This piece about hydration and heat is an update of one CBF posted on June 8, 2014. It includes a video shot on Aug. 15, 2010, when we lived in Colorado. The information is still true and relevant for our pickelballers, trekkers, nudist volleyball player, and others who exercise under the hot summer sun.
Dehydration is an unavoidable result of exercise because body heat rises right along with work rate. Mild dehydration is no big thing because we can get to water in a matter of seconds at home or in the gym. But it can be a different story if you are outside on a sweltering day, especially more than an hour.
Older adults are at increased risk for dehydration because our kidneys do not regulate fluid retention as well, nor do we feel thirst as early as younger people as body fluid levels drop. Certain medications, like diuretics, can also accelerate dehydration.
The human body cools by routing blood to 3 million sweat glands that are distributed along a fine network of vessels near the surface of the skin. This incredible control system tries to maintain the core body temperature at a normal 98.6 degrees F. Just 8 to 10 degrees above this magic number, you are delirious or dead due to heat exhaustion or heat stroke. Everybody needs to know their warning signs. (Also, read “Surviving the Extremes: What Happens to the Body and Mind at the Limits of Human Endurance” by Kenneth Kamler, M.D He was the medical support on several expeditions into hostile environments. You’ll learn fascinating details about the body’s response to heat as he tells the unlikely survival story of an ultra-marathoner who got lost during a race in the Sahara Desert.)
Be aware that although dehydration may not result in full-blown heat stroke, even mild dehydration can cause early symptoms such as dizziness, vertigo, muscle cramps, and mental confusion. These don’t kill you, but they increase the odds of accidents and falls.
Sweating indicates your body is regulating internal heat stress, which is a good thing. However, there’s a cost. As you dehydrate, your blood plasma decreases, causing the blood to thicken which then causes your heart to beat harder. Your brain also signals your blood vessels to open as wide as possible to help dissipate the heat, which in turn further taxes your heart to continue to deliver an adequate supply of blood to the muscles in need of oxygen and nutrients to fuel your movement. Lastly, there’s the loss of those important electrolytes.
You cannot replace all the fluids at the same rate that they are lost. It takes time for anything you drink to transit through your gut and into the bloodstream. Even for young people, thirst lags dehydration. For this reason, the American College of Sports Medicine recommends drinking 1/2 to 1 cup of water for every 15 minutes during exercise, whether you are thirsty or not.
If you are exerting more than an hour, you can also add an electrolyte like a little sodium or potassium, which is basically what’s in sports drinks like Gatorade. If you are seriously over-heating, drinking fluid alone will not lower your core body temperature.
Research shows that even drinking ice-cold water has little impact, though it tastes good. The only way to lower your core temperature is to lower your metabolic rate, which means you must slow down or stop altogether.
Remember your muscles, including your heart, are 70% water, so drink, drink, drink.