Hyponatremia or Overhydration—who is at risk?

Hyponatremia has gained media attention in the last few years, but it is important for athletes to realize that dehydration is much more common and 'overhydration' is a risk mostly associated with ultra-endurance sports and not sports events lasting less than 2 hours such as hockey, basketball and soccer, or shorter hikes/runs.

Hyponatremia, a low concentration of sodium in the blood, has become more prevalent in athletes as more people are participating in endurance sports lasting more than 3 hours such as marathons. Such prolonged activity and excessive sweat production increases the risk of an athlete having too little sodium in their blood stream during training and competition. The hyponatremia or 'overhydration' associated with prolonged exercise arises primarily from fluid overload, under replacement of sodium losses, or both. When blood sodium concentration falls to abnormally low levels, a rapid and dangerous swelling of the brain occurs, that can result in seizures, coma, and death.

Because sodium is lost in sweat, it is important for individuals who exercise at high intensity to get adequate sodium before, during and after exercise, especially as they continue to drink water. This is even more critical during ultra-endurance competition, for those who are slower (i.e. more than 4 hrs, and therefore have more time to drink excessively) and for smaller athletes who can dilute their blood plasma more quickly than larger individuals.

Fatal hyponatremia in athletes is rare, but it has claimed the lives of marathon runners and military recruits, and should therefore be brought to the attention of sports/health professionals. However, it is clear from epidemiological (population) data that hyponatremia associated with prolonged exercise is quite rare. It is very important to keep the risk of 'overhydration' in perspective. For example one study of 'higher risk' athletes who developed symptomatic hyponatremia were participating in distance running events of  42 km and triathlons lasting 9–12 h. In these events, symptomatic hyponatremia still only occured in 0.1–4% of the participants.

Conversely, dehydration has been estimated to occur in up to 80% of athletes in activities such as team sports, tennis, as well as endurance events, and remains to be the primary challenge to an athlete’s physiological homeostasis or performance.  There appears to be little physiologic need to replace electrolytes during a single exercise session of moderate duration (e.g., less than 2 to 3 h), particularly if sodium was present in the previous meal.

Excessive drinking is a key risk factor for hyponatremia, and this risk can be reduced by making certain that fluid intake does not exceed sweat loss and by ingesting sodium containing beverages or foods to help replace the sodium lost in sweat. As a safeguard athletes can include sodium containing sports drinks for exercise lasting more than 1-2 hours, especially during activity in the heat when sweat losses are greatest. Sports drinks are also beneficial by means of supplying carbohydrates when muscle fuel stores may run low during intense training or competition lasting more than 1-2 hours.

Many athletes however, may prefer to drink only water, and should be aware of misleading advice to 'always drink as much as you can'. The amount of water an athlete should drink depends on their volume of sweat and the sodium concentration of their sweat, both of which can vary depending on aerobic fitness, exercise intensity and ambient temperature. The following outlines position statements of various organizations:

American Dietetics Association, Dietitians of Canada, and American College of Sports Medicine (2000): "Athletes should drink enough fluid to balance their fluid losses. Two hours before exercise, 400 to 600 ml (14 to 22 oz) of fluid should be consumed, and during exercise, 150 to 350 ml (6 to 12 oz) of fluid should be consumed every 15 to 20 minutes depending on tolerance."

American Academy of Pediatrics (2000): "Before prolonged physical activity, the child should be well hydrated. During the activity, periodic drinking should be enforced (e.g., each 20 minutes 150 ml [5 oz] of cold tap water or a flavored salted beverage for a child weighing 40 kg (88 lbs) and 250 ml [9 oz] for an adolescent weighing 60 kg (132 lbs)), even if the child does not feel thirsty. Weighing before and after a training session can verify hydration status if the child is wearing little to no clothing."

National Athletic Training Association (2000): "To ensure proper pre-exercise hydration, the athletes should consume approximately 500 to 600 ml (17 to 20 oz) of water or a sports drink 2 to 3 hours before exercise and 200 to 300 ml (7 to 10 oz) of water or a sports drink 10 to 20 minutes before exercise. Fluid replacement should approximate sweat and urine losses and at least maintain hydration at less than 2% bodyweight reduction. This generally requires 200 to 300 ml (7 to 10 oz) every 10 to 20 minutes."

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