In 2002, the Armed Forces Medical Examiner in Rockville, Maryland reported that three US military recruits were killed by drinking too much water. The colonel states: "In September 1999, a 19-year-old Air Force recruit collapsed during a 5.8-mile walk, with a body temperature of 108 degrees Fahrenheit." The doctors concluded he had died of both heat stroke and low blood sodium levels as a result of overhydration. You get low salt blood levels from not getting enough salt, not from taking in too much water. During January 2000, a 20-year-old trainee in the Army drank about 12 quarts of water during a 2- to 4-hour period while trying to produce a urine specimen for a drug test. She then experienced fecal incontinence, lost consciousness and became confused, then died from swelling in the brain and lungs as a result of low blood sodium. In March 2001, a 19-year-old Marine died after a 26-mile march, during which he carried a pack and gear weighing more than 90 pounds. Although he appeared fine during the beginning stages of the 8-hour walk, towards the end he began vomiting and appeared overly tired. He was then sent to the hospital, where he fell into a coma, developed brain swelling and died the next day. That's what happens when you don't take in salt when you replenish water lost from sweat.
In the first case, the colonel is describing heat stroke, a sudden uncontrolled rise in body temperature that cooks the brain. Prevention of heat stroke is to stop exercising when your circulation is compromised. For example, as your temperature starts to rise, your hot blood burns your muscles and your muscles hurt and burn. If you continue to exercise, your circulation is compromised when you become extremely short of breath and no matter how hard you breathe, you can't catch your breath. If you ignore this sign, your temperature rises above 106 and your brain is damaged, you get a headache, see spots in front of your eyes, hear ringing in your ears, feel dizzy and pass out. So prevention is to stop exercising when you feel unusually short of breath.
Lack of fluids set you up for heat stroke, so you are supposed to drink fluids all the time when you exercise in hot weather. You cannot depend on thirst to tell you when you are dehydrated because you won't feel thirsty from exercise until you have lost between two and four pounds of fluid and by then, it is too late to catch up on your fluid deficit. What the colonel doesn't mention is that you need to take salt also when you exercise. If you take water and no salt, the water goes into the brain to cause swelling and death. If you take salt with your fluids, the concentration of salt inside and outside your brain remains equal. If the recruits take salt, the salt keeps the water distributed between the cells and the fluid. When you exercise vigorously, particularly in hot weather, you should drink plenty of the fluids that you like the best and eat foods that contain lots of salt.
IMPORTANT: Read my report on HYPONATREMIA, too much water.
Military Medicine 2002;167:432-434.
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