Hyponatremia and Sodium Loss

There's no need to try to superhydrate preexercise; your body can absorb just so much fluid. The kidneys regulate water balance by adjusting urine output—from a minimum of about a tablespoon to a maximum of about 1 quart (1 L) per hour. If you overdrink, you then may need to (inconveniently) urinate during exercise. A wise tactic is to drink up two or more hours before exercise; this allows time for your kidneys to process and eliminate the excess. Then drink again 5 to 15 minutes preexercise.

For the most part, frequent trips to the bathroom simply inconvenience people who drink too much water. But in some cases, drinking too much water can actually be lethal if it dilutes body fluids and creates a sodium imbalance. A condition known as hyponatremia occurs when blood sodium levels become abnormally low. In general, hyponatremia that occurs in events that last less than four hours is caused by overdrinking water before, during, and even after the event. Hyponatremia that occurs in endurance events that last more than four hours is often related to extreme sodium loss. Athletes affected by extreme sodium loss tend to be those who exercise more than four hours in the heat. During exercise and heat stress, the kidneys make less urine. Therefore, if athletes overhydrate during exercise, their bodies may not be able to make enough urine to excrete the excess volume.

Athletes likely to experience a sodium imbalance caused by extreme sodium loss commonly include slow marathoners, triathletes, ultrarun-ners, and unfit weekend warriors who have a higher sweat loss of sodium than their fit counterparts. These athletes may diligently consume high amounts of preexercise water plus consistently drink water during the event. As a result, they accumulate too much water by consuming water faster than their bodies can make urine, and they end up with a relative excess of water compared with sodium. The plain water dilutes their electrolyte balance and makes matters worse.

Hence, with extended exercise, be sure to replace sodium losses with more than just sports drinks. Sports drinks generally contain too little sodium to balance sweat loss. Choose endurance sports drinks and salty snacks (e.g., pretzels, V8 juice, olives, and pickles), salt sprinkled on foods, soup, and even salt tablets. (Note that some salt tablets, such as Endurolytes, offer only 100 mg of sodium per tablet.) Your target should be 250 to 500 milligrams of sodium per hour, the amount in 20 to 40 ounces (0.6 to 1.2 L) of Gatorade, for example. Most athletes get too much salt in their daily diet, so use sports drinks appropriately, before and during exercise that lasts for more than an hour, not as a standard mealtime beverage. Also, remember that the more you train in the heat, the less sodium you lose because your body learns to conserve sodium as well as other electrolytes (see table 8.1).

Table 8.1 Electrolyte Content of Sweat in Unfit and Fit Subjects

Electrolyte in sweat

Unfit, unacclimatized

Fit, unacclimatized

Fit, acclimatized


3.5 g/L

2.6 g/L

1.8 g/L


0.2 g/L

0.15 g/L

0.1 g/L


0.1 g/L

0.1 g/L

0.1 g/L


1.4 g/L

1.1 g/L

0.9 g/L

Adapted, by permission, from T. Noakes, 2003, Lore of running, 4th ed. (Champaign, IL: Human Kinetics), 214.

Adapted, by permission, from T. Noakes, 2003, Lore of running, 4th ed. (Champaign, IL: Human Kinetics), 214.

As you can see, if you are neither fit nor accustomed to exercising in the heat, you will lose twice as much sodium as you would if you were fit and familiar with exercising in the heat. Even if you are fit, you should be mindful of sodium losses if you are not used to exercising in the heat—such as happens if you live in Alaska and run a marathon in Hawaii.

The symptoms of hyponatremia include feeling tired, bloated, nauseated, and headachy. Any of these symptoms may become increasingly severe. A person with hyponatremia may also experience swollen hands and feet, undue fatigue, confusion and disorientation (due to progressive swelling of water in the brain), a decline in coordination, and wheezy breathing (due to water in the lungs). Blood sodium levels that drop too low can lead to seizures, coma, and death. To prevent hyponatremia, people who will be exercising for more than four hours in the heat should observe the following guidelines:

  • Avoid water loading before the event.
  • Eat salted foods and fluids (soup, pretzels, salted oatmeal) 90 minutes before you exercise. This dose of sodium results in water retention in your body. This extra fluid not only can help you exercise longer but also may make the exercise seem easier and more enjoyable (Sims et al. 2007).
  • Consume an endurance sports drink with higher sodium amounts than the standard sports drink during extended exercise in the heat that lasts for more than four hours.
  • Consume salty foods during the endurance event, as tolerated (V8 juice, broth, pickles, cheese sticks).
  • Stop drinking water during exercise if the stomach is "sloshing," as may happen if you drink more than a quart (32 oz, or 1 L) of water per hour for extended periods.
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