The Risk and Reality of Hyponatremia
Robert Murray, Ph.D
Gatorade Sports Science Institute
Hyponatremia is a disorder in fluid-electrolyte balance that results in abnormally low plasma sodium concentration (<135 mmol/L; normal = 138-142 mmol/L). A sustained decrease in plasma sodium concentration disrupts the osmotic balance across the blood-brain barrier, resulting in a rapid influx of water into the brain. This causes brain swelling and a cascade of increasingly severe neurological responses (confusion, seizure, coma) that can culminate in death from rupture of the brainstem.
The lower and faster blood sodium falls, the greater the risk of life-threatening consequences. A decrease in plasma sodium concentration to 125-135 mmol/L usually results in little more than gastrointestinal symptoms such as bloating and nausea. Below 125 mmol/L, the symptoms become more severe and include confusion, throbbing headache, wheezy breathing, swollen hands and feet, unusual fatigue, and incoordination. When plasma sodium concentration drops below 120 mmol/L, seizure, coma, and death become more likely. However, some athletes have survived hyponatremia of <115 mmol/L, while others have died at >120 mmol/L.
Hyponatremia can be caused by dozens of different factors, but in athletes excessive drinking is often, but not always, the common denominator. Even in the absence of other physiological provocations, excessive drinking alone can result in hyponatremia, as has occurred in people who have ingested large volumes of fluid (e.g., 3 liters of fluid in an hour) in attempts to hasten micturition for drug tests.
During exercise, there is an elevated risk that excessive drinking will result in hyponatremia. There are two reasons for this. First, urine production is decreased during exercise, reducing the body's ability to excrete excess water. Second, sodium loss (via sweat) is increased, so it becomes easier to dilute the body's sodium content.
Overdrinking water during either rest or exercise will unavoidably result in a temporary drop in plasma sodium concentration. The kidneys respond to this overload by increasing urine production to allow plasma sodium concentration to return to normal. Under most circumstances, abnormally large volumes of water must be ingested to create a hyponatremic state, such as in the case of the female marathon runner who drank 10 liters of water the night before a race. Unfortunately, this aberrant behavior does occur in some individuals who mistakenly assume that overconsumption of water is not dangerous. Although dehydration remains the primary challenge for the vast majority of athletes, hyponatremia should be recognized as a possible threat to those athletes who go overboard in their hydration practices.
DRINK TO STAY HYDRATED, DON'T OVERDRINK -- Excessive drinking dramatically increases the risk of hyponatremia. It is vital not to overdrink before training or competition, because doing so can lower blood sodium before the event begins. Also, don't overdrink during or after exercise. Your fluidreplacement plan should be designed to avoid or at least minimize dehydration. Weight gain during training or races is a sure sign of overdrinking. A simple way to gauge fluid needs is to weigh nude before and after a workout. A minimal weight loss indicates an effective fluid-replacement regimen. Weight loss of more than 1 - 2 pounds indicates dehydration and a need for drinking more during your next exercise session. Gaining weight during exercise is a sure warning sign of excessive drinking, so fluid intake should be reduced during subsequent bouts of exercise.
MAINTAIN A SALTY DIET to make certain you replace all of the salt lost during training - which can be considerable in some athletes. During a long race (e.g., more than four hours), consider eating salty snacks such as pretzels, especially if you are a salty sweater (skin and clothes caked in white residue).
FAVOR SPORTS DRINKS over water during training and competition, to keep your body hydrated, fueled and salted. The flavor of a sports drink will encourage you to drink enough to stay hydrated while providing the electrolytes to help replace what is lost in sweat during long periods of exercise.
RECOGNIZE WARNING SIGNS of hyponatremia. When in doubt, stop exercise, stop drinking, and seek medical help fast.