Drink Before Thirst to Avoid Dehydration
When you compete in sports, you need to drink before you feel thirsty because you slow down and lose power long before you have any signals to tell you that you are dehydrated.
In warm weather, trail runners raced 12 km (7.2 miles) much faster when they took fluids (Journal of Athletic Training, March-April 2010). With fluids, they averaged 53.1 minutes compared to 55.7 minutes without fluids. Immediately after the race, the dehydrated runners had signs of greater body stress such as heart rates six beats per minute faster and intestinal temperatures .22 degrees C higher.
You won’t feel thirsty during exercise until you have lost between two and four pints, or two to four pounds. Thirst is a very late sign of dehydration. You sweat during exercise, and since sweat contains much less salt than your blood, you lose far more water than salt during exercise. As blood salt levels rise higher and higher, they trip off special osmoreceptors in your brain to tell you that you are thirsty. Since it takes a long time for blood salt levels to rise high enough to tell you that you are thirsty, you will be severely dehydrated long before you feel thirst. During long sports competitions, you need to take sugar as well as fluid because running out of sugar stored in muscles slows you down. The only mineral that you need to replace during exercise is common table salt. Water or your favourite drink plus food containing sugar and salt are just as effective as any sports drink to maintain endurance and prevent heat exhaustion.
The best exercise drink is the one that tastes best to you, because that’s what you will drink the most (International Journal of Sport Nutrition and Exercise Metabolism, January 2002).
On very rare occasions, drinking too much fluid causes death from a condition called hyponatremia. It almost always occurs in people who attempt events that are beyond their training levels. They run out of energy, slow down and expend more effort drinking fluids than in maintaining their pace. This condition is caused by drinking too much fluid, not by excessive loss of salt in sweat or by the stress of exercising. The extra fluid expands blood volume and dilutes blood salt levels. This causes blood salt levels to drop to low levels, 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 and the athlete has been drinking often during the event. All people who are confused, pass out or have seizures should be sent to a hospital immediately. Hyponatremia requires skilled management because the first impulse of an inexperienced physician is to give intravenous fluids, which dilute blood salt levels further, causing more brain swelling that 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. 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 (Clinical Journal of Sport Medicine, July-August 2005). 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 who are out of shape should limit fluid intake, probably to less than two large water bottles per hour. When you exercise for more than an hour, particularly in hot weather, you need fluid, salt and sugar. We drink Pepsi Cola and eat salted peanuts.
Dr. Gabe Mirkin’s Fitness and Health E-Zine June 27, 2010
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