Hyponatremia should never happen to you.

For the last 40 years, sports medicine experts have told athletes in endurance events that they should take fluids frequently during events lasting more than one hour. However, three years ago, a 28-year-old woman collapsed and died after finishing the Boston Marathon. Her blood salt levels were extremely low and she died from a condition called hyponatremia. This summer, a policeman training for bicycle duty died of the same condition. In July 2005, sports medicine experts issued a warning to all athletes from the First International Exercise-Associated Hyponatremia Consensus Development Conference (Clinical Journal of Sport Medicine, July/August 2005.)

I have never seen this syndrome in well-conditioned athletes. It has been reported almost exclusively in less-fit, slower and novice athletes, and is far more common in women. Hyponatremia is caused by drinking too much fluid and is not caused by excessive loss of salt in sweat or by exercising. When people with psychiatric problems force themselves to drink huge amounts of water while sitting still, they also can die of hyponatremia, only in this case, it is called water intoxication.

The excess fluid expands blood volume and dilutes blood salt levels. This causes blood salt levels to be very low, while brain salt levels remain normal. Fluid moves from an area of low salt concentration into areas with high salt levels. So fluid moves from the bloodstream into the brain, causing brain swelling. Since the brain is enclosed in the skull, which is a tight box, the brain expands and has nowhere to go, so it is squashed to cause headache, nausea, and blurred vision. Since these are the same symptoms caused by pure dehydration with normal blood salt levels, the only way to diagnose the condition is with blood tests.

As blood salt levels drop even lower, the person becomes confused, develops seizures and falls unconscious. You should suspect hyponatremia when the event takes more than four hours, the athlete is in her first ultra-long endurance event, and she has been drinking a lot as she exercises. All people who are confused, pass out or have seizures should be sent to a hospital immediately. The condition requires skilled management because the first impulse of an inexperienced physician is to give intravenous fluids, which dilute blood salt levels further and swell the brain and can kill the patient.

How much fluid should you drink? You will not become thirsty during exercise until you have lost between two and four pints of fluid, so you can't wait for thirst to encourage you to drink. Dehydration makes you tired and it is unlikely that you can replace the lost fluid during a race after you have become thirsty. Blood has a much higher concentration of salt than sweat, so when you sweat, you lose far more water than salt. This causes blood salt levels to rise. Thirst is controlled by certain cells in your brain called osmoreceptors which are stimulated to make you thirsty only after blood salt levels have risen considerably. So you will not become thirsty until you are significantly dehydrated.

The American College of Sports Medicine recommends a limit of 1200cc (5 cups, 2.5 pints, a little over 1 quart, or 2 average size water bottles) per hour, but for a person who is not exercising near his or her maximum, this could be too much. A person exercising near his capacity and not slowed down by fatigue probably does not have to worry about limiting fluid intake. He is working so hard at maintaining intensity, he doesn't have enough time to drink too much. On the other hand, people slowed down by fatigue or those out of shape, should limit fluid intake, probably to less than two water bottles per hour. If you are exercising for more than an hour, you should also replace salt, either with salted sports drinks or salted foods.