It seems as if we could never drink enough water. After all, aren't we always being told to drink more water?

That's not always the case for ultra-endurance athletes who spend long hours exercising. Sometimes, drinking too much water can actually be a problem. If they flood their bodies with excess water, they may fall victim to a potentially fatal condition called hyponatremia.

What Causes Hyponatremia?

Hyponatremia is low sodium concentration in the body, specifically the extra-cellular fluid, which is the blood and the fluid around the body's cells.

Normal sodium levels run between 135-145 milliequivalents per liter (mEq/L). Hyponatremia is generally defined as a sodium level of 135 mEq/L or below.

When sodium concentration in the blood is critically low, symptoms like muscle weakness, disorientation, headache, fatigue, and nausea may occur. The most severe symptoms include seizures, respiratory arrest, coma, and death.

When Does Hyponatremia Happen?

Hyponatremia is more likely to occur in the following situations:

  • Exercising in hot climates rather than cold ones
  • During long distance endurance events such as marathons and triathlons
  • During events that require high effort

Who Is Most at Risk for Hyponatremia?

Ultra-endurance athletes who enter such events as 100-mile races and Ironman competitions are most at risk. Hyponatremia can occur as early as four hours into an event, but it's more likely to happen after six, eight, or 10 hours of exercise.

Non-elite marathon runners are also at risk. A study in the New England Journal of Medicine found that out of 488 runners who volunteered blood samples after completing the Boston Marathon, 13% had hyponatremia and 0.6% had critical hyponatremia. Risk factors for developing hyponatremia included: gaining substantial weight during the race, having a racing time greater than four hours, and having a low or high body mass.

How Can Endurance Athletes Prevent Hyponatremia?

Endurance athletes need to take preventive measures to head off hyponatremia.

Say Yes to Salt

Ultra-endurance athletes may want to consider adding more salt to their diets so they begin events with a good supply of sodium. This is not necessary for most recreational athletes, since most diets typically contain enough sodium to make up for that lost when sweating.

Choose Drinks Wisely

All athletes should drink water before their events, but when you've been exercising for more than an hour, sports drinks or other fluids with sodium are better choices.

Experts disagree on how much fluid to drink during endurance events. One recommendation is to consume 5-10 ounces (150-300 milliliters) of fluid every 15 minutes. However other experts recommend that athletes avoid exceeding 13-26 ounces (400-800 millileters) per hour. As you gain experience, you will begin to know your own thirst needs and drink accordingly.

Weigh In

Athletes should weigh themselves before and after their long-distance events. In fact, some long-distance events may require that you do so. In most cases, dehydration is more likely to be a problem than hyponatremia. By weighing themselves, athletes will know if they're drinking too much or too little.

Checking body weight at the same time every day before and after an endurance event will help athletes gauge how much fluid they need to replace or if they've consumed too much fluid.