If you ever felt like a workout had drained you to exhaustion, consider the Gatorade ad that's currently playing on TV. It shows footage of a guy staggering into the home stretch of a race, then collapsing like an inflatable man who's just been pierced by a BB. He tries to crawl to the finish line but stumbles like a downer cow and goes horizontal as medics rush in to help.

It's spine-chilling stuff, a mix of horror and inspiration -- especially when you learn that the footage is real, shot at the 1997 Ironman Triathlon in Kona, Hawaii. The racer, Australian Chris Legh, now 32, told us what happened that day -- or at least the parts he remembers.

"From the start I was having problems keeping fluids down. I kept chasing [trying to drink to replace lost fluids], but puked it all back up." After finishing the 2.4-mile swim, Legh started the 112-mile bicycling segment about six minutes behind the leaders.

"I caught them, and there was only about four of us left," Legh said. "Imagine a 24-year-old athlete at the lead of the Ironman: You're not going to give up."

Then came the last leg: a full marathon, 26.2 miles of running. "I thought if I just shut my eyes and made it to the end, I'd be all right. I came up about 50 meters too short."

Robert Murray, director of the Gatorade Sports Science Institute in Barrington, Ill., who worked with Legh on training and hydration after the 1997 race, said the Australian was near death when he collapsed in Kona. "Part of his large intestine had become necrotic" -- by which he means "dead" -- "due to dehydration."

Dehydration causes blood volume to drop and, in severe cases, the body cannot deliver sufficient blood to all organs and muscles. To preserve flow to the most vital organs, like the brain, the body first ceases delivery to less-crucial areas, like the intestine, Murray explained.

"In Chris's case, it was an almost fatal mistake of passing up aid stations and neglecting to take in fluid. You get so far along [in dehydration] that you lose the power of decision-making."

Legh agrees. "From about 10 miles out [from the finish] I knew I was in trouble," he said. "From about eight miles out, I can only remember patches. I'm glad I can't really remember all of it."

He returned to racing five months later, winning a half-Ironman and, one month later, placing second in a full Ironman. In 2004, he won an Ironman race.

The take-away for weekend warriors? Proper hydration is crucial.

As we've reported before, ideally you will start each exercise session adequately hydrated and either drink enough during exercise (a little bit every 10 to 15 minutes) or refuel very soon after finishing to replace fluid lost via sweat. You should weigh the same after an exercise session as you did before.

For longer bouts -- say, more than an hour -- in which you sweat a lot, replacing sodium during exercise is key, either with a sports drink or a salty snack.

As we have also noted, endurance athletes and those partaking in unusually time-consuming physical activities face a slight risk of hyponatremia -- over-hydrating to the point of dangerously diluting blood-sodium levels. But this risk is almost nonexistent for typical exercisers who engage in moderate exercise for 30 to 90 minutes at a stretch. Marathon runners and serious outdoor jocks should learn about the condition.

No chat today; back next week. E-mail: move@washpost.com.

-- John Briley

During the 1997 Ironman Triathlon in Kona, Hawaii, Australian Chris Legh learned about the risks of dehydration the hard way. He collapsed 50 meters before the finish line.