Bryon Powell November 18, 2011 TWEET COMMENTS 3

Keeping the Glass Half Full

Improve performance with proper hydration and electrolyte replenishment

Most runners realize that staying hydrated is paramount to success in longer events, but how much ...

Photo by Glenn Tachiyama

Most runners realize that staying hydrated is paramount to success in longer events, but how much fluids is enough? Too much? Just a two- to three-percent loss in body weight from perspiration can result in a 10-percent drop-off in endurance performance. Marathoners may be familiar with the concept of cardiac drift, which happens when dehydration decreases blood volume, leading to a reduction in oxygenated blood flow. The result is a slowly increasing heart rate even while the runner maintains a consistent effort. Cardiac drift is worsened by hot temperatures, when blood is drawn from the muscles to the skin to assist with cooling.

Beyond decreasing your endurance performance, dehydration can also lead to cramps, digestion issues, such as diarrhea and vomitting, and blisters due to increased rubbing in fluid-deficient tissue. Extreme dehydration can lead to chronic, even life-threatening health conditions. To hydrate properly, you need to know how much you sweat, how to balance water and electrolytes and how to drink on the run.

Fluid Loss

Always related to running intensity, sweat rate also varies based on several environmental factors, including temperature, relative humidity and altitude. The most effective way to measure sweat rate is to conduct a sweat test.

First, weigh yourself naked before a 60- to 90-minute run. Do not eat, drink or use the bathroom until you have completed your run. Post-run, re-weigh yourself naked. The difference between your starting and finishing weight is your net sweat loss. Divide that number by the number of minutes you ran, and multiply the result by 60 to determine your sweat rate in ounces per hour.

You may want to conduct multiple sweat tests to determine your sweat rate under a variety of conditions. If you wish to determine your hydration rate with a particular goal race in mind, conduct the sweat test under conditions that mimic probable race conditions.

As you gain experience, you will be able to estimate how much fluid you should be consuming in various situations. No matter how much you are sweating, though, it is difficult for the body to process more than 24 to 28 ounces of water per hour.


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