You suffer heat stroke when your body temperature rises so high that it cooks your brain, causing you to pass out. This is particularly likely to happen when you exercise, particularly in hot weather, because food is converted to energy by a series of chemical reactions, each of which release heat. The sum of the reactions convert more than 80 percent of the energy into heat, while less than 20 percent is actually used to drive your muscles. That means that the harder you exercise, the higher your temperature can rise, and your body has to work very hard to keep your body from overheating. Since more than 80 percent of the energy used to power muscles is lost as heat, the more intensely you exercise, the faster you convert food to energy and the more heat you produce. That means that your heart is already working very hard to pump oxygen to your muscles to help convert the food to energy. Now that you are producing more heat, your heart also has to work much harder to dissipate the heat by pumping more hot blood from your over-heated muscles to you skin where sweat evaporates to cool the blood. This double duty of pumping oxygen to muscles and heat away from muscles can be too much for your heart. Your temperature can rise too high and you can die from high body temperature.
In the relatively cool environmental temperature of 50 F, healthy marathon runners can have body temperatures as high as 103.8 F. Weight lifters often have temperatures of 101 F during workouts in a warm gym. One runner who was still conscious is reported to have developed a temperature of 107.8 F after finishing a marathon, but most people cannot tolerate temperatures that high.
Several factors increase our chances of developing a heat stroke such as when the outside temperature and humidity are high, you are not in shape, you take certain medications or are sick or dehydrated. Aspirin does not keep your temperature from rising during exercise because aspirin lowers fever by making you sweat. When exercising intensely, you may already be sweating at your maximum and not be able to increase your sweating. So taking aspirin will not increase sweating and your temperature can rise even higher.
To protect yourself from heat stroke when you exercise, start out slowly and gradually increase your pace. This gives your body time to circulate the heat to the skin where heat can be dissipated. Drink fluids long before you are thirsty. Once you are thirsty, you are already dehydrated, having lost at least 2 pounds of fluids. Drink whatever you like best at least every 15 minutes and know the symptoms of rising body temperature. When your temperature rises above 102, your muscle often start to burn, when your temperature is over 104 you will usually become short of breath and when your temperature rises above 105, you will often have signs of brain distress, such as a headache, blurred vision, ringing in your ears, dizziness, nausea and passing out.
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4) Gordon. Am J. Card1985;55:74D-78D.
5) Tandberg. NEJM 1983(April 21);308(16):945-946
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