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, thus lowering their sodium levels, in rare cases, they may fall victim to a potentially fatal condition called hyponatremia.

Experts define hyponatremia as low sodium concentration in the body, specifically the extra-cellular fluid which is the blood and the fluid around the body's cells. "When someone becomes hyponatremic, their sodium level falls below normal," says Amy Roberts, Ph.D., sports scientist at the Boulder Center for Sports Medicine in Boulder, Colorado.

Normal sodium levels run between 135-145 milliequivalents per liter (mEq/L), says Lawrence E. Armstrong, Ph.D., professor of exercise and environmental physiology at the University of Connecticut. Hyponatremia is generally defined as a sodium level of 135 mEq/L or below.

What Causes Hyponatremia in Athletes?

Because the condition cannot be safely duplicated in laboratory studies, little is known about the causes of hyponatremia. Armstrong says experts have three main theories.

  • Theory #1: The extracellular fluid becomes diluted because the sweat volume and the sweat sodium concentration are very high, Armstrong says.
  • Theory #2: An anti-diuretic hormone known as AVP (arginine vasopressin), a hormone that causes the body to retain water in the urine and extracellular fluid, reduces sodium levels. AVP secretion causes water retention so that rather than losing fluid, people retain it, Armstrong explains.
  • Theory #3: Armstrong supports a third theory which proposes that sodium losses are moderate or normal in sweat and urine. "Yet because a large volume of water or diluted fluid is consumed and subsequently retained in the body," he says, "the sodium concentration of the blood is reduced to critical levels."

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?

Several conditions have to be present for hyponatremia to occur. First, the athlete has to be sweating. So it's more likely that this will happen in hot, humid environments rather than hot, dry climates, Roberts says.

By far, though, the most important factor is the duration of the activity. Recreational athletes aren't likely to experience hyponatremia, or what's also known as water intoxication. "Hyponatremia is overdramatized," says Mark Baugh, Pharm.D., author of Sports Nutrition: The Awful Truth. "Unless you're in a sport where you just have to keep running for hours to win the race, you're not going to have this problem."

Still, because of the growing popularity of endurance events such as marathons and triathlons, there may be an increase in hyponatremia-related collapses during and after these events.

Who Is Most at Risk for Hyponatremia?

Ultra-Endurance Athletes

Ultra-endurance athletes who enter such events as 100-mile races and Ironman competitions are most at risk. Baugh says that 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. Even then, it's a rare occurrence. In a race of 1,000 runners, there might only be one or two athletes who develop clinical symptoms of hyponatremia, Armstrong says.

Other Endurance Athletes

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

Baugh suggests that ultra-endurance athletes salt their food more than the average person. "Don't be shy with the salt," he says. "Start with a good supply of sodium in your body." (Recreational athletes, don't fear. Armstrong says the United States diet adequately supplies the amount of sodium lost in sweat.)

Choose Drinks Wisely

All athletes should drink water before their events, but once you've been exercising for more than an hour, sports drinks or other fluids with sodium are better choices. "The whole goal is to replace water at the exact rate that it's being lost and to add sodium to that ingested fluid," Roberts says. That's hard, though, because most people become thirsty after they're already dehydrated.

Experts disagree on how much fluid to drink during endurance events. One recommendation is to consume 150-300 mL (milliliters) of fluid every 15 minutes. However other experts recommend drinking according to thirst, but not exceeding 400-800 mL per hour.

Weigh In

Athletes should weigh themselves before, during, and after their long-distance events. "The safest way to consume fluids is not to overdrink and not to underdrink," Armstrong says, adding that 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.

Armstrong recommends checking body weight at the same time every day before and after an endurance event. Doing this will help athletes gauge how much fluid they need to replace or if they've consumed too much fluid. For every pound lost, they should drink one pint of fluid. As Armstrong says, citing a popular phrase, "A pint's a pound the world around."