Understanding Exercise-Associated Hyponatremia

A Conversation with Dr. Bob Murray

The importance of starting, and remaining, well-hydrated during physical activity – especially in warm environments – is well recognized among sports experts. Proper hydration not only helps maintain important physiological functions, it also reduces an athlete’s perception of physical effort, and increases exercise capacity.

However, it is essential that athletes understand the importance of avoiding excessive fluid intake and inadequate sodium intake during prolonged physical activity, which can lead to a relatively rare, but potentially life-threatening condition called exercise-associated hyponatremia.

To better understand exercise-associated hyponatremia and how to prevent it, The Coca-Cola Company Beverage Institute for Health & Wellness (BIHW) talked with Dr. Bob Murray, one of the world's leading experts in sports nutrition and hydration.

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  • What is meant by the term exercise-associated hyponatremia?

    • Dr. Murray: Hyponatremia refers to a potentially dangerous reduction in the sodium concentration in the blood. When hyponatremia occurs with exercise, it is referred to as exercise-associated hyponatremia.

      Hyponatremia is also a lesson on how too much of a good thing, in this case water, can quickly become a very bad thing. Most hyponatremia that occurs during or following exercise is a result of the ingestion of too much water. Close

  • How much water is too much?

    • Dr. Murray: Cutting right to the science of the question, hyponatremia defines the consumption of too much water.  For Example, hyponatremia during exercise is often, but not always, associated with weight gain caused by drinking too much water.

      Under normal circumstances, thirst is a good indicator of fluid needs such that at the end of each day, we usually return to normal hydration. But during exercise, thirst often lags behind fluid needs, so athletes should be encouraged to drink to keep pace with sweat losses. It’s only in rare circumstances that people become hyponatremic by forcing themselves to drink well beyond the point at which thirst is quenched.Close

  • Why is it important for health professionals to understand hyponatremia?

    • Dr. Murray: Although exercise-associated hyponatremia is not a big risk from an epidemiological perspective, it can be deadly for individuals. That’s why it’s important that health professionals understand this disorder. For example, although there have been seven documented cases of hyponatremia-related deaths of marathon runners over the past two decades, it’s important to realize this condition is not limited to athletes.

      Hyponatremia has taken the lives of soldiers who overdid hydration during long marches, a college freshman who had to drink without urinating as part of a hazing ritual, two young girls forced to drink water by their parents as punishment, a bicycle police officer who overdid his water intake during a day of training, and others who over-consumed water while at rest or during physical activity.Close

  • How does ingesting too much water cause blood sodium concentrations to drop?

    • Dr. Murray: Drinking too much water too quickly will inevitably cause blood sodium levels to fall because humans can drink faster than they can urinate. Now, a small drop in sodium levels is usually not a problem – people will normally stop drinking when no longer thirsty and the excess water will be urinated off.

      However, when people vastly overdo their water intake before, during, or after exercise, blood sodium can quickly drop to dangerously low levels because the body simply cannot get rid of the excess water fast enough to keep blood sodium at a safe level.Close

  • What are symptoms of hyponatremia, especially associated with exercise?

    • Dr. Murray: Not surprisingly, some of the symptoms of hyponatremia are what you’d expect from drinking too much water: abdominal bloating, swollen fingers and toes, swollen ankles, and swollen wrists. Other symptoms are consistent with the dangerous swelling of the brain caused by hyponatremia: unusual fatigue, throbbing headache, loss of coordination, aggressive behavior, confusion, seizures, and coma.

      Physicians and nurses who staff the medical tents at marathons and triathlons see hundreds of athletes on race day and hyponatremia is just one of many race-related problems they must keep in mind. Fortunately, a simple blood test is all that is needed to confirm hyponatremia and begin treatment.Close

  • Who is most at risk for exercise-associated hyponatremia?

    • Dr. Murray: Exercise-associated hyponatremia is most common in small female endurance athletes who avidly drink water before and during endurance events such as marathons.

      But large athletes are not immune to hyponatremia – and anyone who drinks enough water in a short period of time is at risk. This happened a few years back when an NFL lineman drank an estimated four gallons of water in one day during summer training camp and ended up with severe hyponatremia. He was fortunate to survive.

      In addition, just as sweat rate differs, the amount of salt in sweat varies from one person to another. The term “salty sweating” refers to excess sodium in sweat. Individuals with salty sweat may be at greater risk for hyponatremia. Clues to excess salt in sweat include salty white residue on dark clothes worn during exercise and sweat that stings eyes, abrasions or cuts.Close

  • Is it possible to be both dehydrated and hyponatremic at the same time?

    • Dr. Murray: Yes, strangely enough, it is possible, even though it seems counterintuitive. Hyponatremia is most often a disorder of drinking too much, which is why hyponatremia is sometimes referred to as water intoxication. But dehydrated athletes can also suffer from hyponatremia if they lose enough sodium in their sweat and drink enough water to dilute the blood to hyponatremic levels.

      For example, if an athlete loses 10 liters of salty sweat during an Ironman-distance triathlon and drinks eight liters of plain water, the athlete will be both dehydrated and hyponatremic due to the combination of salt loss and water intake.Close

  • How much and what should athletes drink during exercise?

    • Dr. Murray: Individual hydration needs vary quite a bit because people sweat at different rates, even during the same activity. That’s why coaches should encourage athletes to periodically weigh themselves before and after practices. Weight loss signals drinking too little, while weight gain signals drinking too much. After weighing in and out for a few practices, athletes get a quick sense of how much they should drink to minimize weight loss (dehydration) during exercise. Some athletes may need to drink only six ounces of fluid each hour, while a teammate may need 48 ounces or more to maintain hydration. When sweat losses are light and performance is not a concern, drinking plain water is fine. But on those occasions when sweat losses are high and athletes want to get the most from their bodies, sports drinks provide additional benefits.Close

  • How can athletes monitor hydration status?

