Hydration for Dummies

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Sunday, 27 June 2010 20:15

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.