Under normal conditions, the body maintains its fluid balance — lose fluids through sweating, for instance, and your body responds with the feeling of thirst. You drink. If you swallow more liquid than needed just then, your body typically responds by excreting the excess through urination. But should you consume so much fluid that your body cannot easily rid itself of the surplus, you dilute sodium levels in your blood. Osmosis then draws water from the blood into body cells to equalize sodium levels, and those cells swell. At that point, you have hyponatremia. If the cellular bloating occurs in the brain, it can be fatal.
Today, most knowledgeable coaches and exercise experts warn athletes not to overdrink. “You should drink only when you need to, when you are actually thirsty,” said Dr. James Winger, a professor of family medicine at Loyola University Medical Center, who has studied the hydration habits of athletes.
If you’re not sure whether you’re drinking too much or too little during exercise, try weighing yourself before and after a lengthy workout, experts advise. If you have lost more than about 3 or 4 percent of your body weight, you’re probably flirting with dehydration and might want to drink a bit more next time. But if you have gained weight or your fingers seem swollen and your rings tight, you’re most likely drinking too much and should moderate intake.
Most important, listen to your body’s signals, Dr. Winger said. “Thirst is a very reliable indicator,” he said, of whether and how much to drink.