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.

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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.

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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.

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Dehydration Causes Cramps

Although pro cyclists routinely become dehydrated during races we rarely read about cramps in the peloton. In lab experiments, dehydration does not cause cramps.

Drink Early and Drink Often

In the mistaken belief that dehydration affects performance and causes cramps, we are taught to drink before a workout and to continue drinking frequently during the workout. Develop the habit of drinking enough throughout the day so that you are fully hydrated—you urinate every few hours with good output. Don't force yourself to drink on the bike.

More: The New Rules of Hydration

Drink Before You're Thirsty

We're also told to drink before we're thirsty to be sure we take in enough fluid. Sports scientists now recommend drinking just enough to satisfy thirst in order to avoid hyponatremia. Your body is marvelously effective at self-regulating. If you become significantly dehydrated, your thirst mechanism will kick in.

Water Is All You Really Need

Taste your sweat—does it taste like clear water? No! In addition to replacing the water you lose in sweat, you need to replace the sodium. A liter of sweat contains about 800 mg of sodium—half the recommended daily intake of sodium. So after a sweaty workout, don't spare the saltshaker. A quart of your sweat contains 115 mg of potassium, only about 2.5 percent of the recommended daily intake of potassium.

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Sports Drinks Provide the Necessary Electrolytes

Yes, but only to an extent. The American College of Sports Medicine recommends that a sports drink contain 125 to 175 mg of sodium per 8 fluid ounces and 20 to 48 mg of potassium per 8 fluid ounces. Most sports drinks don't provide nearly enough sodium, the one electrolyte you really need to replace. If they had enough sodium they'd taste terrible. Read the ingredients on the label of your sports drinks.

Electrolyte Depletion Causes Cramps

If you cramp, you may have been told to eat a banana for potassium, take Tums for the calcium or a supplement for magnesium. The amount of each of these electrolytes in sweat is minute compared to what your body stores, and depletion doesn't cause cramps. Experts continue to disagree as to whether sodium depletion causes cramps or not.

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Pouring Water Over You Is Effective

Your body cools itself like your car engine and radiator. Blood flows through your core organs, gaining heat and flowing through your skin to produce sweat. Most of the cooling comes this way, not from your hot skin directly radiating heat to the already hot atmosphere. If you have limited water, drink it instead of dowsing yourself.

Carbonated Beverages Are Bad

Carbonated beverages are absorbed just as quickly as non-carbonated ones. However, carbonated drinks may make you feel full sooner, so you drink less.

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Caffeinated Drinks Cause Dehydration

The amount of caffeine in a couple of cups of coffee, tea or 3 to 4 sodas isn't enough to cause dehydration. If you need to urinate after drinking a caffeinated beverage, it's because of the fluid you ingested.

Beer Is Good for Rehydration and Glycogen Replacement

Unlike caffeine, alcohol—even in moderation—is a diuretic. Only about 1/3 of the calories in beer come from carbs (the source of glycogen). The majority are empty calories from alcohol.

The bottom line:

• Develop the habit of drinking frequently during the day and evening so that you are fully hydrated before exercise.
• During exercise drink whenever you are thirsty, but not more.
• After a sweaty workout, particularly if your clothes are white with salt, eat salty snacks.

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