| Don't Let Dehydration
Dampen Your Workout
Federal governments and health professionals
are encouraging people to exercise more in order to combat obesity
and to prevent chronic health conditions. But both experienced
athletes and beginning exercisers need to be careful about exercising
in hot weather conditions.
Even though sweating profusely
may make you feel like your workout is in full swing, severe dehydration
can be dangerous because the body can't function without a certain
amount of water and electrolytes, which are minerals such as sodium
that are lost in sweat and urine.
People need to apply common sense in order to stay hydrated
during or after exercise, and serious athletes should take extra
precautions. Below, Michael Sawka, chief of Thermal and Mountain
Medicine at the U.S. Army Research Institute of Environmental
Medicine in Natick, Mass., discusses the effects of dehydration
and how it is best avoided.
How do you know you are dehydrated?
People generally refer to dehydration as a reduction in body water
below normal levels. The first thing you'll probably experience
is thirst. There is not a real precise relationship to how thirsty
you feel and how dehydrated you are. Usually when you get the
sensation of thirst, you're already somewhat dehydrated. You may
get a headache. You may feel dryness of the mouth. If you are
exercising or changing posture, you could feel dizzy. If you are
in hot weather or exercising in the heat, you may feel hotter.
Your skin may feel warmer. You would be urinating less frequently
and smaller volumes, so your urine would be dark in color because
it would be more concentrated.
There is also some evidence that both your physical and mental
performance capabilities decrease as a result of dehydration.
You may not be as sharp in terms of some of the types of complex
cognitive functions that you have to do. So there are a variety
How can dehydration affect one's health?
Acute dehydration will increase your risk of heat exhaustion and
heat stroke. [Heat exhaustion causes heavy sweating, fainting
and vomiting, and heat stroke occurs when the body temperature
rises to 106ºF or above.] Although it is not as well studied,
there is evidence that shows that chronic dehydration can have
a variety of different affects on chronic diseases, including
urinary tract infections, and kidney stones and gallstones. Some
evidence indicates that dehydration may be related to susceptibility
to bladder and colon cancer. But that evidence is not real strong.
What are common causes of dehydration?
Many types of stress will cause under-drinking and lead to dehydration.
Heat exposure and exercise are common causes. When you exercise,
a normal response is to sweat to regulate your body temperature.
You lose body water because you sweat more. And if you're exercising
in hot weather, you have a greater requirement for sweating because
you depend more on evaporation of sweat for body cooling. As a
result, people can become dehydrated from physical exercise, particularly
physical exercise in the heat.
People can become dehydrated in other ways as well. One other
way is through medications. For example, blood pressure drugs
such as diuretics are dehydrating because they work by decreasing
your total body water.
It's also common to see dehydration as a result of diarrhea
How much water does one need in a day?
How much water you need in a day varies. It depends on a lot of
factors: age, activity level, the environment you're exposed to.
For a normal healthy person, generally within reason, short-term
under-consumption is not too much of a problem, unless you're
physically active, because your kidneys will act to reduce your
urine output to conserve water. Likewise, you don't have to worry
about taking in too much fluid because your kidneys will remove
what you don't need. Over-consumption can become a problem, however,
during prolonged exercise (several hours) because urine output
For healthy adults, if you're expending about 3,000 calories
a day, the minimal amount you should take in would be about three
quarts of water a day, roughly three liters. It doesn't matter
if the water is contained in food or beverages.
For a very active person in very hot weather, such as an agricultural
worker or maybe a soldier out in the field in hot weather, the
requirements could be substantially higher. The government is
going to be putting out some guidelines about this later in the
Who is particularly at risk for dehydration?
There is some evidence that children may be more susceptible to
dehydration and that the adverse consequences may be more marked.
The elderly and people who are sick are at risk because of their
health. The elderly tend to under-drink anyway.
