Hydration for Endurance Athletes: How Much Water do You Really Need?

Below are a few droplets of less-controversial hydration information to help you quench your thirst, perform well, and stay out of the medical tent when you are doing extended exercise in hot weather.

  • Our bodies can deal with transient underhydration that lasts from 4 to 8 hours. In contrast, chronic dehydration leads to health issues—such as what happens when elderly people are trapped in hot apartments during a heat wave.
  • Most runners feel thirsty at about two percent dehydration. At that point, they'll start looking for water. Ultrarunners can maintain performance at three percent dehydration. (To determine your percent dehydration, weigh yourself naked before and after your workout. A 1-pound drop equates to a loss of 16-ounces of sweat; two percent dehydration equates to a 3-pound sweat loss for a 150-pound runner.
  • Thirst is a powerful fluid regulator. Noakes disapproves of the advice to drink before you are thirsty because that can create problems with overhydration. Yet, others contend drinking on a schedule can help endurance runners maintain proper hydration, as long as they do not aggressively over-hydrate, but rather replace fluids according to their sweat losses (as learned during training via pre-and-post exercise weigh-ins).
  • Exercise-induced hyponatremia (low blood sodium) occurs when runners drink excessively during prolonged exercise. It can also occur when even dehydrated endurance athletes lose significant amounts of sodium in sweat. Data from 669 ultramarathoners indicates 15 percent experienced low blood sodium. Of those, 24 percent were overhydrated, 36 percent were dehydrated, and the rest were in fluid balance (but not sodium balance).
  • The amount of sodium lost in sweat varies from person to person. Some runners are salty sweaters. Runners accustomed to exercising in the heat retain more sodium than unacclimatized runners. (Compare the saltiness of your sweat on the first hot day of spring vs. in the fall.)
  • Athletes lose relatively more water than sodium, so under standard conditions, the blood sodium level can actually increase during exercise (unless you over-hydrate). But with abnormally high sodium losses, such as during an ultramarathon, blood sodium can be low even in a dehydrated runner. Hence sodium replacement can be a wise idea.
  • The amount of sodium in a sports drink is small—and unable to counter the dilution of body fluids that occurs with over-drinking. The 220 milligrams of sodium in 16 ounces of Gatorade is far less than the 1000 milligrams of sodium in 16 ounces of sweat loss.
  • Noakes says evidence is lacking to prove that runners who cramp have low serum sodium or are more dehydrated than non-crampers. He suggests muscle cramps are related to fatigue, not sodium deficiency. If sodium deficiency were the problem, wouldn't the entire body cramp, not just one muscle?
  • Exercise-induced muscle cramps occur in muscles that perform repetitive contractions. Runners who get cramps tend to be those who do high-intensity exercise, as well as those who have a history of cramping. Note: Many exercise scientists believe there are two types of muscle cramps: some related to fatigue, others related to sodium imbalance. The science of cramping lacks a clear consensus.
  • Stopping exercise to stretch resolves muscles cramping. (Stretching also resolves nocturnal cramps.)
  • A 2.5 oz. mouthful of (salty) pickle juice has been shown to alleviate muscle cramping within 90 seconds of drinking the pickle juice. This rapid benefit is unlikely due to changes in blood sodium levels—too quick. Noakes speculates drinking the (acidic) pickle juice triggers a reflex in the throat that lessens or stops the cramps.
  • A runner who collapses after the finish line is most likely experiencing blood pressure changes—not severe dehydration. When exercise stops, the heart stops pumping enough blood to the brain; the runner collapses. What to do? Noakes advises to quickly raise the athlete's feet and pelvis above the level of their heart. This aids the return of blood to the heart and rapidly corrects the situation—without any IV fluids.

More: 4 Key Hydration Tips for Runners

So what's a sweaty endurance runner supposed to do during prolonged exercise? Learn your sweat rate and drink accordingly.

If fluid in your stomach starts "sloshing", stop drinking! The body can absorb about 600 to 1,000 ml/hour (women/ men).

Adding carbohydrates and sodium to the water enhances fluid absorption as well as palatability and performance. Consuming "real" foods (salty pretzels, pickles, chicken broth, ham-cheese-mustard wrap) during endurance events can do a fine job of providing needed electrolytes.

Just don't get too aggressive with water or sodium intake—and have fun!

More: Runners Electrolytes and When to Replace Them

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