How Much Does Dehydration Affect Performance?

Diana Nyad vomited constantly during her 110-mile marathon swim from Cuba to Florida. She was so sufficiently dehydrated that she received IV fluid afterward. Yet she swam very well, maintaining a steady stroke and rhythm for the full 53 hours.

For decades marathoners raced 26.2 miles without drinking in cool or hot conditions.

More: Cycling Hydration Myths

Pro stage racers ride so hard that their guts can't absorb enough fluid to replace all that they are losing in sweat. Race rules also restrict when a rider can get a bottle toward the end of a stage. Although somewhat dehydrated, the pros sprint quite well.

But dehydration hurts performance, right?

Proper management of hydration and electrolytes is important in both hot and temperate conditions. But scientists are learning more and more about hydration, electrolytes and sports—and much of what we were taught isn't exactly the best advice.

Hydrate or Die

The average male's body is 60 percent water, the average female 50 percent and the typical athlete another 10 percent on top of that. Obviously if we don't replace this, we die. However, almost all of the heat-related deaths every summer are shut-ins living in homes with no AC. Your body has about 2 quarts (liters) of free water in your intestines. You don't even start to feel thirsty until you you've lost 1.5 to 2 quarts of water.

More: Stay Hydrated During Anaerobic Performance

More accurately: overhydrate and risk dying. Dilutional hyponatremia is a more serious condition than dehydration. That's when you drink so much fluid that your blood sodium is diluted to a dangerously low level.

Dehydration Leads to Collapse

We've all seen pictures of runners collapsing at the end of a marathon or triathlon. This must be because the runner is dehydrated, right? When a runner stops, the pulse and blood pressure fall so significantly that the sudden decrease in blood that gets to the brain causes the runner to faint. This has nothing to do with dehydration.

More: 7 Tips for Fueling on the Bike

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