Dehydration -- also called "hypohydration" by exercise physiologists -- can ruin your running, not just in terms of health hazards but also in terms of your running pace.
Your core body temperature during exercise in the heat can increase by 0.12 to 0.25 degrees Celsius (0.12-0.25C) for every 1% of body mass that is lost due to dehydration.
For example, suppose that you weigh 200 pounds before beginning to exercise in the heat but then lose 2 pounds (1% of body mass) due to dehydration. For reference, that 2-pound deficit is approximately equivalent to having a drinking deficit of 32 fluid ounces (4 cups) of water. Then you should expect your core body temperature to rise by up to 0.45 degrees Fahrenheit (0.45F).
As another example, suppose that you weigh 135 pounds before beginning to exercise in the heat but then lose 4.05 pounds (3% of body mass) due to dehydration. Then you should expect your core body temperature to rise by up to 1.35F.
Your body reacts to a rise in core body temperature by increasing your skin blood flow and your sweat rate. But dehydration leads to a reduction in the overall water volume in your body, which leads to a reduction in central blood volume, which results in a reduction in skin blood flow.
So your body responds to a dehydration-driven rise in core body temperature by raising your heart rate -- by some 3 to 5 beats per minute for every 1% of body mass that is lost due to dehydration while exercising in the heat.
Returning to the previous example, if you weighed 135 pounds before beginning to exercise in the heat but then lost 4.05 pounds due to dehydration, then you should expect that your heart rate would increase by 9 to 15 beats per minute!
Is a dehydration-driven increased heart rate sufficient to solve the problem of a dehydration-driven rise in core body temperature? In other words, can your heart simply speed up to prevent a dehydration-driven drop in skin blood flow and thereby cool your body back to its optimal core body temperature?
"No" is the answer, unfortunately.
You see, a rise in heart rate leads to decreased "filling time" in the heart, which translates to a decrease in what cardiologists call "stroke volume" -- the volume of blood that is pumped with each beat of the heart.
The results of this cascade of events include:
a stress response;
an increased perception of effort;
an anticipatory regulation of running pace; and,
a decrease in percentage of total work completed.
Put simply, the more dehydrated you become during a run, the slower your pace will become.
So, contrary to the "Stay thirsty, my friends!" advice from an actor in a long-running series of TV commercials for a particular beer, "Stay hydrated, my friends!" is the best advice for runners!
Author Resource:-
Kirk Mahoney, Ph.D., loves to walk and run, and his SpryFeet.com website provides practical research for runners and walkers. By going to http://www.SpryFeet.com/Reports/, you can get his FREE "Pace Tables for Runners and Walkers" special report, letting you look up paces needed to complete several different race distances within given durations and for different micro-level-pacing methods.
(c) Copyright - Kirk Mahoney, Ph.D. All Rights Reserved Worldwide.
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