    • Dr. Murray: In addition to periodically weighing themselves before and after practices, athletes can monitor the color and volume of their urine. In a well-hydrated person, urine looks more like lemonade than apple juice. When dehydrated, the body tries hard to hold onto fluid, so urine volume is small. An athlete who excretes a small volume of dark urine is most likely dehydrated. Some sports teams have color-coded urine charts hanging above urinals and in stalls to remind athletes to pay attention to urine color.Close

  • How reliable is monitoring urine to gauge hydration status?

    • Dr. Murray: In a laboratory setting, measures of urine color, volume, and specific gravity or osmolality provide useful and reliable information about a person’s hydration status. In the locker room, reliability is not as important. From a practical perspective, encouraging athletes to pay attention to urine color helps increase their awareness of the importance of staying well-hydrated.Close

  • Should athletes ingest sodium during exercise?

    • Dr. Murray: The body loses sodium every time a person sweats, so it’s easy to see why sodium is a critical nutrient for athletes who regularly work up a sweat.

      For most people, sodium loss during exercise is relatively minor and is easily replaced through foods and beverages. But it’s important to keep in mind that many athletes train twice each day and sweat a lot in each training session. And not just athletes—construction workers, soldiers, miners, road workers, and farmers often sweat all day long. Sodium is the primary electrolyte in sweat, and daily sodium losses can be quite large, easily exceeding 10 grams. Ingesting sodium during exercise has a number of benefits such as maintaining desire to consume fluids, which is an important response in combating dehydration.  Sodium ingested during exercise also helps maintain blood volume and reduce urine output, two other critical responses that aid hydration.Close

  • What about other electrolytes?

    • Dr. Murray: Sweat also contains chloride, potassium, calcium, and magnesium, as well as smaller amounts of other minerals such as iron and zinc. While sodium is lost in far greater quantities than the other electrolytes, total electrolyte loss can be large in circumstances where sweat losses are large.Close

  • When do sports drinks offer advantages over plain water?

    • Dr. Murray: A well-formulated sports drink is simply water with additional benefits for athletes. Keep in mind that sports drinks are roughly 94% water, with carbohydrates and electrolytes making up most of the remaining 6%.

      For athletes, there are many occasions when sports drinks provide benefits beyond water. For example, voluntary fluid intake is better with sports drinks than with water, reducing the risk of dehydration. Sports drinks do a better job of maintaining blood volume and reducing urine loss than water. And sports drinks provide a quick source of carbohydrate energy that active muscles can use to keep going longer and stronger. When athletes work up a sweat during prolonged, intense physical activity and want to get the most out of their bodies, sports drinks offer additional benefits that plain water doesn’t provide. For individuals who want to replace fluid and electrolytes but not calories during exercise, zero-calorie sports drinks are an option.Close

  • Is there a role for pre-event and post-event food intake?

    • Dr. Murray: Overall food intake helps replace electrolytes, such as sodium and potassium that are lost in sweat.  And having a well-timed meal or snack prior to practice or competition helps maintain energy stores so we can exercise longer and feel better while doing so. Meals and snacks are also a great opportunity for athletes to get additional fluid on board.Close

  • Can you address the uses and benefits of zero-calorie sports drinks that contain electrolytes related to exercise-associated hyponatremia, yet contain no carbohydrate?

    • Dr. Murray: Zero-calorie sports drinks are a great hydration option for those who want to replace fluid and electrolytes but not calories during exercise. Electrolytes play an important role in hydration, including maintaining a normal blood sodium level.Close

  • In summary, how can physically active people reduce their risk for exercise-associated hyponatremia?

    • Dr. Murray: The best hydration advice for anyone who works up a sweat – regardless of the activity – is to drink enough during physical activity to minimize dehydration, but don’t over-drink.

      The very best way to gauge fluid needs is to weigh in before and after physical activity. Weight loss signals dehydration, especially loss of more than two percent of body weight. Weight gain is a sure sign of over-drinking and a clear warning to cut back on fluid intake. Athletes who sweat for prolonged periods should also be encouraged to consume sports drinks with sodium during exercise and to avoid restricting dietary sodium.Close

  • Also of Interest
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Biography

Bob Murray, PhD, FACSM

Dr. Bob Murray is founder and principal of Sports Science Insights, LLC. A native of Pittsburgh, Dr. Murray earned his B.S. and M.Ed. degrees in Physical Education at Slippery Rock University. He served as Assistant Professor of Physical Education and Head Swimming Coach at Oswego State University from 1974-1977 before returning to graduate school for his Ph.D. in exercise physiology from The Ohio State University. He served as Assistant/Associate Professor of Physical Education at Boise State University from 1980 to 1985 before relocating to Chicago to help found the Gatorade Sports Science Institute. An author of numerous publications in scientific texts and journals, Dr. Murray is a Fellow of the American College of Sports Medicine and served on the ACSM Board of Trustees. After leaving GSSI in April 2008, Dr. Murray founded Sports Science Insights, LLC, to help companies and organizations maximize the value and impact of scientific knowledge in exercise science and sports nutrition.

The views and opinions expressed by the experts and organizations quoted in this article are their own and do not necessarily represent the views of any institution or association to which they belong, nor The Coca-Cola Company.