Seeing water intoxication clearly. .

hyponatremia, or water intoxication, is a rare but serious condition that occurs when plasma sodium levels fall dangerously below normal. If a patient's sodium level dips low enough, the condition results in massive brain swelling and is considered a medical emergency.

The causes of hyponatremia can be boiled down into two categories: consumption of too much fluid over too short a time span, or inordinate sodium loss through bodily sweat. It is possible for these two factors to be in play at once, which exacerbates the risk and can also make the condition at times appear contradictory.

Salty sweaters who in addition sweat excessively are at a higher risk. However, excessive fluid intake for a normal sweat rate will lead to inadequate fluid loss and therefore hyponatremia as well. It is useful, then, to think of the two factors as largely independent: If you are an excessive, salty sweater, your sodium loss will be greater than that of a moderate sweater. This is what makes it possible for hyponatremia to occur even in athletes who are simultaneously dehydrated.

At the same time, if a person is overdrinking--that is, not using the amount of water they are consuming for their level of exertion--they will dilute their plasma sodium concentration and therefore be more prone to the condition. This is what makes it possible for a person to become hyponatremic without exercising at all.

The more fluid a person consumes per kilogram of weight, the higher the likelihood of overdrinking. Therefore, smaller runners are more at risk. Women should take care to avoid consuming fluid like men.

Four- and five-hour marathoners are at increased risk, too. These often first-time runners tend to be hypervigilant, and may not be exerting themselves enough to justify all their drinking. Slow finishers also have more time to consider stopping for water as they slowly pass each aid station. And at more moderate speeds, these same people are likely to get that much more fluid into their mouths, as opposed to down the fronts of their shirts.

Early signs that you are hyponatremic include feeling bloated, accompanied by nausea and vomiting. This is usually followed by visible bloating, including an inability to rotate one's wedding band or an uncharacteristic lack of "play" in the wristwatch. At this stage you should seek medical attention. More advanced stages can result in respiratory distress, seizures, coma and, finally, brainstem herniation and death.

Simply stated, fluid intake during a marathon should never exceed fluid loss. Weigh yourself at home and record it on your bib the morning of the event. Expect to lose up to 2% of your premarathon body weight; any more and you are likely dehydrated, but gaining weight is a dangerous sign you're overdrinking. Favor sports drinks over water to gain back some of the sodium lost in sweat. In an Ironman competition, however, this may be insufficient. It's a good idea to carry pretzels to eat along the course.

Be aware of how much you're drinking and how much you're sweating on race day. Remember that, while dehydration is a far more common problem, things aren't as simple as "the more fluid the better." What's more, there is little need for rapid, massive post-race rehydration. You're not going to be running a marathon the next day. Save that impulse for multi-event sports.

To view American Running and AMAA's brochure, The Right Way to Hydrate for Marathons, go to www.amaasportsmed.org/programs/education.htm.

(Adapted from "AMAA Experts Examine Hyponatremia's Challenging Characteristics at Boston," by Jeff Venables, AMAA Journal, 2003, Vol. 16, No. 2, pp. 6-8)