Too much water can be dangerous: How to avoid hyponatremia

In blind taste tests, most people can't tell the difference between bottled and tap.
A recent study published in the New England Journal of Medicine and reported on by the New York Times has many runners worried. The study was conducted by the medical staff of the Boston Marathon, which found that an alarmingly high percentage of the finishers of the 2002 Boston marathon had developed hyponatremia -- a potentially fatal condition -- during the race.

What it is

Hyponatremia, or water intoxication, results when the sodium concentration of the blood falls too low due to prolonged sweating combined with excessive fluid consumption. Symptoms include dizziness, muscle cramping, confusion, and stomach bloating. Severe cases can lead to seizure, coma, and even death.

Hyponatremia occurs most often during marathons and long triathlons, when athletes are exercising longer and drinking more than they do in training. Non-competitive runners who require more than four hours to complete a marathon are more susceptible to hyponatremia than faster athletes. Smaller women may be most susceptible.

Among the 488 runners tested for the study, 13 percent were hyponatremic, and a total of three runners were considered dangerously so. The reason? These runners had consumed an average of three liters of fluid while completing the race -- enough to actually gain weight despite all the sweating they had done.

No longer a rare condition

It is widely believed that hyponatremia has become more common in recent years for two main reasons. The first reason is that hydration authorities, spurred on by the sports drink industry, have given athletes bad advice to "drink as much as possible" during exercise.

The second reason is that the so-called "second running boom" has brought primarily non-competitive runners into the sport. These runners often require more than four hours to complete a marathon and are therefore more susceptible to hyponatremia.

How to avoid it

Although it is becoming more common, hyponatremia is very easy to avoid. Just follow these guidelines:

  1. Drink sports drinks only during prolonged running. Don't drink water. Sports drinks contain sodium and therefore have a smaller diluting effect on the blood than water, which has no sodium. Sports drinks also provide energy in the form of easily absorbed carbohydrates and, in some cases, protein.
  2. Don't force yourself to drink more than you're thirsting for. Drink at a comfortable, natural rate, which is between 4 and 6 ounces every 12 to 15 minutes for most runners engaging in vigorous exercise.
  3. Learn your sweat rate. Weigh yourself without clothes immediately before and immediately after a run that mimics the conditions (temperature, pace, etc.) of your upcoming event. Don't drink during the workout and keep it reasonably short to minimize dehydration. Divide your weight loss in ounces by the workout duration in hours.

    For example, suppose you weigh one pound (16 ounces) less after a one-hour run. This means your sweat rate under these conditions is about 16 ounces per hour. Practice drinking at 60 to 80 percent of this rate in workouts, then duplicate this pattern in your event.

There's no cause to be alarmed about hyponatremia. As dangerous as it may be, you really have to go out of your way to develop this condition. If you simply drink at a comfortable rate, and use a good sports drink exclusively instead of water, you'll be fine.

Matt Fitzgerald is the managing editor of and author of "Triathlete Magazine's Complete Triathlon Book" and "Runner's World Guide to Cross-Training."

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