It's probably not news to you that drinking water or other fluids while exercising for prolonged periods in warm weather -- or for that matter, in more benign temperatures -- is vital to preserving both performance and health. What may be less clear is how drinking water while exercising affects your heart rate, and how this in turn affects your performance and well-being.
Heart Rate in Humans
The National Institutes of Health reports that a normal pulse for an adult at rest ranges from 60 to 100 beats per minute. Many factors can affect this figure, including stress, physical illness and hydration. It is not uncommon for elite athletes to have resting heart rates under 40, because their hearts have adapted to pumping more blood to the body with each contraction. Maximum heart rates during exercise vary widely between people and decline with age. Most healthy young adults can approach or exceed 200. Exercisers typically aim for target heart rate "zones" that are based on a function of maximum heart rate. Loosely speaking, exercising at 60 percent to 75 percent of maximum is considered from easy to moderate, 75 to 85 percent moderate and above 85 percent hard.
Fluid Turnover During Exercise
If people didn't sweat, there would be no shifts in body-fluid balance during exercise and this whole topic would be moot. But in fact, the amount of water you can lose during exercise when it's warm is startling -- up to three to four pounds per hour, according to exercise physiologist and former Olympic marathoner Pete Pfitzinger. At that rate, you need to drink as much as a half-gallon of water an hour to offset sweat losses, a daunting task for most people. If you're going to be exercising more than an hour, you'll want not plain water but something with carbohydrates in it. Beverages with a carbohydrate concentration of about 6 percent to 8 percent are absorbed from the gastrointestinal tract as quickly as water is; most commercial sports drinks fall within this range.
Effects of Hydration on Heart Rate
When the amount of fluid in your body decreases during exercise, your heart rate increases. The reason is simple: When there is less water in your system -- represented as blood volume -- then your heart pumps less blood per beat, meaning that it needs to beat more quickly to maintain total cardiac output. So, if you don't drink any water or other beverage during exercise, your heart rate will steadily increase at a given, fixed level of exertion as the amount of water in your body decreases from losses in the form of sweat.
When you become sufficiently dehydrated as a result of not replacing fluids lost during exercise from sweating, then a condition called hypovolemia -- abnormally low blood volume-- results. Pfitzinger says every 1 percent loss in body weight as a result of water loss translated into a 3 percent decline in performance. More ominously, continuing to exercise in the face of serious losses of fluid -- think 4 percent to 5 percent of your body mass -- can result in potentially fatal heat stroke. So, do tank up on the go, even if you don't feel thirsty, as is often the case even in athletes who have already lost a good deal of body water.
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