Preventing and Treating Hyponatremia in Endurance Athletes

By Scott Black, M.D., M.S.

For years, endurance athletes have been cautioned about the dangers of dehydration and overheating during exercise.  They have been encouraged to drink liberally during physical activity to avoid these hazards.  Now, a new buzzword has crept into the sport vocabulary and we’re being encouraged to hydrate more cautiously to avoid the complications of hyponatremia.

 Hyponatremia literally means “too little sodium in the blood” and in the last eight years, it has been responsible for at least eight deaths in endurance events.  Though the experts are still debating the cause, there is general agreement that it represents an imbalance of water and salt in the blood.

Mild hyponatremia can be hard to distinguish from fatigue, early heat illness, or low blood sugar.  Athletes with low blood sodium can initially develop a headache along with irritability.  They may feel “puffy” and develop swelling in their hands and feet.  Nausea, vomiting, and abdominal bloating are also frequent early complaints.  If not recognized, worsening hyponatremia can bring about loss of balance, confusion, seizures, and progressively, coma and death.

The development of moderate or severe hyponatremia generally will mean the end of competition for that day and might even involve a trip to the hospital for management.  Obviously, these are two outcomes that most athletes would rather avoid, so it is very important that we do all we can to prevent its occurrence.

The single most important thing that any athlete can do to avoid hyponatremia is to drink the right amount of fluid before and during exercise.  Given our earlier fears about dehydration, we have often consumed as much fluid as we could hold just before and subsequently during a race.  Water stops are frequent in most events and we’re often encouraged to drink more fluid.  There is no doubt that drinking too little and becoming dehydrated can significantly impair performance during a race.  Drinking too much, however, can lead to all of the problems associated with low sodium.

Recognizing this, the American College of Sports Medicine (ACSM) has recently updated its guidelines for fluid replacement during physical activity.  The goal of drinking prior to exercise is to begin the physical activity with the proper balance of water and salt, not to begin “super-hydrated”.  Athletes should drink slowly, starting about 4 hours prior to a race and consume between 0.08 and 0.1 ounces for every pound they weigh.  For example, a 150-pound triathlete should consume between 12 and 15 ounces over the 4 hours before an event.  The ritual pre-race trip to the porta-john should produce clear, light-colored urine if you are properly hydrated (and not taking vitamins).

During exercise the goal changes to preventing excessive dehydration and avoiding over-hydration and potentially dangerous changes in electrolytes.  Fluid intake should be matched as carefully to sweat loss as possible.  Individual sweat rates vary and the best way to know your sweat rate is to measure it during a workout.  Weigh nude then exercise for one hour at race intensity and in as close to the same heat and humidity as you expect race day.  Following the workout, remove your wet clothes, dry off and weigh nude again.  Your weight loss in that hour is a close estimate of your sweat loss. Sixteen ounces of sweat weighs about one pound, so if you lose 1½ pounds during the hour of exercise, your sweat rate is 24 ounces per hour.  You would need to consume fluid at that same rate to stay properly hydrated. If race conditions are more hot and humid or if you exert yourself harder, your sweat rate will increase and you will need to make allowances in your fluid intake.  Likewise, cooler conditions or lower levels of exertion produce less sweat loss and require less fluid intake.

In more extreme events, such as long-course triathlon, it is also possible to lose excessive sodium in sweat so it becomes important to replace salt as well as water over the course of the event.  Substituting sodium-containing sports drinks for water is one way to take in salt during a race.  Eating salty foods like pretzels or broth especially during the last half of an event can be beneficial as well.  Some athletes take salt tablets to replace lost sodium.  Regardless of your sodium-replacement strategy, make sure that you practice it in your longer workouts.  Salt tablets or even salty food can be hard on the stomach and you don’t want to try these for the first time on race day.

In conclusion, as an athlete preparing for an Ironman event you have spent countless hours training.  Proper hydration and salt balance during the event will increase your chances for a strong finish.  It is well worth the small amount of extra effort to plan and practice your fluid and salt replacement strategy in advance.

Dr. Black is an avid runner, cyclist, and triathlete.  He Received a Master’s in Exercise Physiology and MD from University of Kentucky.  Dr. Black Completed a Family Medicine residency at UK and later a Sports Medicine fellowship at Wake Forest University.  He is also Certified as an Exercise Specialist by the American College of Sports Medicine.

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