If you are running in temperatures over 70 degrees, or over 60 degrees if the humidity is high, then staying properly hydrated can become a challenge. You need a strategy for preventing dehydration during today's run, and for minimizing the cumulative effects of hot weather running.

Before workouts and races, concentrate on drinking enough fluids to ensure you are fully hydrated. Do not just rely on your thirst-your body's thirst mechanism is imperfect. Also, you cannot just sit down and drink a half gallon of fluid at one sitting and assume you are fully hydrated. It takes time for your body tissues to absorb fluid. To top off the tank, the American College of Sports Medicine recommends drinking about half a liter of fluid (just over 1 pint) about 2 hours before exercise to help ensure adequate hydration and to allow time to excrete excess water. Drinks containing sodium are more readily retained by the body.

How much you should drink during your runs depends on the heat and humidity, and how far you are running. The maximum amount you should drink is the amount that can empty from your stomach. Research has shown that most runners' stomachs can only empty about 6-7 ounces of fluid every 15 minutes during running. If you drink more than that, the extra fluid will just slosh around in your stomach and not provide any additional benefit. You may be able to handle more or less than the average, however, so experiment with how much liquid your stomach will tolerate.

Weigh yourself before and after running and calculate how much weight you lost, then drink with the objective of bringing your weight back up to normal. Your blood and other fluids help to remove waste products and to bring nutrients to tissues for repair. Replacing lost fluids as quickly as possible after running will speed up your recovery.

What to drink

There are 2 excellent options-water and carbohydrate replacement drinks. Nothing beats water for pure hydration. The advantage of replacement drinks with approximately 4-6% carbohydrate is that they are absorbed as quickly as water, and also provide energy. The carbohydrates can help your performance during workouts or races lasting longer than 1 hour. Drinks containing the correct levels of sodium are absorbed more quickly and help to prevent dehydration. You should avoid caffeinated drinks like coffee, tea and colas because caffeine is a diuretic and actually leaves you less hydrated than before. Similarly, beer and other alcoholic beverages may help calm your nerves, but are counterproductive to hydration. If you drink caffeinated drinks or alcohol, drink extra water to balance out the dehydrating effect.

Can you take in enough fluid to prevent dehydration on a hot day?

Let's figure it out. Sweat versus intake. On a hot day you may lose 4-5 pounds of water an hour. We already estimated that your stomach can absorb about 28 ounces, which is a little bit less than 2 pounds per hour. That leaves a deficit of 2-3 pounds per hour. You can't drink enough to keep up, so the farther you run, the greater your fluid deficit will be.

Look at running in the summer as another of nature's challenges. Be flexible with your training schedule. Run at the time of day when the weather is the least taxing on the body, and be prepared to face the physiological facts. On a hot, humid day, slow down your pace from the outset rather than waiting until your body forces you to slow down. By using common sense and staying well hydrated, you can safely enjoy running through the summer.


Related articles:
Summer running
Factors that affect the heart rate during training
Running safely through the summer
Pre-cool to run fast in the heat
What additional precautions should the runner take while training under warm weather?
Eat & Drink for Distance Running Performance
Heart rate, recovery, carbohydrates

Text copyright by Pete Pfitzinger
Pete Pfitzinger is an exercise physiologist with over 20 years of coaching experience, Pete adheres to the principle that every runner is unique and that training programs must be tailored to the athlete's individual strengths and weaknesses. 

Pete Pfitzinger is co-author of two successful books:

Road Racing for Serious Runners
Road Racing for Serious Runners
Buy it here

Advanced Marathoning
Advanced Marathoning
Buy it here

This article has informational purpose and  isn't a substitute for professional advice.

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