By Peter Pfitzinger | DistanceCoach.com
Warm weather 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.
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.