How to Hydrate: A Special Report that Answers the Question Why Should I Drink Water

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Later, when she had recovered, the doctors asked what she’d had to drink recently. She thought. The day before, another scorcher, she had lounged by the pool with her kids. One of them had knocked over her full glass of iced tea, and she had never refilled it. At dinner, she had sipped a glass of red wine with her pasta. She realized she had had nothing else to drink all that day; on race morning all she had was a couple sips of water.

“Jill might have been 2 percent [mildly] dehydrated before she even started the race,” said Sam Cheuvront, Ph.D., an exercise physiologist, when he heard her story. (A 10-kilometer, or 6.2-mile, race is usually too short to produce significant dehydration by itself.) This is not terrible. But, said Cheuvront, “86 degrees and humid and trying to run a 10K fast—even if you’re marginally dehydrated, in a hot environment the ­effects are exacerbated.”

In Jill’s case, that meant deeper dehydration, and a dehydrated body can’t cope as well in hot weather. (In fact, for every 1 percent of body mass lost due to dehydration, your body temperature rises by about a third of a degree Fahrenheit.) This most likely led to Jill’s heat exhaustion, which probably caused her collapse.