Q. IÂ’ve always been told that itÂ’s important to drink plenty of fluids during exercise, and it seems to work for me. But now IÂ’ve read that too much water can be very dangerous. Which is right?
A. Both are right: good hydration is important, but overhydration can be hazardous, even lethal. Fortunately, common sense and moderation will protect you from both extremes.
Until the late 1970s, most athletes were advised not to drink before or during exercise. The idea was to avoid bloating and to improve performance. Some coaches may have also believed that withholding fluids would Â“build character.Â” It didnÂ’t work. In fact, dehydration increases the risk of muscle cramps, heat exhaustion, and heat stroke during exercise in warm weather. And even in mild weather, dehydration can leave exercisers grumpy or groggy for hours afterward.
When the hazards of dehydration became apparent, experts began to encourage fluids during exercise. A common recommendation was to drink 20 to 40 ounces per hour of exercise. But these guidelines were formulated for elite male athletes whose high-intensity exercise produced lots of fluid loss in sweat. And they were intended to apply to exercise lasting two hours or less.
As a result of these guidelines, athletes began to drink. Spurred on, in part, by manufacturers of sports drinks and bottled water, they drank more and more. And some drank too much, producing water intoxication and hyponatremia (low blood sodium levels). Hundreds of cases and a number of deaths have been recorded in medical journals.
The tragic death of a female runner in the 2002 Boston Marathon called attention to the problem and has stimulated advice against drinking too much during exercise. The caution is justified and the advice is important, but itÂ’s not easy to drink enough to get into trouble. The typical victim of water intoxication is a runner who is out on a marathon course for over four hours and who consumes enough fluids Â— often well over 3 quarts (96 ounces) Â— to gain weight during the race. That is way, way too much.
How much should you drink? In hot, humid weather, you can lose over a quart of sweat in an hour of moderate to intense exercise; in cool weather, much less. Plan to drink two to three cups of water an hour, but boost the amount if you are sweating heavily. If you drink a quart, or even two, during a long summer tennis game or hard workout, you wonÂ’t get into trouble. Extra salt is not required to prevent hyponatremia, and unless you get way behind in your fluid replacement, sports drinks wonÂ’t be any better than water. The trick is simply to avoid drinking too much fluid.
Best of all, evaluate your own needs. Drink when you feel dry or thirsty, but donÂ’t arbitrarily force down huge amounts. To learn precisely what you need to stay in balance, weigh yourself before and after exercise, stripped down to avoid the effects of sweaty clothing. For each pound you lose, youÂ’ll need a pint of fluids. You can also keep an eye on your urine output. If your urine is scant and concentrated, youÂ’re dry, but if itÂ’s clear and copious, you are fully hydrated. Remember, though, that if you gain weight, feel bloated, or experience nausea and vomiting, youÂ’re on your way to hyponatremia and big trouble.
Medicine is a science, but its teachings often seem to swing like a pendulum. If you stay centered and use common sense in evaluating advice, you wonÂ’t be left high and dry Â— or, for that matter, all wet.
Â— Harvey B. Simon, M.D.
Editor, Harvard MenÂ’s Health Watch
Exercise: A program you can live with
Hundreds of studies conducted over the past 50 years show that exercise helps you feel better and live longer. Exercise: A program you can live with answers many important questions about physical activity, from how your body changes through exercise to what diseases it helps prevent. It will also help guide you through starting and maintaining an exercise program that suits your abilities and lifestyle. Throughout, you'll find advice on being a savvy consumer when it comes to fitness products, as well as useful tools and tips designed to help make exercise work for you.
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