With the summer months approaching many of us will turn to the outdoors for our exercise fix. We know that it is important to drink enough water to replace the fluid lost by sweating during vigorous workouts in order to avoid becoming dehydrated. With sales of bottled water and sports drinks on the rise it is easy to find fluids just about anywhere there are food products. Many of us were brought up on the idea that drinking large amounts of water is safe and the more the better especially when exercising in hot, humid conditions.

This thinking is generally correct. Many of us do need to hydrate ourselves sufficiently during intense, long-term exercise. However a new problem is on the rise and it results from drinking too much water during exercise. The problem is a condition known as hyponatremia which results from having a too low concentration of sodium in our blood.

Hyponatremia occurs when the concentration of sodium in the blood is too low (less than 135 milliequivalents per liter). Symptoms of hyponatremia include nervous system problems such as tiredness, lethargy, confusion, irritability, seizures and coma.

According to a study in the Archives of Pathology and Laboratory Medicine (1) that examined causes of collapse in marathon runners in the 2003 Boston marathon it was found that a significant number of runners suffered from hyponatremia.

Hyponatremia appears to be more prevalent in the slower marathon runners completing races in greater than 4 hours and those who use hyperhydration techniques before events.

Hyponatremia is difficult to diagnose without measuring blood sodium levels because the symptoms are similar to dehydration. It is very dangerous to give fluids to someone with hyponatremia and it could cause swelling of the brain and even death. Other signs of too much water intake included weight gain of greater than 1 kilogram that was correlated with water intake of greater than 3 liters.

The researchers recommended limiting water intake during a marathon to between 400-800 ml per hour. These recommendations could also be applied to other forms of vigorous exercise such as bicycling, hiking or dancing.

Another recommendation from the American College of Sports Medicine is to drink fluids at regular intervals rather than at one time and consume salty snacks to replace lost sodium.

Hyponatremia can be a very dangerous problem and is counterintuitive to our belief about drinking lots of fluids during extreme exercise. In the coming summer months it is important for athletes, trainers and coaches to be aware of this problem in order to avoid serious consequences.
References:

1. Kratz, A., et. al. Sodium status of collapsed marathon runners. Arch Pathol Lab Med. 2005 Feb:129(2):227-30