This year's London Marathon, held in late April, was miserable. Temperatures hit an unseasonably warm 72 degrees. Thousands limped into medical tents, exhausted and overheated, and more than 600 of the 36,396 runners dropped out. But the race's lone casualty, a 22-year-old fitness instructor, wasn't felled by dehydration. He drank himself to death.
Known medically as hyponatremia, water intoxication was, until this spring, believed to be caused solely by overhydrating. Simply drink more than you lose by sweating or urinating, the theory went, and you put yourself at risk of a coma and, in extreme cases, death.
But a Harvard University study published in May's American Journal of Medicine found a link between hyponatremia and a hormone triggered by overexertion that shuts off urination and causes the body to retain excess water. That means anyone participating in taxing physical feats can be susceptible to water intoxication, regardless of conditioning. The good news is there's a simple way to predict whether you're at risk. The bad news? It hurts: "You need to re-create race conditions in training," eliciting the cascade of hormones that can occur in a long, hard race, says Arthur Siegel, M.D., the lead author of the study. Here's how:
1. Weigh yourself before exercising.
2. During workouts, drink as you normally do.
3. Push hard--until you hit the wall. Curse Siegel.
4. Weigh yourself again postworkout.
5. You should have maintained or lost weight.
The results: Up to a 3 percent drop in weight is normal. (More than that shows dehydration.) If, however, you've gained weight, there's a problem. You're retaining fluid. Find your optimal H20 intake by repeating the workouts (train with a buddy to be safe) and adjusting how much you drink until you've found the formula that keeps your weight steady or slightly lower, postworkout. Then race. You'll not only be prepared in terms of hydration, you'll be in race-winning shape.
Timing is everything when it comes to hydration: "The duration of your exercise and when you plan to drink should dictate the kind of beverage you choose," says Edward Coyle, Ph.D., director of the Human Performance Laboratory at the University of Texas, at Austin. Here, Coyle's picks for what to drink and when. --Orlee Maimon
SPORTS DRINKS have the optimal balance of H20 and carbs.
When to drink up: Before and during one-hour-plus workouts--or
if you start to fatigue.
RECOVERY DRINKS contain protein and more carbs. When to drink
up: Within 30 minutes after a long, hard workout to aid muscle recovery.
H20 is vital. When to drink up: If you're well-sated and only exercising for 30 to 45 minutes, extra carbs won't help your performance. Good ol' water will do the trick.