The physically active populations are also at risk. When you
dehydrate, the thermal and cardiovascular benefits you get from
adjusting to a high temperature, and high physical fitness, are
greatly compromised. So these very fit people who go out and run
hard, but decide they're not going to drink for whatever reason,
lose these advantages during exercise.
At what point do electrolytes need
to be replaced?
It depends. If you have diarrhea or vomiting, you can lose an
awful lot of mineral electrolytes, so you should replace them
right away. If you're performing exercise in temperate conditions
for less than two hours, it may not matter. But if you're exercising
in the heat, you probably want to start replacing electrolytes
The general rule of thumb is if you're exercising and having
high sweat rates for any prolonged period — let's say over
an hour — you probably then want to replace the electrolytes
at a rate proportionate to what you're losing. Sports drinks contain
electrolytes in concentrations proportionate to what is lost in
sweat by a moderately trained athlete.
Should people replace electrolytes
with a sports drink?
The National Academy of Sciences has looked at sports drinks,
and they have their place. I think that it's fair to say that
sometimes they're better than water and sometimes they're not.
When you are doing high intensity exercise of a prolonged nature,
the carbohydrates and the electrolytes sports drinks have can
provide advantages. Because they contain sodium, they stimulate
thirst and make it easier to hold the water that you ingest, and
they provide the energy that's needed to sustain physical exercise.
Athletes participating in hot weather training should consider
But for the average person maybe just going out and playing
a game of tennis or something like that, replacing carbohydrates
and electrolytes is probably not a concern.
How can you monitor your hydration
One ways to check hydration is by monitoring your body weight.
Generally, if you take your body weight every morning, it is relatively
constant. If you take your body weight and it's down all of a
sudden a lot one morning, you're probably dehydrated.
Another thing that you can do is monitor your urinary habits.
If you're urinating more frequently than usual, and if it's relatively
clear, you're probably very well hydrated. If you're urinating
infrequently and it's dark, you're probably dehydrated. But there
is no precise relationship between urine color and dehydration.
How can people who exercise avoid dehydration?
First of all, you should drink a nice tall glass of fluid maybe
an hour before you exercise. If you're still thirsty have another
one so that you have some idea that you're starting out hydrated.
You want to stay away from carbonated beverages and those with
high fructose because they can give people GI problems.
But overall, once you're post-exercise, drink whatever you want.
If it's liquid, it's probably pretty good. An important point
is that most people fully rehydrate their bodies at mealtime.
So if you're concerned about hydration, one of the worst things
you can do is skip meals. It's important that you have standardized
meals in a comfortable environment with plenty of fluids available.
Should you avoid caffeine before exercise?
Not if you're a regular caffeine drinker. Coffee makes you urinate
more, but the overall effects are not that significant in normal
coffee drinkers. I wouldn't prehydrate on black coffee, but if
you want to have some coffee it's not a big deal.
Is dehydration associated with heat
stroke and heat exhaustion?
One of the things that you often see with dehydration is elevated
body temperature. If a person is exercising in the heat, this
causes greater than normal elevation in the body temperature,
and cardiovascular strain.
Many years ago, when people didn't pay as much attention to
replacing fluid, dehydration was very often associated with heat
exhaustion and even heat stroke. However, now we pay a lot better
attention to replacing fluids and dehydration, so the risk of
these conditions is lower. However, modest dehydration —
a loss of 2 percent of your body weight — can significantly
reduce your ability to work out in hot weather.
What would your advice be in terms
of avoiding exercise on hot days?
You can exercise on hot days. You don't want to overdo it like
anything else. Clearly you should try to exercise in the cool
of the morning or the evening hours. But if you're like most people,
you exercise when you can.
If you're going to exercise in heat, use moderation. If you're
sick or have health problems, you may want to not exercise or
use caution. Don't go out on the hottest day and push it. Instead,
scale back your activity levels. For example, you can walk instead
of run, or run slower or for a shorter period of time. You can
be active, but you have to gauge how much you do depending on
the exposure and the amount of strain on the body. Use common
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